AlllDO TTbnt.

UtL INST OF STANDARDS & TECH RXC,

A1 11 009961 96

egg, Rosalie T/Economlc evaluation ol IHS U58 V119.1979 C.4-NBS-PUB-C 1979

NBS BUILDING SCIENCE SERIES 119

Economic Evaluation of Windows

in Buildings: Methodology

U.S. DEPARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS °*
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901 . The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Mechanical

Engineering and Process Technology^ — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of ,computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundatiort for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. ^Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
MatfonaF Bureau of Standards APR '^ V

NBS BUILDING SCIENCE SERIES 119

Economic Evaluation of Windows

in Buildings: Methodology

Rosalie T. Ruegg

Robert E. Chapman

Building Economics and Regulatory Technology Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1979
Library of Congress Catalog Card Number: 79-600042

National Bureau of Standards Building Science Series 119

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 119, 118 pages (Apr. 1979) CODEN- BSSNBV

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $3 Stock Number 003-003-02042-7
PREFACE

The work covered in this report has been conducted within the framework

of a National Bureau of Standards (NBS) Interdisciplinary Research

project on the energy-related performance of windows. This effort has

been supported in part by NBS and in part by the Energy Research and

Development Administration (Mode 2 of Contract E(49-l) 3800), jointly

with the Department of Housing and Urban Development (Contract No. RT

193012), in conjunction with the Building Energy Performance Standards

Program.

COVER: (illndom -in hou^Q^ and In o{)^i.cii6 cjin kavo, ImponXant ejJi^ecXi on tk(UA tl{)2.-tlm2. (io6t^ and benejj-iti.
ACKNOWLEDGEMENTS

The authors wish to thank all of those persons in the Center for

Building Technology who contributed to the preparation of this report.

Special appreciation is extended to Dr. Harold Marshall, Chief of the

Applied Economics Program, for his assistance throughout the project;

to Dr. Stephen Weber, Dr. Belinda Collins, and Mr. Heinz Trechsel for

reviewing the draft report and making many helpful comments; to

Ms. Kimberly Hockenbery for her assistance with the computer analysis;

and to Ms. Bobbie Cassard and Ms. Cynthia Broussalian for their assistance in preparing the tables, graphs, and figures. Appreciation is

also extended to Dr. Douglas Shier of the Center for Applied Mathematics

for his excellent review of the report.

iv
Economic Evaluation of Windows in Buildings: Methodology

by

Rosalie T. Ruegg and Robert E. Chapman

Abstract

This study, which is one part of a National Bureau of Standards

interdisciplinary project on windows, is aimed at improving the costeffectiveness of window selection and use in buildings. It develops and

illustrates a life-cycle costing evaluation model and computer program

for assessing for alternative window systems the net dollar impact of

acquisition, maintenance and repair, heating and cooling energy gains

and losses, and artificial lighting and daylighting trade-offs. The

method is applicable to the evaluation of many different window sizes,

designs, accessories, and uses, both for new and existing residential

and commercial buildings. Two step-by-step examples of evaluating

selected window alternatives in a residence and in an office building

in Washington, D.C. serve to illustrate the application of the method.

A companion report, A Regional Economic Assessment of Selected Window

Systems , presents the results of eight additional residential case studies and eight additional commercial case studies. While the emphasis of this report is on the method of evaluation, the companion report

focuses on summarizing the results of a regional analysis in a form that

will be convenient for use by building owners, operators, designers,

financiers, and builders, those whose interest centers on the actual

implementation of research results.

Key Words: Building economics; daylighting; economic analysis; energy

conservation; engineering economics; life-cycle costs; solar

heat gain; thermal efficiency; window; window management

V
TABLE OF CONTENTS

Page

Preface iii

Acknowledgements iv

Abstract v

List of Tables viil

List of Figures x

SI Conversion xii

1. INTRODUCTION 1

1.1 Background 1 1.2 Purpose 4

1.3 Scope and Approach 4

1.3.1 Delineation of Costs and Benefits Treated. . . 5 1.3.2 Choices in Design and Use 7 1.3.3 Method of Treatment 8

1.4 Organization 10

2. LIFE-CYCLE COST METHODOLOGY 11

2.1 General Description of the Life-Cycle Cost Method . . 11

2.2 Development of the Model 13

2.3 Flowcharting of the Computer Program 17

2.4 Graphical Representations 22

2.5 Limitations of the Model 26

3. ILLUSTRATION OF THE METHODOLOGY: RESIDENTIAL CASE STUDY . 27

3.1 Basic Assumptions 28

3.2 Envelope Costs 32

3.3 Energy Costs 35

3.3.1 Energy Costs for Unmanaged Windows Not Used

for Daylighting 36

3.3.2 Energy Costs for Managed Windows Not Used for

Daylighting 41

3.3.3 Energy Costs for Unmanaged Windows Used for

Daylighting 41

3.3.4 Energy Costs for Managed Windows Used for

Daylighting 45

vi
Page

3.4 Life-Cycle Costs 48

3.4.1 Life-Cycle Costs for Unmanaged Windows Not Used

for Daylighting 48

3.4.2 Life-Cycle Costs for Managed Windows Not Used

for Daylighting 48

3.4.3 Life-Cycle Costs for Unmanaged Windows Used

for Daylighting 52

3.4.4 Life-Cycle Costs for Managed Windows Used

for Daylighting 55

3.5 Implications of Results 59

4. ILLUSTRATION OF THE METHODOLOGY: COMMERCIAL CASE STUDY. . . 65

4.1 Basic Assumptions 65

4.2 Envelope Costs 70

4.3 Energy Costs 72

4.3.1 Energy Costs for Unmanaged Windows Not Used

for Daylighting 72

4.3.2 Energy Costs for Managed Windows Not Used for

Daylighting 73

4.3.3 Energy Costs for Unmanaged Windows Used for

Daylighting 75

4.3.4 Energy Costs for Managed Windows Used for

Daylighting 78

4.4 Life-Cycle Costs 78

4.4.1 Life-Cycle Costs for Unmanaged Windows Not

Used for Daylighting 78

4.4.2 Life-Cycle Costs for Managed Windows Not

Used for Daylighting 81

4.4.3 Life-Cycle Costs for Unmanaged Windows Used

for Daylighting 83

4.4.4 Life-Cycle Costs for Managed Windows Used

for Daylighting 83

4.5 Implications of Results 87

5. SUMl^lARY, CONCLUSIONS, AND SUGGESTIONS FOR FURTHER RESEARCH . 93

SELECTED REFERENCES 95

APPENDIX - Computer Program for Life-Cycle Cost Evaluation of

Windows (BASIC Language) 97

vii
LIST OF TABLES

Table Page

1.1 Estimated Number and Cost of Windows Used in New

Residential Construction in a "Typical" Year 2

1.2 Costs and Benefits of Windows 6

2.1 Elements in the Life-Cycle Cost Model for Windows. . . 14

2.2 Sample Computer Printout From Window Life-Cycle

Costing Computer Program: Energy Costs, Envelope

Costs, and Total Costs 20

2.3 Sample Printout of Life-Cycle Cost Optimization,

Payback, and Fuel Price Sensitivity Routines:

Minimization of Life-Cycle Costs 21

3.1 Assumptions for Residential Case Study 31

3.2 Residential Case Study: Acquisition Cost of a Window in Excess of the Cost of a Windowless Wall. . . 33

3.3 Residential Case Study: Cost of Window Accessories. . 34

3.4 Residential Case Study: Maintenance and Repair Costs. 34

3.5 Life-Cycle Costs When Windows Are Unmanaged and Not

Used for Daylighting 50

3.6 Life-Cycle Costs When Windows Are Managed and Not

Used for Daylighting 51

3.7 Life-Cycle Costs When Windows Are Unmanaged and

Used for Daylighting 53

3.8 Least-Cost Window Systems When Windows Are Unmanaged

and Used for Daylighting 54

3.9 Life-Cycle Costs When Windows Are Managed and Used

for Daylighting 56

3.10 Least-Cost Window Systems When Windows Are Managed

and Used for Daylighting 57

4.1 Assumptions for Commercial Case Study 68

4.2 Commercial Case Study: Acquisition Costs 70

4.3 Commercial Case Study: Cost of Window Accessories . . 71

vill
Table Page

4.4 Commercial Case Study: Cleaning and Insurance

Costs 72

4.5 Life-Cycle Costs When the Window System is

Unmanaged and Not Used for Daylighting 80

4.6 Life-Cycle Costs When the Window System is

Managed and Not Used for Daylighting 82

4.7 Life-Cycle Costs When the Window System is

Unmanaged and Used for Daylighting 84

4.8 Least-Cost Window Systems When Windows Are

Evaluated Without Window Management But With

Daylighting 84

4.9 Life-Cycle Costs When the Window System is

Managed and Used for Daylighting 85

4.10 Life-Cycle Costs When the Window System is

Managed and Used for Daylighting; Testing the

Sensitivity to a $5/Ft Shutter Cost 88

ix
LIST OF FIGURES

Figure Page

2.1 Flowchart of A Computer Program for Evaluating the

Life-Cycle Costs of Alternative Windows 18

2.2 Alternative Relationships Between Window Envelope

and Energy Costs 23

2.3 Window Life-Cycle Cost as a Function of Window Area

and Orientation 25

3.1 Plan of House Used in the Case Study 29

3.2 Window Sizes in Relationship to the Size of the

Exterior Wall of the House 30

3.3 Base-Year Energy Costs for the Residential llodule

with North (N) or South (S) Facing Unmanaged Windows

Not Used for Daylighting 37

3.4 Life-Cycle Energy Costs at Constant Energy Prices for

the Residential Module for North (N) and South (S)

Facing Unmanaged Windows Not Used for Daylighting . . 39

3.5 Life-Cycle Energy Costs at 12% Escalation for the

Residential Module for North (N) and South (S) Facing

Unmanaged Windows Not Used for Daylighting 40

3.6 Base-Year Energy Costs for the Residential Module

for North (N) and South (S) Facing Managed Windows

Not Used for Daylighting 42

3.7 Life-Cycle Energy Costs at 12% Escalation for the

Residential Module for North (N) and South (S)

Facing Managed Windows Not Used for Daylighting ... 43

3.8 Base-Year Energy Costs for the Residential Module

for North (N) and South (S) Facing Unmanaged Windows

Used for Daylighting 44

3.9 Life-Cycle Energy Costs at 12% Escalation for the

Residential Module for North (N) and South (S)

Facing Unmanaged Windows Used for Daylighting .... 46

3.10 Base-Year Energy Costs for the Residential Module

for North (N) and South (S) Facing Managed Windows

Used for Daylighting 47

X
Figure Page

3.11 Life-Cycle Energy Costs at 12% Escalation for

the Residential Module for North (N) and South (S)

Facing Managed Windows Used for Daylighting 49

3.12 Graphical Determination of Least-Cost Window

System for the Residential Module 58

3.13 Key Results 60

4.1 Window Sizes in Relationship to the Size of the

Exterior Wall of the Office Module 67

4.2 Life-Cycle Energy Costs for the Office Module for

North (N) and South (S) Facing Unmanaged Windows

Not Used for Daylighting 74

4.3 Life-Cycle Energy Costs for the Office Module for

North (N) and South (S) Facing Managed Windows Not

Used for Daylighting 76

4.4 Life-Cycle Energy Costs for the Office Module for

North (N) and South (S) Facing Unmanaged Windows Not

Used for Daylighting 77

4.5 Life-Cycle Energy Costs for the Office Module for

North (N) and South (S) Facing Managed Windows Used

for Daylighting 79

4.6 Graphical Determination of Least-Cost Window System

for the Commercial Office Module 86

xi
SI CONVERSION

In view of the presently accepted practice of the building industry in

the United States, common U.S. units of measurement are used throughout

this paper. Because the United States is a signatory to the Eleventh

General Conference on Weights and Measures, which defined and gave

official status to the Metric SI system, the following conversion factors

are given to assist users of SI units.

Length 1 in = 0.0254* meter

1 ft = 0.3048* meter

Area 1 in^ = 6.4516* x

10""^ meter^

1 ft^ = 0.0929 meter^

Volume 1 in^ = 1.638 x 10"^ meter^

1 gal (U.S. liquid) = 3.785 x 10"^ meter"^

1 liter = 1.000* x 10"^ meter^

Energy 1 Btu (International Table) = 1.055 x 10"^ joule

Power 1 Btu/hr = 0.2930 watt

Temperature °c = — (Op _ 32)

Illumination 1 ft candle (fc) = 10.76 lux

* Exactly

Facing page: ExtoAZon. ihadlng d(ivZc2J>

and -lyiteAioA. dtap^AlOyi o/te tMo it/uite.gZ2yi,

ion. K2.daci.ng tho. znaAgy co6t6

Mindom .

xii
1. INTRODUCTION

1.1 Background

This report is concerned with the impact of window selection and nse

on the life-cycle costs of office and residential buildings. Particular

attention is given to the effect of windows on heating, cooling and lighting costs and to the cost effectiveness of alternative energy conserving

strategies. Not all window strategies for conserving energy are necessarily cost effective when their life-cycle costs, including purchase,

installation, and maintenance costs, are taken into account. The

emphasis here is on a method of identifying energy conserving window

strategies that are also cost effective.

As shown in Table 1.1, nearly 27 million windows costing more than

$1 billion are estimated to be installed yearly in new residential units

alone. It is estimated that these 27 million windows will subsequently

1
O OJ <-{

to n5 -H O rH .-H O 0-1 *-> O t-" C -H

M)C 3 0)

O 0) > <]) C Q) o- ta

(A

C ^

0) CT^ <D -H (t

O > ,H -u 0)

a; 4-t

u E J-"

G. < « E o

-HO

W 0)a

CiO o

QJ OJ w at

x: oo o n) e

4J c ^ o

CO > 60 c c u <u c o

T) ^ U

CO -O

W

13 00 (0

<U 4JC £ rH

a. CO f-' cn (0 o c

t-H XI CO x: o

OJ OJ f-H 4-1 -H > ^ 3 -H OC

OJ

O CO 3 0) S XI3

a;

u c c

00 -H to c s e

•r-l G.^

-a T3 tc •

CO OJ ^ ^

OJ CO

4-J £10 uj 3O OJ I 3 X) ^ ; O O

-O cei ; -H OJ oo * ' 3 3 OO to o o

CO 01

(1)

H jr x; t-i

C 4J ,-1 00 i-J •HO) CO *•-' (0

CU E -H nH Z w S

OO

O 4-' QJ

C O.O 4-1

D. S co -a -o

•HC OJ

4-» CO 4J

O 3 O

C

C 2
require an expenditure of almost $27 million each year for maintenance

(cleaning), about $80 million on the average for yearly repair costs.

According to the results of this study, these windows may substantially

raise or lower the energy costs of the buildings in which they are installed, depending on how they are designed, sized, located, fitted,

and used.

Table 1.1 illustrates the potential costs of windows in new residential

buildings only. If the total United States stock of residential and

nonresidential buildings were considered, the yearly costs of acquisition, replacement, maintenance and repair, and energy would total many

billions of dollars. According to an ERDA-s ponsored window research

program at the Lawrence Berkeley Laboratory in California, about 5 percent of the total national energy consumption is due to windows.^ This

loss amounts to about one-fourth of the total energy used for heating and

cooling buildings in the U.S. each year, the equivalent of 1.7 million

barrels of oil per day averaged over the year. According to another

recent article, the "development of more efficient windows (is) a high

priority on ERDA's (DoE's) energy conservation checklist."

While the costs associated with windows can be greatly affected by their

selection and use, architects, builders, and building owners often find

it difficult to know what is the cost-effective choice regarding windows.

The cost-effective choice is often obscured by the multitude and diversity of effects from windows, by the difficulty of measuring certain

effects, and also by the fact that there are many available alternative

window designs, sizes, and accessories (hereafter referred to collectively as "window systems") which may differ significantly in their

costs and benefits. In many cases neither intuition nor attention

For example, results of applying the model developed in this study in

a number of case studies showed the yearly energy costs for a typical

single family residence to rise or fall by 25 percent by having a window as compared to not having a window, depending on how the window

was sized, oriented, and used. [See Rosalie T. Ruegg and Robert E.

Chapman, A Regional Economic Assessment of Selected Window Systems , National Bureau of Standards, NBSIR (In press).

S. M. Herman and S. D. Silverstein, "Energy Conservation and Window

Systems," Efficient Use of Energy; The APS Studies on the Technical

Aspects of the More Efficient Use of Energy , Part III, LC 75-18227,

1975.

"Science and the Citizen," Scientific American , April 1977, p. 58.

Engineering News-Record , April 21, 1977, p. 12.

3
to single attributes of windows, such as U values, is adequate to answer

the following kinds of questions: (1) Which window system and size will

be most cost-effective? (2) How much will it cost to enjoy the benefits

of a good view? (3) How will window orientation affect costs? (4) How

much should be spent to up-grade the thermal characteristics of windows?

and (5) What window accessories will it pay to add? A better method than

subjective judgment is needed to evaluate the diverse effects associated

with different window strategies in order to arrive at economically sound

decisions.

Economic analysis offers a logical approach to bringing together and

summarizing, using common measures, the architectural, thermal, and, to

a lesser extent, the psychological consequences of alternative window

strategies. Although complete quantification may not be possible, mea- surement of even part of the different kinds of benefits and costs in common dollar terms should substantially improve the ability of the

building community to make informed decisions regarding windows.

1 . 2 Purpose

The broad purpose of this study is to promote cost-effective energy con- servation in buildings through improved window selection and use. The

specific purposes are (1) to provide a conceptual model for determining

the impact of alternative window systems on life-cycle capital, maintenance, and energy costs; (2) to provide a computer program that can be

used by the building community to exercise the method of evaluation;

(3) to illustrate the use of the method by applying it in two selected

case studies; and finally (4) in carrying out the above tasks, to lay the

analytical groundwork for the future preparation of a comprehensive set

of general guidelines for window selection and use. It develops and

illustrates the economic evaluation method that is subsequently applied

in a companion report for 18 regional case studies.

1.3 Scope and Approach

A focus of this report is on determining the impact of alternative

window systems on the energy costs of a building. The main focus, however, is on determining the total life-cycle costs of providing alternative window systems. The inclusion of capital, maintenance, and repair

costs enlarges the scope of the report from energy-efficient windows to

cost-effective windows. By measuring the cost effects of alternative

U = coefficient of thermal transmission. See "Design Heat Transmission

Coefficients," ASHRAE Handbook of Fundamentals (New York: American

Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

,

1972) pp. 347-371.

Rosalie T. Ruegg and Robert E. Chapman, A Regional Economic Assessment

of Selected Window Systems .

4
window systems, sizes, orientations, accessories, and management for different scenarios, the model can provide cost information which will be of

interest to designers of new buildings who are relatively free to select

complete window systems, as well as to owners of existing buildings who

are constrained to making their existing windows more economically efficient.

To provide a broader perspective of window performance than previous

studies, the window research at the National Bureau of Standards was undertaken as an inter-disciplinary effort. In addition to the economists, the research team included architects, a thermal engineer, and a research psychologist. This economic evaluation of windows draws upon

the results of the companion architectural, thermal, and psychological

studies for its technical underpinning in these other areas. In the

following description of the approach, the particular contributions from

the other disciplines are noted.

1.3.1 Delineation of Costs and Benefits Treated

The first task in the economics study was to define the kinds of costs

and benefits that may arise from windows. These are listed in Table 1.2.

This list provided a basis from which to select the items for economic

ana lysis

.

"Costs" and "benefits" as listed in Table 1.2 are defined broadly to

ecompass any sacrifice or gain , respectively, from windows, in terms

of money, goods, leisure time, safety, prestige, comfort, or pleasure.

Costs of windows arise from the purchase and installation of the windows,

their maintenance and repair, their heat loss during the heating season

and heat gain during the cooling season, and from undesirable effects on users such as privacy loss, noise, hazards, visual distractions, and

reductions in usable interior wall space. Benefits of windows arise from

passive solar heat gains during the heating season, the availability of

natural ventilation when needed, beneficial daylight, and other desirable

features, such as view out and contact with the outside world, enhancement of the appearance of the interior and exterior of the building, and

improvement of the morale of building users. An indication of the importance of some of these costs and benefits to users of buildings was provided by a recent literature survey of user reactions to^windows made

by the psychologist member of the NBS window research team.

For an overview of the inter-disciplinary project, see Belinda L.

Collins, Rosalie T. Ruegg, et_. a]^. , A New Look at Windows , National

Bureau of Standards, NBSIR 77-1388, January 1978.

Belinda L. Collins, Windows and People , National Bureau of Standards,

Building Science Series 70, June 1975.

5
TABLE 1.2

Costs and Benefits of Windows

Costs Benefits

Purchase and Installation

Maintenance and Repair Costs

Passive Solar Heat Gain

Daylight

Undesirable Heat Loss and Gain

Safety Hazard

Noise and Visual Distractions Which

Reduce User Comfort or Productivity

Undesirable Light, Glare, and

Contras t

Loss of Privacy

Inflexibility in Interior Space

Arrangement

Natural Ventilation

Higher User Productivity

User Sense of Weil-Being

View Out

Enhanced Interior and Exterior

Appearance

User Source of Information

Safety Aid for Emergency

Entrance and Egress

Undesired Air Infiltration

It is assumed that the owners of buildings will desire to maximize their

net benefits (i.e., benefits minus costs) from the combination of glazed

and opaque surfaces in their exterior walls. Ideally, an economic analysis would look for the solutions which maximize net benefits. However,

this would require the assigning of dollar measures to each of the items

listed in Table 1.2. While there is some precedence for developing

dollar measures for safety and psychological factors in other areas, it

is difficult to develop dollar measures for windows which are broadly

applicable.

For this reason, the focus here is on those costs and benefits whose

effects can be measured in dollars with a relatively high degree of confidence. These include the purchase and installation costs, thermal gains

6
and losses, energy savings through daylighting, and repair and maintenance costs. These are the first three cost items and the first two

benefit items listed in Table 1.2. In addition, insurance and tax effects

for window systems in commercial buildings are treated.

This focus on owning and operating costs does not mean that the equally

important, but difficult to measure, psychological effects are totally

ignored. The net dollar costs or savings provided by the life-cycle

cost approach can be regarded as measures against which decision makers can compare their estimates of the value of these other effects. While

a large element of judgment remains in the decision, the decision maker

need not rely completely on subjective choice. The development of eco- nomic measures for those types of window effects not presently covered

in the economic evaluation model would further guide the selection of

cost-effective windows.

1.3.2 Choices in Design and Use

There are numerous choices available in the selection, location, and use

of windows. Many of these choices will affect the energy usage and

life-cycle costs of buildings. In their recent report. Window Design

Strategies to Conserve Energy , the architects working on the NBS interdisciplinary window project catalogued the following six broad areas of

opportunities for making windows more energy efficient: (1) site selection; (2) use of exterior appendages; (3) choice and treatment of frame;

(4) choice of glazing material; (5) use of interior accessories; and

(6) treatment of the building interior.

Within these six groups, thirty-three specific "strategies" for making

windows more energy efficient are explained. These include, for example,

the reduction of winter heat loss through the use of multiple glazing.

Although the economic evaluation model is capable of incorporating the

cost effects of natural ventilation, these effects are not included in

the case illustrations of the model because at the time of this report

the NBS thermal analysis model, which was used to obtain the thermal

data needed for the economic model, did not treat natural ventilation.

A general reference to the different types of window design; the

available materials for glazing, frames, and weatherstripping ; the

various kinds of fittings; the features of design and construction

which affect the performance of windows; their costs; and available

window accessories is provided by H. E. Beckett and J. A. Godfrey,

Windows; Performance, Design, and Installation (New York: Van

Nos trand Reinhold Company, 1974).

Robert Hastings and Richard Crenshaw, Window Design Strategies to

Conserve Energy , National Bureau of Standards, Building Science

Series 104, June 1977.

7
storm sashes, edge-sealed transparent roll shades, and windbreaks; the

reduction of night-time heat loss through the use of tight-fitting

draperies, opaque roll shades, and insulating shutters; and the reduction

of solar heat gain in summer by the use of heat-absorbing and reflective

glass, shade tress, and exterior appendages such as sun screens and

awnings. Their report describes each strategy; explains the physical

phenomena that account for the effect of the strategy; gives the energy

and non-energy advantages and disadvantages of the strategy; discusses

aesthetic factors to consider in adopting the strategy; indicates the

approximate acquisition cost of the strategy; and provides results of

applicable laboratory studies, illustrative examples, and references to

relevant literature.

The economic evaluation model presented in Section 2 is suitable for

analyzing the cost effectiveness of most of the strategies detailed in

the NBS report on window design strategies . Similarly, the economic

evaluation model is capable of receiving energy data based on comprehensive analysis of the impact of the strategies on the thermal and lighting

characteristics of a building. The model's main limitation in this regard

is the difficulty in obtaining comprehensive energy data.

In the case illustrations presented in Sections 3 and 4 of this report

and in the regional analyses presented in the companion report, a limited

number of window strategies are examined. These include choice of window

size; choice of orientation; choice of single, double, or triple glazing;

use of two interior accessories —Venetian blinds and insulating shutters;

and use of windows for daylighting.

1.3.3 Method of Treatment

The economic evaluation model—a life-cycle cost model—provides a means

of assessing the dollar consequences of choosing alternative options for

windows. The model enables us to include relevant costs or savings

occurring in the past, present, and future, and to take into account

changing prices over time. It brings together the costs of purchasing,

installing, maintaining and repairing, and operating a specified window

system in a given type of building with a specified orientation, geographical location, and mode of use.

The data required for the model are the purchase and installation

costs of the window systems under consideration and of the wall system

which would be used in lieu of the windows; the sizes of the windows

under consideration; the current maintenance and repair costs per unit

size; the different quantities of energy used in the building space for

The only strategies which the model is not capable of handling are

those which affect the energy performance of the other elements of

the building. An example would be the addition of a fence as a wind

barrier. The model cannot apportion capital costs between the window

and the other affected building elements.

8
each of the alternative window options and cases of use; the current

price of energy and the expected rate of future price escalation; the

technical efficiency of the mechanical heating, cooling, and lighting

systems; the interests rate(s) which indicates the opportunity cost of

money; and the life expectancy of the window.

For the purpose of the case illustrations in this report and the

regional studies in the companion report, purchase, installation, maintenance, and repair costs of the previously listed window options were estimated by consulting manufacturers, builders, and written sources.

The operating (energy) costs were obtained by applying a thermal evaluation model developed as part of the NBS inter-disciplinary study of

windows.''' Realistic values for energy costs, system efficiency, interest

rates, and expected life were assumed.

2 A computer program, written in BASIC, is used to apply the life-cycle

cost model. The program calculates life-time energy costs for the

interior space with the designated window system, the life-time additional

costs of the envelope with the designated window system (as compared with

having no window), and the life-time costs of any designated accessories

used with the window system, and then sums the results. These calculations are reiterated for specified sizes of the designated window system

and for specified orientations. For each orientation, the least-cost

window system and size are then identified from among those alternatives

for which data are entered into the program. For each least-cost window

system, the program computes the net present value of costs and savings,

the years to discounted payback (i.e., the elasped time until cumulative

savings offset the purchase and installation costs of the window system),'^

and the break-even rate of future fuel price escalation (i.e., the fuel

price escalation rate for which the combined costs of accessories, energy,

and the windows for the least-cost window system are identical to the

corresponding costs associated with having no window).

For a description of the model and case results see, Tamami Kusuda and

Belinda L. Collins, Simplified Analysis of Thermal and Lighting Characteristics of Windows: Two Case Studies , National Bureau of Standards,

Building Science Series 109, February 1978.

"BASIC" is an acronym for Beginners All-purpose Symbolic Instructions

Code. For a description of the use of BASIC, see BASIC LANGUAGE , Honeywell Software Series 400, Honeywell Information Systems, Inc.,

August 1971.

The payback calculation can be made only if the life-cycle cost of

the least-cost window is less than that associated with having no

wl ndow

.

9
1 . A Organization

The report is organized in five major sections and has a supporting

appendix. Section 2 presents the methodology. It provides an overview

of life-cycle costing, presents the economic evaluation mode for windows,

and formulates a computer program to implement the model, (The computer

program is described in the text and is listed in detail in Appendix A.)

In Section 3, the method is applied step-by-step in a case study for

a residence in Washington, D.C. In Section 4, the method is again applied

step-by-step in a case study for an office building in Washington, D.C.

Four measures of life-cycle costs are provided, based on four different

levels of thermal analysis.

The final part. Section 5, gives a summary, conclusions and suggestions

for further research.

Facing page: Tkz intoACLdtLv d computQA

ph.0Qn.am deAcAA-bud h<in.<i can be a6ed to

tivataato. thd tl()Q,-c,y(iZ(i ao^ti a^^ocilaJidd

u)Ajth lAXlndow-i) i,n mw and dXAJitlng

n.2^-lddniyiat and commz^Kilcdi buAZdJ^ng^

.

10
2. LIFE-CYCLE COST METHODOLOGY

2.1 General Description of the Life-Cycle Cost Method

Life-cycle cost analysis is an economic evaluation technique which is

used to measure costs over the life of a system. The technique can be

applied to alternative systems (investment choices) in order to compare

them on an equivalent basis and select the most cost-effective system.

The technique involves four main steps: (1) the identification of the

relevant cost items for each alternative, (2) the determination of the

amounts and timing of cash flows, (3) the conversion of cash flows to

a common point in time, and (4) the calculation of net life-cycle costs

by summing the discounted cash flows.

I

11
In the evaluation of windows, the time adjustment of cash flows, step

three, is necessary because windows give rise to cash flows that are

spread over time. While purchase and installation costs, i.e. , "first

costs", are incurred at the outset, other effects, such as maintenance

and repair and heat losses and gains, are usually more-or-less continuous

over the life of the window. Given that the life of the window generally

corresponds rather closely to the life of the building in which it is

located, the cash flows associated with a window may extend over a period

of 20 to 100 years.

There are two factors in the time adjustment: (1) possible changes in

prices and (2) the time value of money. With respect to price changes,

it is necessary to ensure that all costs and benefits are stated in terras of comparable prices. This requires that past and future prices be

adjusted for purely inflationary or deflationary price changes. One way

of adjusting "current" (inflated or deflated) dollar amounts to "constant"

(level purchasing power) dollars is to apply a price index^ to the current

dollar amount. For example, based on the 1975 annual average wholesale

price index for industrial commodities (1975 = 171.5, where 1967 = 100),

a window component that cost $100 in 1967 would cost roughly $172 in

1975.^

Assumptions as to the useful life of a building vary greatly. For

example, a recent study of educational facilities assumes a life of

40 years for a school. [Educational Facilities Laboratories, The

Economy of Energy Conservation in Educational Facilities , (New York:

Education Facilities Laboratories, Inc., November 1973), p. 10.] A

recent report on public buildings suggests the assumption of a 50 year

life for purposes of analysis, but acknowledges that some buildings

will have a shorter economic life because replacement will become cost

effective and that some buildings will have substantially longer lives

than 50 years. [Booz-Allen and Hamilton, Inc., Life-Cycle Costing in

the Public Building Service , Volume 1, (A Report Prepared for the

General Services Administration, Public Building Service), undated,

pp. 1-2 and IV-2 . ] A recent study of university buildings in Canada

uses a 60 year cost horizon. [ Report on Building Life Costs , (a report

prepared by the Council of Ontario Universities), Toronto, Ontario,

November 1973, p. 1.14.] Because of the uncertainty associated with

future energy prices, as well as with intended use of the building,

the period of the analysis is often set substantially less than the

expected physical life of the building. As is explained in the discussion of assumptions in Section 3.1, for example, the "period of

analysis" for purpose of the case illustrations in this report is

defined as 25 years.

A price index is a statistical measure of the average change in prices over time for a given item or "market basket" of items.

U.S. Department of Labor, Bureau of Labor Statistics, Monthly Labor

Review , Volume 99, Number 2, February 1976, p. 93.

12
It is also necessary to take into account changes in prices of particular

goods or services relative to general price changes. Altered supply and

demand factors can cause some prices, such as for energy, to rise more sharply than is warranted by changes in the purchasing power of the

dollar in general. A life-cycle cost analysis needs to take into account

any "real" price changes that occur over time.

With respect to the second factor, the time value of money, it is necessary to adjust cash flows to incorporate the opportunity cost which is

incurred by using resources for the purpose at hand rather than for the

best alternative purpose (i.e., the foregone earnings on alternative

investments). The concept of opportunity cost applies regardless of

whether there is inflation or deflation and regardless of whether borrowed

or equity funds are used, as long as alternative productive investments

are possible. If there is a positive opportunity cost, an individual

investor or firm will prefer to delay expenditures (costs) and hasten

receipts (benefits). The opportunity cost is generally stated as a compound interest rate referred to as the "discount rate." For example, if our opportunity cost were indicated by a discount rate of 10%, compounded

annually, we would just break even if we spent $385.54 today to save $1,000 ten years from now. (That is, $385.54 invested at 10% for ten

years = $1000). Similarly, we would be indifferent, other things being

equal, between paying $564.47 now and paying $1000, six years from now.

Interest formulas, which are listed and explained in most engineering

economics textbooks, can be used to discount cash flows to a common time

for comparison. Interest factors derived from the interest formulas, and

also contained in most engineering economics textbooks, can be used to

simplify the discounting calculations.^

2 . 2 Development of the Model

Table 2.1 lists the main elements in the life-cycle cost model for

windows. The impact of windows on energy consumption is the most difficult item of information to estimate. The NBS thermal model, ^ used to

estimate energy consumption for use in the life-cycle cost model, takes

into account climate; thermal resistance of glazed and unglazed portions

of exterior wall; window size, directional orientation of the windows;

the effects on energy usage of window accessories, internal heat loads

such as equipment, people, and lights; and operational factors including

See, for example, Gerald W. Smith, Engineering Economy: Analysis of

Capital Expenditures , 2nd Ed. (Ames, Iowa: The Iowa State University

Press, 1973) pp. 47 and 50.

^ Ibid . , pp. 575-621.

Tamami Kusuda and Belinda Collins, Simplified Analysis of Thermal and

Lighting Characteristics of Windows: Two Case Studies , National Bureau

of Standards BSS 109, February 1978.

13
TABLE 2.1

Elements in the Life-Cycle Cost

Model for Windows

Type of Cost Type of Information Required

Energy Energy Sources

Quantities Used

Unit Prices

Tax Rates and Rules

Acquisi tion Purchase Price of Windows, Walls,

and Accessories

Installation Price of Windows,

Walls, and Accessories

Tax Rates and Rules

Maintenance, Repair,

and Replacement

Cleaning Cost per Unit Area

Painting Cost per Unit Area

Window Size

Insurance Costs

Insurance Reimbursables

Nonreimbursable Repair Costs

Tax Rates and Rules

changing the thermostat and utilizing daylight. The thermal model, how- ever only applies to a single room in a larger structure and does not

take into account natural ventilation effects of operable windows.

An algebraic statement of the life-cycle cost model is given below.

The model is designed to assess the life-cycle cost implications of

1

The model as stated in equation 2.1 is deterministic. As such, it assumes that the timing and values of all cash flows are known. In

the event that the timing or values of future cash flows are non- deterministic but governed by a probability distribution (i.e., the

timing and values of cash flov7S are random variables) it is necessary

to use probabilistic methods to calculate the present value of the

window system cost. [See A. Reisman and A. Rao, "Stochastic Cash Flow

Formulae Under Conditions of Inflation," The Engineering Economist , Vol. 18, No. 1, Fall 1972, pp. 49-69; and Young, D. and Contreras, L.

,

"Expected Present Worths of Cash Flows Under Uncertain Timing," The

Engineering Economist , Vol. 20, No. 4, Summer 1975,

14

pp. 257-268.]
windows in either residential or commercial buildings. It enables the

determination of the least-cost window system and its savings relative

to alternative window systems and relative to the wall with no windows.

The model also enables the calculation of time to payback and the performance of sensitivity analysis. Assessing sensitivity to fuel prices is

particularly important because of the uncertainty attached to present

forecasts of long-term increases in fuel prices. The model is also able

to assess the impact that different tax structures and depreciation

allowances have on the cost-effectiveness of window systems for com- mercial buildings.

Present Value Costs = ENERGY COSTS

PV = [E^ X + E(. X C(. + Ele X C^^] X R(l) x T(l) +

PURCHASE AND INSTALLATION

+ [PW + IW - A x CW + ACC X [PB^ + IB^ + PS^ + IS^] ] x T(2) +

MAINTENANCE, REPAIR, AND REPLACEMENT

+ [n^ X R(2) + Mg^ X R(3) + INS^ x R(2)] x T(l) 2.1

where

,

PV = the present value of the acquisition, maintenance and repair

costs for the window and its accessories, plus the energy costs

for the interior space with the designated window system.

E^, Eq, Ej^g = the quantities of energy required for heating, cooling,

and lighting and equipment. These quantities incorporate values for the efficiency of heating, cooling

and lighting systems.

Cy, C^, Cj^g = the current prices per unit of the energy sources used

for heating, cooling, and lighting and equipment.

R(1) = the uniform present value factor for a series of end-of-period

sums changing by a constant rate, FPE, per period. It adjusts

fuel prices for future escalation and sums the per unit expenditure over the expected life. It is defined algebraically as

R(l) = Z (l+FPE)t

^=;L 1+DIS

where, FPE = the rate of fuel price escalation,

DIS = the discount rate, and

L = the expected system life in years.

15
T(l) = the proportion of operating expenses remaining after taxes.

For residential case applications T(l)=l.

PW = the purchase price of the window,^

IW = the framing and installation costs associated with the window,

A = the area of the window in square feet.

CW = the cost per square foot for the windowless exterior wall,

ACC = 1 if management accessories are used; 0 otherwise.

PB^ = the purchase price of Venetian blinds of area A.

IB^ = the installation price of Venetian blinds of area A.

PS^ = the purchase price of a thermal shutter of area A.

IS^ = the installation price of a thermal shutter of area A.

T(2) = a factor which adjusts for the present value of capital

depreciation allowances in computing taxable income.

= the cleaning cost in the base year for a window of area A.

R(2) = the uniform present worth factor, used to calculate the

present value of constant yearly recurring costs, i.e.

,

cleaning and insurance expenditures. It is defined as

L

R(2) = I t=l (1 + DIS)'^

Mg^ = the repainting and recaulking costs for a window area A

assumed to occur in the fifth year, but evaluated in base

year dollars. (^^g^ is equal to 0 in the commercial case application)

.

R( 3) = the uniform present worth factor used to find the present

value of constant future expenditures that occur periodically

but not yearly, i.e., repainting and recaulking every fifth

year. It is defined algebraically as

Prices would reflect taxes, contractor discounts and contractor markup

for overhead and profit.

16
L/5

R(3) = Z I t=l (1 + DIS)

INS^ = the annual insurance cost in the base year for a window

of area A. (If yearly recurring repair and replacement

costs are not available, insurance premium costs can be

used as a proxy for these costs. If the data are available, insurance costs should be expressed inclusive of

nonreimbursable expenses and exclusive of reimbursable

expenses .

)

(Note: R(2) and R(3) are based on M^, Mg^, and INS^ being given in constant dollars and DIS representing a "real" discount rate ajusted to

exclude inflation. If, however, the analysis is performed in current

dollars, using "nominal rates that include inflation, it would be necessary to include a price inflation escalation rate in R(2) and R(3),

similar to R( 1 ) .

)

2 . 3 Flowcharting of the Computer Program

To perform a comparative evaluation of alternative window systems,

window sizes, and orientations, and to perform sensitivity analysis,

equation 2.1 must be applied a number of times. To facilitate these

calculations a computer program was developed. The program, written in

BASIC computer language, is designed to provide users flexibility in

specifying the values of the critical variables upon which analysis is

based. This approach was taken in order to satisfy the dual objectives

in developing the evaluation model: to provide a method which others

can apply to their particular problem of window selection, and to provide

a method for performing the case studies of this and the companion report

Figure 2.1 shows a detailed flowchart of the computer program. At the

beginning of the program the operator is called upon to enter the values

of the rate of fuel price escalation (FPE) over the specified life cycle

and the discount rate (DIS), and to indicate the types of glazing and

building that will be considered. The program automatically assesses the

life-cycle cost impacts of single and double glazing; if triple glazing

is to be considered 1 is entered; otherwise 0 is entered. To specify

whether the building type is residential or commercial, the variable CO

is assigned a value of 0 or 1, respectively. If a commercial application

is specified, the program will request that an income tax rate be entered

If greater flexibility on the part of the user is desired, additional

variables now given in the program may be changed to input variables.

For example, expected life is given in the program as 25 years, but

the program could easily be modified to allow the life to be entered

by the user.

17
< i < ^ I i i 1
After the values for the key variables have been entered, the computer

reads thermal and cost information from its data bank. The data bank

is entered at the outset by the user and consists of three sets of

theirmal data: (1) the heating requirement in therms, (2) the cooling

requirement in therms, and (3) the number of kilowatt hours required for

lights and equipment; and four sets of cost data: (1) the cost per unit

of energy for heating and cooling and of electricity for lighting,

(2) the purchase and installation cost of the window less the cost of

a comparable wall system, (3) the costs of maintaining, repairing, and

replacing the window system, e.g., cleaning, painting, caulking, and

insuring, and (4) the cost of window accessories. Using these data and

the values of the variables entered by the user, the computer program

calculates and prints three sets of life-cycle cost values for each

window size, glazing type, and orientation: one set for life-cycle

energy costs; one set for life-cycle purchase, installation, maintenance, repair and replacement (insurance) costs (referred to hereafter

as "envelope costs"); and one set for total life-cycle costs (the sum

of the first and second sets).

A sample printout of the life-cycle cost tables is shown in Table 2.2.

The first two sections of the table give the life-cycle energy costs for

single and double glazing, respectively, for designated window sizes and

orientations. The third section of the table gives the life-cycle envelope

costs. The fourth and fifth sections of the table give total life-cycle

costs, again for first single and then double glazing.

Table 2.3 shows the additional outputs of the computer program: (1) the

identification of the window system which minimizes life-cycle building

costs for each orientation, (2) the total life-cycle costs and cost

savings resulting from the least-cost window system, (3) the years

required for the least-cost system to pay back, and (4) the rate of

future fuel price escalation required for the least-cost system to break

even.

To derive the above outputs, first the life-cycle costs are examined

for the first orientation in the computer program, which is south (J=l),

As the window life-cycle costing program becomes more widely used,

thermal and cost data files on selected cities could be established

for referencing directly by the user, by inputting only a city file

ID. This step would greatly simplify usage of the program and reduce

error.

This thermal and cost data should fit the particular case of window

use being examined.

The routine for this computation is contained within the second column

of blocks in Figure 2.1.

19
TABLE 2.2

Sample Computer Printout From Window

Life-Cycle Costing Computer Program:

Energy Costs, Envelope Costs, and Total Costs in Dollars

Designated

Size of Windows

(ft. 2) LIFE-CYCLE ENERGY COSTS FOR SINGLE GLAZING

SOUTH

SOUTHWEST

SOUTHEAST EAST/WEST

NORTHWEST/

NORTHEAST NORTH

0 1218

3060

1628.31

1109.27

1060.78

1023.55

1037.02

1635.64

1122.91

1075.76

1043.92

1066.58

1639.23

1137.07

1094.20

1068.20

1105.46

1631.77

1157.49

1128.09

1124.00

1219.62

1622.22

1152.82

1125.18

1123.32

1227.69

LIFE-CYCLE ENERGY COSTS FOR DOUBLE GLAZING

SOUTH

SOUTHWEST/

SOUTHEAST EAST/WEST

NORTHWEST/

NORTHEAST NORTH

0 1218

3060

1635.64

1096.44

1041.58

982.99

940.89

1639.23

1110.19

1056.12

1000.05

967.31

1631.77

1123.55

1071.56

1021.92

1000.57

1631.77

1125.33

1074.96

1031.23

1017.81

1622.22

118.93

1068.82

1026.51

1015.80

LIFE-CYCLE ENVELOPE COSTS

SINGLE-GLAZED

WINDOW

DOUBLE-GLAZED

WINDOW

miNTENANCE, REPAIR

AND REPLACEMENT

WINDOW

ACCESSORIES

0 12

1830

60

0.00

18.48

20.12

38.25

76.50

0.00

48.08

58.78

108.31

216.53

0.00

42.93

64.40

107.33

214.67

0.00

71.00

84.00

155.00

320.00

TOTAL LIFE-CYCLE COSTS FOR SINGLE GLAZING

SOUTHWEST/

SOUTH SOUTHEAST EAST/WEST

NORTHWEST/

NORTHEAST NORTH

012

18

3060

1628.31

1241.68

1229.30

1344.13

1658.18

1635.64

1255.32

1244.28

1344.50

1677.74

1639.23

1269.48

1262.72

1368.78

1716.62

1631.77

1289.90

1296.61

1424.58

1830.78

1622.22

1285.23

1293.70

1423.90

1838.86

TOTAL LIFE-CYCLE COSTS FOR DOUBLE GLAZING

SOUTH

SOUTHWEST/

SOUTHEAST EAST/WEST

NORTHWEST/

NORTHEAST NORTH

0 12

183060

1635.64

1258.45

1248.76

1353.64

1692.09

1639.23

1272.20

1263.30

1370.69

1718.51

1631.77

1285.56

1278.74

1392.56

1751.77

1631.77

1287.34

1282.14

1401.87

1769.01

1622.22

1280.94

1276.00

1397.15

1767.09

20
TABLK 2.3

Sample Printout of Life-Cycle Cost Optimization,

Payback, and Fuel Price Sensitivity Routines:

Minimization of Life-Cycle Costs

Orientation Area in Sq Ft 18 Single Glazing

Total Cost 1229.30 Total Savings 399.01

Years to Payback 5.4

Percent FPE

^ South orientation (J = 1).

b A negative value indicates that the window system would break even relative to the life-cycle costs of a windowless section of wall even if fuel prices declines by 1 percent per annum. A positive

value means that the system would break even even if fuel prices

increased by the designated percentage rate per annum.

and for the first window size, which is 0 (1=1).^ These are defined

initially as the minimum life-cycle costs. That is, the least-cost

window system for orientation J, M(J), is taken to be the zero window

case for orientation J; hence, initially M(J) = M(1,J). The next window

size (1=2) is then compared with the prior least-cost window. The com- parison is performed by testing whether the life-cycle costs of the new window system are less than the life-cycle costs of the preceding window

system. If the life-cycle costs of the new window system are less, then

a new least-cost window system is designated. The process is repeated

for each glazing type and window size until the ones which minimize

the window system's life-cycle costs are identified. The computer then

prints the following information for the least-cost window system for

the given orientation, J: (1) the orientation, (2) the window area,

(3) the glazing type, (4) the total life-cycle cost, and (5) the lifecycle savings over the zero window case. If the least-cost window

system costs less than an unwindowed section of wall in terms of total

life-cycle costs, the program enters the payback routine." The time to

payback is calculated to the nearest tenth of a year. The program then

Throughout this discussion J refers to the orientation of the window

and I refers to the size of the window.

This routine is contained within the third column of blocks in

Figure 2.1

21
enters the sensitivity routine which assesses the effect of the rate

of fuel price escalation (FPE) on the cost of the least-cost window.

2.4 Graphical Representations

The relationship that exists between life-time energy costs and envelope

costs determines the size of the least-cost window. Figure 2.2, parts a through c, illustrates three of the relationships that may exist between

these two components of costs. On the vertical axes, life-cycle costs

are measured; on the horizontal axes, window area as a percentage of

total wall area is measured. The curves labeled "E" represent energy

costs; the curves labeled "C", envelope costs for a window minus the

cost of a wall of equal size; and the curves "TC," combined energy and

envelope costs. The slopes of the curves indicate the impact on costs

of changing the window size.^

If the window costs more to purchase, install, maintain, repair, and

accessorize than an unglazed portion of the wall of equal size, and

also raises the building's energy consumption as its size increases,

the least-cost window is no window. This relationship is demonstrated

in Figure 2.2a which shows the area of least-cost window, designated

A' , to be zero.

If the window costs less than an equal area of unglazed wall to purchase,

install, maintain, repair, and accessorize, and also reduces energy consumption, the least-cost approach is to glaze the entire wall area. This

relationship is illustrated by Figure 2.2b, which shows the least-cost

window. A', to be 100 percent of the wall area.

A third possible relationship is for the window to cost more to purchase,

install, maintain, repair, and accessorize than an equal area of unglazed

wall, but to reduce energy consumption. In this case, the the energy

savings potential of the window may be sufficient to more than offset

the extra investment costs of the window, and life-cycle building costs

may be minimized by having a window larger than zero, but smaller than

the entire wall area. This relationship is illustrated by Figure 2,2.c.~^

This routine is contained within the fourth column of blocks in

Figure 2.1.

The least-cost window size may be defined in terns of incremental

(marginal) costs as that window size for which the last dollar

investment in a larger window area produces a dollar in life-cycle

energy savings.

Other possible relationships between window envelope and energy costs

include the following: (1) rising envelope (energy) costs might more than offset falling energy (envelope) costs as window size is increased

such that the least-cost window is zero; (2) falling energy (envelope)

(continued on page 24)

22
Oa' area of window 100

(percent of wall)

(PERCENT OF WALL)

Figure 2.2 Alternative Relationships Between Window Envelope and Energy Costs

Notation: E = Energy costs of the interior space with a window of designated size,

C = Envelope costs attributable to a window of designated size,

TC = Combined energy and envelope costs.

23
Life-cycle costs may also be represented graphically as a function of

window orientation. The results convey considerable information simply

and compactly. The graphs shown in Figures 2.3a through c use polar

coordinates to represent the different orientations. For example, the

top vertical line may be designated as north and the bottom vertical

line as south, as is shown in the graphs. The curves that are equally

distant from the center of the graph are designated as equal-cost, or isocost, curves. For example, on Figure 2.3a, the first curve from the

center indicates life-cycle costs of $250, the second indicates $500,

the third $750, and the fourth, $1000. To complete the diagram, a series of curves can be drawn that plot life-cycle costs against orientation for each window size.

To differentiate these curves from the isocost curves, they are referred

to as equal-window area, or "isoarea", curves. On Figure 2.3a, the curve designated "0" is the isoarea curve for a window of size zero. It shows

the life-cycle costs associated with the windowless case for the alternative orientations. The case with no window is shown because the cost

effectiveness of alternative window systems can be measured by comparing

their life-cycle costs with those of the alternative wall investment.

Figure 2.3b shows, in addition to the isoarea curve for the case with no window, two isoarea curves designated "Aj^" and "A2" that represent a single-glazed window system and a double-glazed window system, respectively, both of size A. With these three curves overlaid, it is possible

to construct a least-cost envelope, and to see which of these window

areas and glazing types are least costly with respect to orientation.

The envelope is constructed by making a fourth curve which consists of

the innermost segments of the three existing curves. This envelope,

illustrated in Figure 2.3c by a heavy solid line, shows for each orientation how large the window should be and whether single or double

glazing should be used in order to achieve the lowest life-cycle cost.

In this illustration, for example, when facing north, a windowless wall

results in lowest costs; when facing south, a single-glazed window of

1

(Continued from page 22)

costs might more than offset rising envelope (energy) costs such that

the least-cost window size is 100 percent of the wall area; (3) the

fall in envelope (energy) costs might just offset the rise in energy

(envelope) costs such that there is indifference from a cost standpoint

between having and not having windows. The possibility of nonmonotonic energy and envelope costs means that more than one minimum

may occur for certain cases.

It may be noted that any positive window area could be used as a standard for comparison. However, using the case of no window as the standard has the following advantages; (1) it explicitly allows for the

option of having no windows, and (2) it does not favor any particular

minimal window size.

24
A

Figure 2.3 Window Life-Cycle Costs in Relationship to Window Area and

Orientation

25
area A; and on the east and west, a double-glazed window of area A. By

comparing the least-cost envelope with the other curves, 0, A^ , and A2,

the extra costs may be seen of using the same size and type of window

system for all orientations.

2.5 Limitations of the Model

In interpreting the results obtained from the life-cycle cost model, the

potential importance of factors omitted from the model should not be

overlooked. Factors such as the influence of windows on the physical and

psychological comfort of occupants, the internal and external appearance

of the building, and safety may cause the optimal window to differ from

the least-cost window as defined in the model.

Furthermore, the results of the model apply only to the particular situation described by the data and assumptions used. It should be recognized,

for example, that the NBS thermal model used here to generate thermal data

for the life-cycle cost model is itself based on many assumptions about

thermal loads, climate, functional properties of windows, and user operational behavior. In addition, it omits certain factors such as natural

ventilation, and there is uncertainty about other factors such as its

daylight, artificial light trade-off routine.

The life-cycle cost model nevertheless serves to reduce uncertainty about

the energy costs and economic efficiencies of alternative window choices,

although it does not provide a truly comprehensive measure of all benefits

and costs. The life-cycle costs of alternative window systems can be

viewed as measures against which decision makers can compare the importance of other factors.

Facing page: NnoAty 27 rrUZtlon

iA}i.ndom coiitinQ mon.11 than $1 b-Lttion

OAQ. 2J>tunoJ:2.d to be tn6 tatted tn now

fLQ^lddwtlat uyujU mch ynoJt.

26
3. ILLUSTRATION OF THE METHODOLOGY: RESIDENTIAL CASE STUDY

The following is a detailed illustration of the method of evaluation

developed in Section 2 of this report. The illustration is for selected

window systems in a room of a residence assumed to be located in Washington, D.C. (The room is referred to hereafter as the "residential module.")

The primary purpose of the illustration is to demonstrate the evaluation

approach in sufficient detail to guide others in using the model with

their own data. A secondary purpose is to derive results which are useful

to the building community in selecting, accessorizing, and using windows.

Because the primary objective is that of illustration, a detailed, stepby-step approach is provided.

27
3.1 Basic Assumptions

The residential module is assumed to be a kitchen/ family room in a one story ranch-style house with a full basement. The overall dimensions

of the house are 28 feet wide by 50 feet 6 inches long, and the total

floor area is 1414 square feet. The layout of the house including the

location of windows and doors is shown in the floorplan in Figure 3.1.

The kitchen/family room shown shaded in the floorplan, is used for the

purposes of both the thermal and the economic analysis of the alternative

window systems. The dimensions of the room are 15 feet wide by 18 feet

long by 8 feet high.

The exterior wall of the house has 4" brick veneer over 8" cinder block,

and contains 3-1/2" of blown-in insulation (cellulose). The U value of

the wall is 0.07.

The windows used in the economic analysis are wooden and double hung.

Both single and double glazing are examined for window sizes ranging from

0 to 60 square feet. Figure 3.2 shows the four window sizes examined in

relationship to the size of the exterior wall. Additional assumptions,

such as lighting requirements, heating and cooling system efficiencies,

and occupancy patterns are given in Table 3.1.

The NBS thermal model which is used to provide input to the economic

model assumes that all heat flows entering or leaving the room are through

the exterior wall which contains the window. It is therefore explicitly

assumed that no heat flows occur between the room under examination and

the basement, attic, or adjacent rooms. (Such a system is usually

referred to as adiabatic.)

Furthermore, the thermal model does not take into account the interdependencies between building components that may result in a change in one factor affecting the thermal performance of other factors. To account

for all possible heat flows and thermal interdependencies would require a considerably more sophisticated thermal model than that which was used in

the case studies.

r The simplifying assumptions employed in the thermal model result in

specific limitations for the life-cycle cost measures produced by the

economic model for the case study. One limitation is that the lifecycle costs of the windows for the single room cannot be multiplied by

a constant, say, 6 or 8 depending upon the size of the house, in order

to get the life-cycle window costs for the entire house. One reason is

that the computer program assumes that there is only one exterior wall

in the room. Since all residences have some corner rooms which have two or more exterior walls, the assumption that there are no heat flows

through these walls is overly strict. Another reason is that most houses

do not have basements and attics with regulated temperature. The fact

that the thermal model makes no allowance for the effects of natural ventilation on energy usage further limits the reliability of the life-cycle

cost measures produced by the case study. But the most tenuous aspect of

28
r oo

cco u oo

cco O

in

M

CO

T3

OJin

01mDO

29

TABLE 3.1 Assumptions for Residential Case Study^

Building Specifications Assumptions

Dimensions of Module^

Type of Construction

Exterior Wall Area

Window Size

Window Construction

15' wide X 18' long x 8' high

Block with brick veneer; 3-1/2"

cellulose insulation; U = 0.07

144 ft^

0, 12, 18, 30, 60 ft^

Wood; double hung; weatherstripped

Internal Loads

Lights

Equipment

Air Leakage

Occupancy

Heat Load/person

0.65 watts/ft^

0.52 watts/ft^

0.5 air changes/hour

0.5 persons

260 But/hour/person

System Efficiency Specifications

Furnace Efficiency

Cooling COP"^ Electric Heating

0.65

2.0

1.0

Fuel Type Cos ts

Electricity

Gas

$0.03 per kWh

$0.30 per therm

Operation Conditions

Thermostat Adjustment 72° to 62° F winter nights

78° to 84° F summer nights

Window Management Times

Thermal Shutters

Venetian Blinds

Winter nights

Summer days

^ Due to the difficulty of modeling the thermal exchange between rooms,

only a single room was modeled. The model assumes no heat transfer

to adjacent surfaces such as walls, floors, or ceilings. Study of

windows within a single room of a house may not necessarily reflect

the performance of windows in the house in general.

All loads are averaged over the 16 hour period from 7:00 AM to 11:00 PM. The assumptions underlying the thermal calculations are described in

greater detail in Tamami Kusuda and Belinda Collins, Simplified Analysis

of Thermal and Lighting Characteristics of Windows: Two Case Studies , February 1978, pp. 10-20.

^ COP = Coefficient of Performance.

31
the case study is the assumption made in the thermal model concern daylighting. Although the amount of daylight predicted by the daylight

computer subroutine has now been validated against two other considerably

different computer programs,"*" available amounts of daylight and the con- comitant savings in electric energy for lighting and cooling have not

been adequately validated by laboratory and field testing.

In addition, there are questions about people's actual use of daylighting to supplement or replace electric lighting. Typical lighting

levels and duration of use in residences are not well established, nor

is the human response to available daylight well determined.

A further limitation of the case study is that it compares daylighting only with general lighting of the room by incandescent bulbs;

for other solutions, such as task lighting, daylighting might have less

value.

3.2 Envelope Costs

In general, purchasing and installing windowed areas in a home is more

expensive than providing windowless walls. However, because windows

displace portions of the wall, they raise initial building costs by substantially less than their full purchase and installation costs. To

estimate their additional acquisition costs, window costs are compared

with wall costs in Table 3.2. This table shows that purchase and installation of good quality wood windows are estimated to add between $18 and

$76 to initial building costs for single-glazed windows and from $48 to

$216 for double-glazed windows, depending on their size. Window prices

are distributor-quoted for the Washington, D.C. area. The wall costs

are the estimates of a builder of homes in the Washington, D.C. area.

If management devices are used, additional acquisition costs are

incurred. Costs of Venetian blinds and wooden shutters based upon

averages of currently quoted prices in the Washington, D.C. area are given in Table 3.3.

Window areas also usually require somewhat more maintenance than

windowless wall areas. Estimates of window cleaning costs, based on a current cost of $0.10/ft^, are shown in the first row of Table 3.4.

Estimates of maintenance and repair costs based on scraping, recaulking,

and repainting once every five years at a current cost of $1.50/ft , are shown in the second row of Table 3.4.

The other computer programs are LUMEN II and GLIM, two more complex

programs used for lighting calculations in the United States and in

Great Britain.

A research program is currently underway at NBS to validate both the

predicted daylight amounts and the energy savings.

32
TABLE 3.2

Residential Case Study:

Acquisition Costs of a Window in Excess of the

Cost of a Windowless Wall

Dollar Cost By Size of Window

Component 9 12 ft^

9 18 ft^

9 30 ft^9 60 ft^

Windows^

Single Glazed 52,20 70.70 122.55 245.10

Double Glazed 81.80 109.36 192.61 385.23

Wall^ 33.72 50.58 84.30 168.60

Window Cost Less Wall Cost^

Single Glazed 18.48 20.12 38.25 76.50

Double Glazed 48.08 58.78 108.31 216.63

^ Purchase prices are list retail prices

,

reduced 10 percent to reflect

a typical builder's discount, for good quality wood double-hung windows,

provided by a distributor in the Washington, D.C. area. A typical con- tractor markup of 25 percent for overhead and profit is then applied to

the purchase price. Prices are for single and multiple units of windows

of a size which comes close to providing the designated areas of the

exterior wall in glazing. The 12 ft^ area is provided by a 3' x 3' 11"

window; the 18 ft area, by a 3' x 6' window; the 30 ft area by two

3' x 5' windows, and the 60 ft^ area, by four 3' x 5' windows. An

Installation cost of $5.00 per window or pair of window is used, based

on an estimate given by a home builder in the Washington, D.C. area. The installation cost and the purchase cost are then added together

to get the acquisition cost.

Costs of non-windowed wall areas corresponding in size to the windowed

areas are based on a price of $2.81/ft^ as estimated by a home builder

in the Washington, D.C. area. The wall section is assumed to be face

brick veneer over 8" cinder block with building paper sheathing, 3 1/2"

of cellulose insulation, and 1/2" of painted interior drywall.

The additional costs incurred for windows are calculated by taking the

difference between the costs of windows and the costs of a comparably

sized wall area.

33
TABLE 3.3

Residential Case Study:

Cost of Window Accessories

Dollar Cost by Size of Window

Type of Accessory 12 ft^ 18 ft^ 30 ft^ 60 ft

Venetian Blinds^ 17.00 20.00 36.00 72.00

Wood Thermal Shutters^ 42.00 51.00 96.00 192.00

^ Prices shown are average of several quoted prices. Installation is

assumed to be done by the homeowner at negligible costs.

^ Estimates are those of a Washington area building contractor for building,

installing, and finishing solid, tight-fitting wooden shutters. (Prices

quoted by custom drapery shops in the area were considerably higher.

)

TABLE 3.4

Residential Case Study:

Maintenance and Repair Costs

Current Dollar Cost by Size of Window

Type of Maintenance

and Repair 12 ft^ 18 ft^ 30 ft^ 60 ft^

(Yearly Cost)

Annual Cleaning at

$0.10/ft2 1.20 1.80 3.00 6.00

Scraping, Recaulking, (Recurring 5th Year Cost)

Repainting^ every

5th Year at

$1.50/ft2 18.00 27.00 45.00 90.00

Costs are based on a large sample of data collected in conjunction

with a lead paint abatement program at NBS (R. Chapman, Economic

Analysis of Experimental Lead Paint Abatement Methods; Phase I , National Bureau of Standards, Technical Note 922, September 1976.)

34
3 . 3 Energy Costs

Due to the uncertainty about future energy prices, energy costs of the

windows are evaluated first based on no future escalation in energy

prices and then on a relatively rapid rate of increase of 12 percent com- pounded annually, to establish a range of possible costs. In addition,

energy costs are assessed for both natural gas, a currently less expensive

energy source, and electric resistance energy, a more expensive energy

source. (The room is assumed to be electrically cooled regardless of

whether heating is by natural gas or electric resistance energy.)

In order to take into account the fact that windows are used in diverse

building situations, their energy effects are evaluated for four different cases.

In the first case (referred to hereafter as the "unmanaged window, not

used for daylighting" case), it is assumed that no energy conserving

devices such as shutters and blinds (hereafter, referred to as "management devices") are installed in the window. The thermal calculations are based on traditional ASHRAE methods. Included in the calculations is the

heat generated by the people occupying the room and by lights and equipment (e.g., televisions, dishwashers, stoves, garbage disposals), as well

as climate-determined heat gains or losses.^ The clima tological data are monthly mean daily temperature and solar radiation figures published by

Kusuda and Ishii.

The second and third evaluation cases are extensions of the first case. The second case (referred to as the "managed window not used for daylighting" case) assumes that management devices are used and that the

desired room temperature is changed between day and night by adjusting

the thermostat. Venetian blinds are used during the summer day to

reduce excessive solar heat gain. Thermal shutters are used during winter

nights to cut heat losses. It is assumed that the thermal shutters are custom fitted into the window opening. The combination of single glazing

and the shutters result in a U value of 0.5; the combination of double

glazing and the shutters result in a U value of 0.2. The resetting of

the thermostat at night permits both heating and cooling costs to be

reduced regardless of whether the room is windowless or has windows.

During the heating season the thermostat is reset from 72°F to 62°F,

and during the cooling season the thermostat is reset from 78°F to 84°F.

ASHRAE, Handbook of Fundamentals , 1972.

In this study a base temperature of 65°F is used for heating degree day

calculations (See ASHRAE Handbook of Fundamentals , 1972); a base temperatrue of 80° is used for cooling calculations.

Kusuda, T. and Ishii, K. , Hourly Solar Radiation Data for Vertical and

Horizontal Surfaces on Average Days in the United States and Canada , National Bureau of Standards, BSS 96, April 1977.

35
In all other ways, the analysis for the second case of window use is

identical to that of the first.

The third case of window use (referred to as the "unmanaged window

used for daylighting" case) assumes that daylighting is substituted for

electric lighting when the quantity of daylight available exceeds 6.7 fc

(72.1 lux) measured at a reference point 15 ft (4.2 m) from the window

and 3 ft (0.9 m) from the floor. This case further assumes that no man- agement devices have been installed and that the thermostat is not reset

at night. Hence, the third case is identical to the first with the

exception of the trade-off between natural and artifical lighting.

In the fourth case of window use (referred to as the "managed window

used for daylighting" case), daylighting is substituted for electric

lighting, window management is practiced, and the thermostat is reset at

night. Thus the fourth case brings together all of the elements of the

first three cases. It is the case that would be expected to have the

greatest potential for energy conservation.

As was indicated above, the third and fourth modes of operation—those

which utilize daylighting—are unfortunately subject to more uncertainty

than those which do not use daylighting. As was explained earlier, this

is because the computer program which calculates the natural/artificial

light trade-off has not yet been adequately validated.

3.3.1 Energy Costs for Unmanaged Windows Not Used for Daylighting

First the energy costs for different window sizes and glazing types are analyzed based on a bare, unmanaged window, not used for daylighting.

Figure 3.3 shows base-year energy costs for single glazing in Part A

and for double glazing in Part B, for two orientations. On the vertical

axis of both graphs the estimated base-year energy expenditure is given

in 1977 dollars. On the horizontal axis the window area in square feet

is given.

Parts A and B of Figure 3.3 show that base-year energy costs rise rapidly

as window area is increased whether it is single or double glazed,

although the rise is more rapid with single glazing. In the base-year,

energy costs of the room heated by natural gas range from $66 for a windowless wall with a southern orientation, to $83 for a 60 ft singleglazed window with a southern orientation. On the north side, costs range

from $66 to $90.

If the room is heated by electricity, the base-year energy costs range

from $70 for a windowless wall on the south side, to $102 for a 60 ft

single-glazed window on the south side. For a northern orientation,

The format used in Figure 3.3 will be followed throughout the chapter.

Recall that these energy costs are for the room indicated on Figure 3.1

and not for the entire house.

36
UJ UJ </) (/) _l _l < <

UJ UJ o o I I

I I

a

UJN

<

-Io

UJ

-I

ffi o

o

ffi I-2

o

CNi

1r—

Ll

Z

<

UJ

< O

ro o

o

00

z

O

o

UJN

<

o

UJ

-Iz

cr<

a.

o

o

ro

cviI-

<

UJ

<

o

00 —

CVJ

O

CVJ O

o

o O

00

o O O O

-IV

I

2^ «

o

O -U)

rH iH 3 0) W

o

o2

O)

en j-j O <U

CO d

QJ -H ^ Pi :2 a

O)

CU ^3 .H

Qj K

-u bC ^

nj M c o

O 'H

<^ e y^ m cj -u 0) m bC I—

I

0 c w

t>C Ph CO

OJ 3 C M

O -H 0

1 CO -U -H

0) rH w w: O

tfl C -H O

($) SISOO A9a3N3 aV3A-3SVa

37
costs range from $70 to $118. Thus, room energy costs in the base year

are increased as much as $24 by a large single-glazed window when gas

heating is used and as much as $48 when electric heating is used. This means that the additional energy costs attributable to windows will be

substantial over the life-cycle.

Part B of Figure 3.3 shows that double glazing reduces the additional

energy costs from a maximum yearly amount of $24 to $9 when gas heating

is used, and from $48 to $14 when electric heating is used.''' Although

double glazing reduces the base-year energy costs of the window, it does

not altogether eliminate the negative effects of increasing window size.

Figure 3.4 shows the annual energy costs of Figure 3.3 converted to a life-cycle basis, assuming no change in future energy prices. Part A

shows that when gas heating is used, even at fixed energy prices, a 60 ft^ single-glazed window can raise life-cycle energy costs for the room by about $175 (from $720 to $895) for southern orientations, and by

about $250 (from $720 to $970) for northern orientations. Life-cycle

energy costs are raised even higher when electric heating is used,

ranging from an increase of about $350 (from $740 to $1090) for southern

orientations, to about $500 (from $745 to $1245) for northern orientations. Part B of Figure 3.4 shows that the increases in life-cycle

energy costs are significantly reduced by the use of double glazing.

The assumption that energy costs will not rise is very conservative in

light of the current energy crisis. With escalating energy prices, the

the energy costs attributable to bare, unmanaged windows could be considerably higher than shown in Figure 3.4. For example. Figure 3.5

indicates the impact on life-cycle costs of energy prices rising at a rate of 12 percent compounded annually.

The reference point (i.e., the room's life-cycle energy costs when it

has no windows) increases from $720 with gas heat fixed in price, to

$2800 with gas heat escalating in price at a 12 percent rate. With

large, single-glazed windows on the north side, the life-cycle energy

costs for the room may now exceed $3700 for gas heating and $4800 for

electric heating. This means that the windows would raise life-cycle

energy costs by between $1,000 and $2,000, depending on the type of

energy used. Part B of Figure 3.5 indicates that the savings potential

of double glazing becomes more important the higher the rate of energy

price escalation.

Similar reductions would result if storm windows are installed initially

or later retrofitted over single-glazed windows.

It was found that annual energy costs increased for all orientations and

for both glazing types as window area increased whenever the case of

window use was assumed to be a bare unmanaged window.

38
o

bJN

<

-IO

UJ

o

o

I-oc<

a.

o

bJ

_lUJ

• UJ

I

in< <

o (9

1 1 z CO

\

\ \ \

<

UJ<

O

O

CO Z

u.z

60

0)

,—1 •H

4J o

^'

,—

j

f—

1

rC 03

• r-' Q 4-1 c i-l CD Q

U-4

(it QJ

C/3 o

4-J Q u to o C/l c

U-l -> •H 5 iH

tfi o

QJ o

O CJ

• r-l 3:

CJ

p-< 'O iH

q; 1-1 >^ 00 *J ao tO o

1-1 <u

0) rH c e [i2 T3

4-J C

c 00 CO CO c

4J •iH OC W O c

c CO •H o 4-1 CO ^—

s

<U

4-1 C3

^ ^

(J) o

4-1 4-1 dJ

cn ,—

1

o O [V] o CO

>^ -O U

c •r-l u CO u

OJ 4-1 c u

QJ

i-H 0)O

>^ c U o CO

1(U tfl M-i CO •H o O

($) SISOO A9a3N3 310X0- Bill

39
a

UJN

<

UJ

-Im

O

o

o

o

00

CM

CViI-

<

UJ

IT<

o

o

<

UJ

or<

o

o

z

o •H

4-1

<u I—

t

•rH I—

1

T3 >^ O CO Q

i-HcC O

•H U-l

QJ T3

•rH :3

CO oc

<u 4-) c

pi o •H

2: iH

CL) O

to O

§

U

O o C •iH •H S-i 4-1 C CJ o -a cu •H OJ I-t

4-1 CO CO

f~H c

CO c

O CO CO c

6^ •H

CNJ c 4-1 1—* •r-l CO O OJ •U CO

CO /-^

CO o

<u CO '—

o w

u

u

3 CJ o •H

^-1 C/1 J-l dJ 4J C T3 CJ U2 C (U CO .-H

(L) w

iHO z 13

>^ C

CJ CO

1(U 4J CO

M-( Sj CO •H O e>

hJ

o o o o o o O O

o o o o o o O O

00 m CVi 0> u> ro O lO lO ro CVJ

IVJ o

LO

u

60

•H

P4

($) SISOO A9d3N3 3"IOAO-3dn

40
3.3.2 Energy costs for Managed Windows Not Used for Daylight Ing

Let us now see how the second case of window use, the addition of window

management devices complimented by a nighttime temperature adjustment,

affects energy costs. The evaluation procedure follows that of the first

case.

Figure 3.6 plots the base-year energy costs for single glazing in Part A

and for double glazing in Part B. In both Parts A and B of Figure 3.6,

the energy cost curves remain approximately horizontal as the window area

is increased, with the exception of single-glazed windows facing north.

This means that the energy costs of the room are not substantially different whether it is windowless or has windows, even large ones, provided

they are not single-glazed windows facing north. The use of management

devices and adjustment of the thermostat have reduced dramatically the

energy costs below those found in the first case.

With no escalation in fuel prices, the life-cycle costs follow about the same trend as the base-year costs, and there is little difference in the

life-cycle energy costs of the room with and without windows. The exception is for single-glazed windows facing north, and even those differences

are not large.

When energy prices rise at a rate of 12 percent per year, life-cycle

energy costs are about quadrupled. The windowless room now requires an expenditure of $2700, as shown by Figure 3.7, Parts A and B.

The energy costs attributable to the windows, however, continue to be

considerably lower than those for the first case. Double glazing,

though still important as a means of reducing energy use, does not result

in as large a savings when blinds and shutters are used and the thermostat

is adjusted at night, as when the windows are bare. The higher the rate

of fuel price escalation, the more important double glazing becomes.

3.3.3 Energy Costs for Unmanaged Windows Used for Daylighting

Let us now examine the third case which includes the utilization of daylight as an alternative to electric lighting, but excludes the use of

window management and thermostat adjustment.

Figure 3.8 shows base-year energy costs for the residential module to

range from $68 with no windows, to as high as $98 with a large, singleglazed window facing north and with electric heat, to as low as $44

with a moderate size, double-glazed window facing south and with gas

heat. In both Parts A and B of Figure 3.8, energy costs fall off quickly

as window area is increased. However, as window area is increased

further, energy costs begin to rise. This pattern of initially falling

and then rising energy prices reflects the fact that once the level of

daylight is sufficient to turn off the electric lights the model ceases

41
o u

_j <-«<

UJ OUJO

I

I I > z

o

Ui

N

<

o

UJ

-I

ffl DO

o

(!)

<

0.

a

UJN

<

O

UJ

-Io

z

10

Io

(0

o

00

o

O

ro

00

(VJ

o oCVJ O

O

o

0>

o

00 o O

to

IV o

CM

<

UJ

q:

<

o

o

CMI-

<

UJ

o:<

o

o

CD

CO bO O 13

•H u iH

0) O O

iH OO

'd CO o [3 O

O •H 13 5-1 .H UJ

CO •H O

•H 12 Q)

4-1 iH

13 W

0) <U

•T3 C3£. 13

•H CO C

03 CO

QJ

Ptj bCd

(1) bO •H

(3 4-1 U •H CO a 0)

?-i CO cc o

M-l 'I?

CO c/:i dJ

J-l iH

CD W

O ^ U

3 o

>^ O H

!-i 4-1 CU 13 •H O

C d +J QJ W CO tH

bO W

•H

cd rH

0) >^ S

>-• CO CO

1 o

0) 4J CO

CO !-i i-l

Cfl o o

pq IS

0)

bC

•H

($)S1S00 A9d3N3 aV3A-3SVQ

42
o

UJN<

-Io

OQOo

03<

Q.o o o O

O o o o O o m ro (M

to lO ro to to ooo

to oo

(VJ oo

00

(M oo

(M o

CsiIu.<

UJ o

ro 5Ooz 2 J

(VJ

<M oo

CNJ»-<

UJ o

to S^Oo = 1

(VJ

U-l ao ,—1 a

-o to o •H

1—(

T—1

ra o

•H

>-i c o

(U U-i X)

•H

t/2

u

OJ t/3 c

t—(

0) 4-1 o o o

CJ

)-i

Wu o •1-1 5 (-1 4-1 C c a o • r-l (U H 1—1

i-i CiJ CO t—I

0) cC OC c o CT3 CO ao

2: c

•H

CNI oc i—(

PI CO H QJ

4-1 cC CO w

u

i-> 0) CO Ou

x:

CJ GO •H U o )-i 0) 4-1 c U

S-i QJ o .—1 (V U2 a 2: -D

>^

C

u CO 1 _c

0) 4-) CT, u-l CO o O

($) SISOO A9a3N3 310A0-3dn

43
o u

Ul UJ

_J _J < <

UJ

1 UJ z

1 w

1 1 z

o

UJN

<

-J

UJ

-I

ffi o

o

<

a.

o

CMI-

<

UJ

q:

<

o

o

CO —

CVJ

o

o

«0

O

to

CM

<

UJ

a:<

o

a

_ CO ze

u 60 O •H

4-J

S-( bO bO O •H d

<+-i iH •H

0) O

rH o o

3

5-1 o O o

M-l •H

!-i rH •U

0) a

•H 0) U

0) Td &

•H o d

Cfi 0)

c^i •H bCc

0) •H

4-1

4-) 0) cC 5C 0)

03 o d

e o

cn c 0) u rHW

ou

•H O

!>% CJ •H

CO U ^ 4-1 U

C 0) W 4-1 iH u O

0) d

t« !T3

1 O

cu W

CO PQ

COm

6C

•H

($) SISOO A9d3N3 aV3A-3SVa

44
to attribute energy savings to daylighting. Undesirable heat flows, on the other hand, tend to increase as window area increases, offsetting

beyond a certain window size the potential gains from daylighting.

The life-cycle energy costs for the third case are particularly interesting because a large potential for savings exists. For example, with

constant energy prices, gas heating and no windows, life-cycle energy

costs for the room amount to $720. Adding a 12 ft single-glazed,

south-facing window causes energy costs to fall to $510. Increasing its

size to 60 ft raises energy costs to $610, still significantly below

the room's energy cost with no windows.

The utilization of daylight becomes more attractive the higher the rise

in the cost of electric lighting. (The heating and cooling costs associated with windows, however, continue to rise, the higher the energy

costs.) Figure 3.9, which assumes that all energy prices increase at

the rate of 12 percent per year, shows the life-cycle costs for the

windowless room to be about $2800. It further shows that, with the use

of daylighting, double-glazed windows facing north side may save as much

as $900 when gas heating and electric lighting is used, and $630 when

electric heating and electric lighting is used. By comparing Figure 3.9

with Figure 3.5 (the first case), we can see the considerable potential

of daylighting for conserving scarce energy resources and for reducing

the homeowner's utility bills.

3.3.4 Energy Costs for Managed Windows Used for Daylighting

In the fourth case, we examine the effects on energy costs of combining

window management with daylighting. Figure 3.10 shows that the management devices and thermostat adjustment keep undesirable heat flows substantially in check, thereby reducing the costs for large windows that were observed in the preceding case when only daylighting was considered.

The mitigating effect of management devices on winter nighttime heat

losses and summer heat gains, plus the beneficial effects of daylighting

result in a significant potential for windows to save energy.

Only with relatively large single-glazed windows on the north side of

the electrically heated room do life-cycle energy costs with windows

exceed the energy costs without windows.

Should a 12 percent rate of energy price escalation prevail over the

next 25 years, the windowless room would require a life-cycle energv

expenditure of $2700. This is almost four times the required energy

costs of the same room if constant prices were to prevail. As is shown

In the trade-off analysis, supplementary electric lighting is assumed

to be added whenever room illumination supplied by daylighting falls

below 6.7 fc at a distance 15 ft. from the window within the room,

such that a minimum illumination level of 6.7 fc is maintained.

45
o

UJ

Q

UJN

<

-IO

Id

_l03o

o

q:<

o.

to<

oI

V)

<

I V)

\ \ \ \

\ \ \ \ \\\

I

I

I

I

/ / J /

V) CO < <

o o 1 1 z V)

\ \

o o O O O O O

O O O O O O O

CM O 00 (0 CM O

lO ro CM CM CM CM CM

o -1 (O

u.

8 5

O

O

CM

O

oo

0) C

iH

•U rC o W)

:s •HiH

I—

I

CO •H O 4-)

J-i a> o

T3 <+-i •H

cc T3

(U QJ Pi

0)

/—

s

o 60 o r: •H

4-1 •H .HO

o

o u

•H 01 •P W) o

03 •H .H C >-l CO 2

4-1 O s CJ M (U W iHW

60c 13

.—

(

•H C

O CO

J-i CO 60C

en •H

4-1 cn 4-1 CO CO o <U o

J-I =) o

60 o •HU

c o

(1) CO t-H w

rHo 13 o CO

1<u cn

^-^ i-i CO •H O

hJ

• cn

(U

>-i

•H o

o

00 o

o

<0

{$) SlS00A9d3N3 310X0-3^11

46
a

UJN<o

UJ

_l

ffi 3oo• • ffi Itr. <

a.o

UJN<

_i

UJ

-Iz

cr

Oo

ro

00 oo o o

00 c o o

lO o o

lO oo

ro

£2 5 _ CM

11. <

UJ<o - ^

CMI-<

UJ

or<ooz

00

•H

4-1 4-1

!-i J= O 60

^; •HtH

>j > o 03

C

M-i

O •H

r-^ CU 1^ o o o

CJu

0) •H

S-i -H tD 4-1 n) O

•H OJ M I—1 C o w

<u c

•H •H c

|2 03 0) Pi T3 bi

OJ

C

0) •H

4-1

u

C

CO

>-l s ffi o

4-1

C o

•H (D

4-J

U

CD 03

w ou a

•H

i-i

u 4-1 0) u c •u w o w

CO c c 1 <u C/3 (0 U

P3O

tHro

0)

»-i 3

60

•H

($) ISOO A9d3N3 aV3A-3SV9

47
in Figure 3.11, life-cycle energy savings increase as single-glazed

windows up to about 30 ft^ in size are added if gas heat is used. Lifecycle energy savings may be as high as $1140 when 60 ft double-glazed

windows are used facing south if gas heat is used.

3.4 Life-Cycle Costs-*-

Let us now see how combining life-cycle energy costs with life-cycle

envelope costs will affect overall life-cycle building costs. We do

this for each of the preceding four cases of window use.

3.4.1 Life-Cycle Costs for Unmanaged Windows Not Used for Daylighting

Table 3.5 shows that combined life-cycle costs, for five orientations,

for unmanaged windows not used for daylighting, both when energy prices

are fixed and when they are escalating at a rate of 12 percent. It can

be seen that in every case, life-cycle costs increase as window area

increases. Although double glazing was shown to be effective in reducing

energy costs, when the extra acquisition costs are also considered, the

two almost exactly offset one another when energy prices are constant,

and the overall life-cycle costs for single and double glazing are nearly

equal. However, with rapid energy price escalation, double glazing

becomes more cost effective than single glazing for all window sizes and

orientations, but particularly for large, north-facing windows. At the same time, the table indicates that life-cycle costs are not raised substantially by adding small windows, particularly small, south-facing

windows

.

A comparison of the upper and lower portions of Table 3.5 demonstrates

the increased weight which would need to be given to window investment

decisions for this case of window use as the price of energy increases.

As the table shows, the life-cycle costs for a south-facing, doubleglazed window of 12 ft^ increases from about $820 at constant energy

prices to about $2,924 when energy prices rise at 12 percent. The entire

increase of more than $2,000 is due to changing energy prices. The appropriate design practice for cost control would be to size windows as small

as possible and use double glazing on all windowed areas,

3.4.2 Life-Cycle Costs for Managed Windows Not Used for Daylighting

The upper portion of Table 3.6 shows the total life-cycle costs, including envelope and energy costs, when energy prices remain constant over

the 25 year period. It can be seen that life-cycle costs are increased

by windows of all sizes, regardless of orientation and glazing type when

they are managed but not used for daylighting. It can also be seen that

"Life-Cycle Costs for Windows" refers to the combination of lifecycle envelope costs given in Section 3.2 and life-cycle energy

costs for the residential module with alternative window systems

given in Section 3.3.

48
o

UJN<

-io

UJ

-I

CD3Ooo

biN<O

UJ

-1oz

to<

»-<

0. <

UJ<oo

I-<

UJ

tr<oo c COo

•H c

0) •H

C

pt; •HrH

0) o

0) o

oc

U

n3

l-i c U o nj •H

u-< ?: 1-1 4-1 C oc O o (U •H •H rH U u wC U

CO

W ^

y c

•H CN 4J 4J iH 3 «t) O <U

4-1 CO

CO

CO c o

4J (U

cn iH o W c

•H

4-1 o

,C •H

>-i 4J i-l 0) •H +J C O w cu rH Q w o a U-l >^ o c U1

OJ

f-H OJ 3

X! 0) •H O

hJ

rH

rHro

(1)

>-i 3

bC

•H

O

O

O

O o o O

<M o 00 <0 lO lO Csl

O o O o o o o o o CD

CM (NJ oo

to

($) SISOO A9d3N3 3n0A0-3Jn

49
00 to lO CO CN 00 0 CNl CO On 0 \0 LO 00 00 CN as rH LO

rH CNI CN CO CO CO

w

<y\ 0 0 ON as

CN 00 <t lo CN 00 00 00 0^ CN 00 as LO

rH CN CM CO CO CO

nj o ,—1 O 4-t US C3 0 c3 CJ^ CO 00 00 CN

nj •-^ CNI ro 00 CO rH vD CN 00

r

(U 4-)w 1

00 00 <7> 00 0^ ,—1

LO

rH CN CNI CO CO CO

<u

Z3QQ o w -3- CN 0 CN VD <f

CN CN CN ,—1

LO CN 3' 00 00 ON CN 00 ,—1

LO

Q) CO ,—1

CN CN CO CO CO

&£) j»( nj

,_( 00 ^ 0 CO <f rH E Q CN in ,—1

CN CN

OJ}

Q 00 00 0^ CN as as ,—1

;3

C rH ,—1

CN CN CNl CO CO

•H

O

d) 4-t

O *H

<C tiC CO 'H U 4-J CD 1—1 *H

J-i o o ctJ -u o Q O 00 0 0 CO LO 00 OD 00

LO q; CN 1

00 vO as CN CO 00 LO

•H }-l rH 00 00 <T^ CN ,—1

CO 0

CO

O [jj (J rH CN CO CO CO <i

4-1

PlJ <D PC CU 03 CJ

m rH CJ^ as as rH LO as < CN CN CO ,—1 0 CO LO as LO H 00 00 0^ CN 00 0 ,—1

CO 0 , "J ,—1

CN CO CO CO <t

4-J 4-J -H O J-J

N o 03 0

•H S in 1-H CO CO a^ as <f CN CN 1^

4J CN CN rH rH rH ,—(

CNI as

C « w 00 00 a^ CN 00 rH CO 00

iH CO

W 0) 4-1 rH CN CO CO CO CO

cO (2 O <D 1 •H Vj

W

OJ CO CO in \D CN 0 rH LO 00 (N

LTi <t 0 rH as 00 00 CO

•H 00 00 CJN CN 00 as CNI 00

CO rH CN CN CO CO CO

00 CJ^ <J- LO LO rH ,—1 0 <}- 00 vO LO LO

f-^ 00 00 rH as 0 CN

rH CN CN CO CO CO < ^ S H 0 fi-i CN 00 0 0 CN 00 0 0

-a ^ 0 rH CO rH rH CO c

•H3

50
T3

0)00

00C ^ 00 •rH C .-1 cu •H O

t-l 4-1 o

<: X. o

00

•H o

i-H •H

>^

U

nj 4-1

a

OO

01 :3 i-i 1—1 O w c M-i <u 13

TD

C

<P nj

CD

U) ED oo

4-1 c

4-1 •H o o iJ U CO

01

01 13

rH

n o to >^ u o 101

4-1

•i-l J oO

O) H

CJ5

J33oQ

T3

0)

00c

(fl 01

1-1 <do

-ac

•H

II 51
when equipped with Venetian blinds and thermal shutters, single glazing

is more cost effective than double glazing except when facing north with

rapidly escalating energy prices.

By comparing Table 3.6 with Table 3.5, it can be seen that the energy

savings from the blinds and shutters are not sufficient to offset their

extra capital costs when energy prices are constant, but more than pay

for themselves when energy prices escalate rapidly.

3.4.3 Life-Cycle Costs for Unmanaged Windows Used for Daylighting

In both the upper portion of Table 3.7 which assumes constant energy

prices and the lower portion which assumes 12 percent energy price escalation, life-cycle costs are shown first to decline and then to increase

as window size is increased. With rapid price escalation, however, even

the large window area is shown to result in lower costs than the windowless wall. The initial decline in costs reflects the electric energy

savings from daylighting. The rise in costs as the window area is

expanded past 12 ft^ reflects the loss in the value attributed by the

model to daylighting once the target illumination level of 6.7 fc is

reached, together with the strong negative effects on heating and cooling

costs of larger unmanaged windows. The life-cycle savings due to daylighting are greater at a high rate of energy price escalation because

the cost of the electricity for lighting is greater.

While there is little difference in the life-cycle costs of single and

double-glazed windows at constant energy prices, double glazing is cost

effective when energy prices rise rapidly. Double glazing is shown to

be more important in this case where windows are unmanaged than in the

previous case where thermal shutters functioned in large measure as a substitute for double glazing.

Table 3.8 indicates: (1) the least-cost window size; (2) the life-cycle

energy costs of the room combined with the differential costs of the

least-cost window system; (3) the savings that result from having the

window as compared with having a windowless room; (4) the years required

for the energy savings of the window to pay back its added cost to purchase, maintain, and repair; and (5) the minimum rate of fuel price

escalation for which the window will continue to pay back. With constant

energy prices, the least-cost window is shown by the upper portion of

the table to be 12 ft^ and single glazed, except for northerly exposures

where it is 12 ft^ and double glazed. These windows can pay for themselves in 1.1 to 2.7 years depending on the orientation. The last column

of Table 3.8 shows that these windows could pay for themselves even if

energy prices were falling.-'-

This information on least-cost window, savings, and payback was not

presented for the preceding cases because the least-cost window was no window.

52
00 a^

m 1^ rsi

i-H 00 o CJN CN CN

Ly~i C7^ o o CNI CN CM CM CM

wz rsi CN \0 o ON CTN nO 00

rsi CT\ iH

O o C^J 00 T3 r~~ tn CTi OO o o ,—1 OJ C 2 CN CN CN CM CM

nj •r-l iH 4-J o 03

m CN ^ CJN OS CO 1 ^ p CM 0^ o in ,—4 CO CO NO

1-1 01

W r-- ON 00 o o •H

J-l CN CN CM CN CM o o o w

in cn

CN 00 ft-) 'IT ON

u~l ON CO

CO CN CM rsj

Ulu

n)

1—1 o o nD

(U 1—1 iH CO o CO CN ON CO in

00 o \0 as CTN CJN (•*-) n)C o CNJ CM c E 00 •H d c 3 -H U5 OO rH 4-J <U C O tn M -H O o

<: 4-1 o u 00 in 00 o\ o c*^ in

1-H CN 1—1 00 (3N 00

cn OO o 0) ^£> CJN (-H

O 1-1 CN CN CN CM cn

O tH (J o

T3 >. 4-1 >1 C « O u

•r-lQ 01 1 W 3 1-1 (1) CN

O OS OO in ON

u bi 14-( CNI CN CN 00 CD cn OS ON

C

O •H r~. MD r~. OS 00 f—1

I-H ON

J TD CN CN CN CN CM

cP

C 0) c 3 n) N o

<U n) iH

CO CO OO 1—1 iJ u 3 C CO 3 u~i u-i CTv in CTN in in .-H CO -i-l 4J CM 00 1—1 r-H o -a- ON c*1 O T3 4-) 0) c w ON 00 ,-H ,—1

CN C30 CJ C (fl 1—1

01 CN CNl CM CM CM

00 •H

0)

x c S-i i-H •1-1 o O U3 CO

CO

w

O \0 00 00

O ^ nO

u o CN 00

O 00 CN in 1 ^ 3 in ^ CTN 00 O CN r»-

OJ cn CN CM CN CN CM

•HhJ

a> 00 CO <r <r CMO in I-H

c/a 1—1 1

a^ I-H <3N in 00 I-H I-H in u-1 OS C o CM r-~ CN CM CN CM CM

CO

0)

CM

O CN 00 o o O IJM r-l en

O CN 00O o

1^ 1-H r-( CO NO C

•H3

53
iI-J e [£) ^

•H P-i 6^ d [j-i ^—

o ..^

4-t

O

CO

CO

i-l CO CO

a)

>-'

CO 00

•H QJ

d

J-' bO U-l X c •H >

•H OC -H i-J CO "C/) •H i—H 6 CO —

1—1 o CD 1—1 O U CO CD cu CO O CO 4-J o U

CO E—

CO

O V-i 3 C!

t= *4-t 4-J o

QJ (J 00 4~i 'O QJ c * C/D 1—1

•pH CD on >^ CO fij 13 M-l CO •H O w j-i kJ §

CO PC C CO o <-\ (U < CO u CO iH H C GO 1 4J CJ •H XI d 4-1 o (--> 2 CO H

CO

4-J CO CO <u W C 0) Oo CjO 1 C CO d u CO •H 0) CO N Oh

cd a) CO >^

0) i-H

i-J <I

O

tsJCNl •H 4-) CO CM

do

•H

4-1 CO

4-1 d

(U •H

!-i O oV cV oV cV oV

CNI

LT, ro 00 o

CO CO CNl 1—1

1-1 I-H rH

CO c CN cr.

r--. 00 OC c^ OC m in in in in

(U 0) 0) <D CU CU CU <u <u

1—1 1—1 1—1 1—1 rH 1—1 iH

60 OC OC XI ub X5 d d d d d d

dd

•H •H •H o o o o o o

CO CO CO

Q o o

CO

W

COI :sCO I oV

OV

in

CN

CV

CN

OV

CN

OV

ro

CN

OC rH o

r-l iH 00 OC 00

00 CO CJ^

C7^ a-) cn CN ON CT C o o 1-1 1—1 CN CN CN

00

COI CO

00I doo

cc 2

54
With rising energy prices, the least-cost window systems are shown by

the lower portion of Table 3.8 to be in all cases double glazed. For

all except northerly orientations, the least-cost window size is 18 ft^;

for northerly orientations, 12 ft is the least-cost size. The additional acquisition, maintenance, and repair costs of the least-cost

windows can be recovered through energy savings in 2.2 to 2.6 years.

As in the previous case, the windows can pay for themselves in the 25

year period even if energy prices fall.

3.4.4 Life-Cycle Costs for Managed Windows Used for Daylighting

Table 3.9 shows the combined life-cycle envelope and energy costs. The

upper portion of the table indicates that when energy prices are constant, single-glazed windows are slightly more economical than doubleglazed windows. Second, it indicates that both single- and double-glazed

windows are more economical^than a windowless wall as long as their area

does not exceed about 18 ft , but that large windows can raise life-cycle

costs substantially. A comparison of the upper portions of Tables 3.9

and 3.7 show life-cycle costs to be greater with the management devices

than without them; that is, when energy prices remain constant, the added

costs of the management devices are not offset by the resultant energy

savings

.

The lower portion of Table 3.9 indicates that if energy prices were to

increase at a rate of 12 percent per year, windows of all sizes and

orientations tend to reduce life-cycle building costs compared to a windowless wall, provided they are managed and used for daylighting.

With constant energy prices, the least-cost window system is indicated

by Table 3.10 to be 12 ft and single glazed. This window system can pay for itself in 6.6 to 7.5 years depending on its orientation.

Savings of between $70 and $90 can result over the 25 year period by

having this least-cost window system as opposed to having no window.

With 12% price escalation, the least-cost window size is shown in

Table 3.10 to be 18 ft . For southerly orientations there is little

difference in the potential savings of single- and double-glazed windows,

although single glazing is slightly more cost effective. For northerly

orientations double glazing is more cost effective. The least-cost

windows can pay for themselves in 4.9 to 6.4 years depending on the

orientation.

The polar coordinate graphing technique introduced in Section 2.4 is

used here to compare the life-cycle costs associated with the singleand double-glazed 18 f t^ windows and the windowless room.

In Figure 3.12, it can be seen that the curve labeled "A", indicating the

life-cycle costs associated with the windowless room, is well oatside of

the curves "B" and "C" which give the costs associated with single- and

double-glazed 18 ft^ window. By closer inspection, it can also be seen that curve B (single glazed) lies slightly inside curve C (double glazed)

for the southerly coordinates and slightly outside curve C for the

55
<:

SI. t c « -H

Ul f-l <: op o c o

CO -H

O S £ U

T3 60 fl a u

•H iH 4J 3 >^ y C O rH S O TS >« c

(0 <a

(0 01 60 owe U 9 -H « <«

r-lC

W

CI 0) SB U CO I <« « u o

COoyoI 3oQ

T3N

(d

1-1 o

0) iH

00c

en o ON lO

0^ o 00 CO o CN to 00 vO vO vO 00 (H ON CN 00 CO CO rH o ON O •<t Qj CN CN rH CM CN <l» S > O (U •How

r*^ CO \0 rH vO 4-) to ON f-H 00 ON o CN CO rH O ^ (U c vO 00 rH o CO ON 00 o rH o ON o

•H CN CN rH CN CN

4J

4J (0 'O c —. o CM lA O^ W iH d o O rH '^ CN 0 04) <d •H \0 00 rH CN CM 00 sO rH o M rH o ON o <r a> O <N CN rH CN CN QJ iH CO U3 o ^ Cd S o o

00 \0 00 00

Q "H ON CO ON rH rH

CO vO rH iH o CO lO •H &0 rH o ON O CO • 0

CN CN r-t CN CM 00 00 "H

•H -H to

I/) o CN o CI

N 4-> (U ON ON lO CO CN CO m CO ^

rH

O 00 o CO rH 00

rH <JN ON o rH 00 •H 'O

CN rH rH CN CO rH <y CN 4) 00 rH (0 CO ^ 'O <u 0O M O

"TS o c

cn LO ON o l/i ON CN 00 CO CO r** \D ON

vO rH 0^ CO CO M-4 rH vO o o rH (U O

CN CN CN CN CN c cj *

r«>.

O rH CN

ON lO 00 CO 0^ •H vD

rHO \0 CO CO cO c rH o O rH vO <u o CN CN CN CN CN <u •o

•H ^ 01 *J OO (U

to c O 00 CO O a s fl. •H cn 00 rH o LT) c CO <v

<u vD vO

O CN CN 00

•H rH o ON o 4J ^

CM CN rH CN CN •H ^ o * O 0

\0 ^

00 CM 00 • ij ON fH CN LO r*«- uO CN CO (y rH sO vO O rH ON uO CO 00

t-H r*^ ON ON o CO 0) CO CO CN rH rH CN CN rH d) • >C CO M 00

to 3 C

CO in iH 00 <r (0 E-r

O CO -H 0^ O <N CO ro o o CO

sC vO o O CO o CO

C CO ;C

rH ON 0\ o CO -H cO CiO CM rH CN CM •H 3 •H 3 rH

0) OJ •H CO rH ^ O

to o o

•w bO 4J C 0) CO •H ^

•H

N -H 3

4= to *J U rH CJ CM OO OJ CO H CN 00 o O C tU to o rH CO •H rH OJ O

CM 00 o o W 00 1 rH

O rH rH ro CO

C ^

O. -H to 00 B CO O c O CJ •H O C rH

CO 0)

O

>.42X o

CQ 4J 4J

O UOf^FXBOSa to

56
o <U iJ i u>

1^ §e

4J

(0OV

(0

(0

0) •J

60^

*H

O

iHU

wu 4U 4J O cJ H

S3 « 6 bO

09 C!

0) 03 60

(0 iX!

(0 to S « . <^ S

(0

'X3

COIw

(0

(U

(JCO

n)

00c

•H •H>

h3 CO <n- CO

(U iH 0 U H

0)

CO

•rt 0o

iH 0)

to

O oH

&0c ^

CO

w

(3OM*J

CO

(S

d) •HMO oV oV

CO 00

set o

CM CM

CO I CO oV oV oV

00

m o rH • • • • • • • • m

CM

00 o 00

CM CM CM oV oV oV00 CO

CO

w

COI

CO oV oV

I—I '>o r>. \£> en r—i r--

O in in CM CM

CM 00 r». kn CO 00 0^ 00

iH CM CM CTi cr> <y\ CT^ vO VO v£> rH iH 1—(

rH

lU (U <u 0) 0) a; 0) (U iH iH rH rH rH rH rH rH rH rH 00 00 00 00 00 oo 00 00 XI c p: fS c c c c 3 D

•H 'H •H •H •H

OO

CO CO CO CO CO CO CO CO o O

00 00 wI

57
Figure 3.12 Graphical Determination of Least-Cost Window System f the Residential Module

58
northerly coordinates, thus indicating slightly lower costs by using

single glazing for southerly exposures and double glazing for northerly

exposures. Costs of single and double glazing are essentially equal for

easterly and northerly applications.

3.5 Implications of Results

The preceding Section 3.4 presented the life-cycle costs of the alternative window systems used in a single room of a Viouse, which in turn

reflects the life cycle costs of the building, for four different cases

of window use. This section summarizes conclusions drawn from the four cases. These conclusions are first listed below in some detail for each

case, the results of the four cases are then compared, and, finally,

the principal findings are summarized in Figure 3.13.

When Windows Are Unmanaged and Not Used for Daylighting

1. The larger the window, the larger its life-cycle cost as well as the life-cycle cost of the building.

2. If windows are used, their costs are lowest when facing

south and highest when facing north.

3. Rising fuel prices raise window costs.

4. If fuel prices escalate rapidly, double glazing is economical for all window sizes examined and for all orientations.

5. If fuel prices remain about constant, it makes little difference from a cost standpoint whether the window is single

or double glazed.

When Windows Are Managed But Not Used For Daylighting

1. The larger the window, the larger its life-cycle costs.

2. If windows are used, their costs are lowest if they face

south and highest if they face north.

3. Window management greatly dampens the negative impact of

energy price escalation on window costs.

4. Life-cycle building costs are not raised greatly by adding

small windows.

5. Double glazing, used in addition to window management, tends

not to be economical except for northerly orientations when

energy prices are rising rapidly.

59
(T) r-l a «j u a o (« n 9 3

iH e 3 n •H (« U t) 9 •H

•H

N

OS eo

k< 0)

B c n 1 <H a B y N 3Oo § c

1-1

O » •o

a)

CO 1 T3

rH c fH «

lU

g 10 3O

IS "ow

4) f CO 5 -H o

60o

•a

a) DC m B 5 M O

TJ i-H 6 Q

v

0)

N

(fl («

(d r-l « Of) uu X 41 e u r-t •H as O B

01

C •w

•H

u

u

01

u 41

Nw 1.1 •H

(8 U) g 0

T3 c B

•H c c B

41 41 01

006 BOg

41

•M

C8OUU

,4

4J DC O 6 3 M O -H O r-l B >. •H rt » c

»H

U 0 o N

W

M « H t-l 60 (0U 3 >. O H

1^

JB.

B » O B

n-l 41 y >

41

4)

U < («

(«

60
When Windows Are Unmanaged But Used For Daylight Ing

1. The life-cycle cost of the building can be reduced by adding

a window If care is taken in Its sizing or orientation.

2. If fuel prices remain about constant, life-cycle building

costs are lowest if a small, single-glazed window facing

south is added.

3. If fuel prices rise rapidly, life-cycle building costs are lowest If a small to medium, double-glazed window facing

south is added.

4. It generally pays to use double glazing for all unmanaged

windows if energy prices are expected to Increase rapidly,

5. Rising energy costs, by increasing the savings of electricity for lighting, tend to Improve the cost effectiveness of a window despite the concomitant rising costs of

heating and cooling.

6. When energy prices rise rapidly, even large windows may

be had at a lower life-time cost than a wlndowless wall

area, provided that the windows are either facing south

or are double-glazed.

7. The net energy savings from Installing a window used for

dayllghtlng can pay back, the additional costs for window

acquisition, maintenance, and repair in just several years.

When Windows are Both Managed and Used for Dayllghtlng

1, The life-cycle cost of the building can be reduced by

adding a window, particularly if energy prices rise

rapidly

.

2. Whether fuel prices remain about constant or rise rapidly,

life-cycle building costs are lowest if a relatively small,

single-glazed window facing south is added.

3. Double glazing tends not to pay even on the north side if

energy prices remain about constant, but if they escalate

sharply, double glazing tends to pay for all window sizes

and orientations examined, except for south-facing, smallto-medium sized windows.

4, The net energy savings from installing a window that is

managed and used for dayllghtlng can pay back the additional costs of window acquisition, maintenance, and

repair in about six to seven years.

61
When Energy Prices Are Constant (All Four Cases of Use

Considered)

1. Life-cycle building costs can be minimized by adding a small (12 ft^), single-glazed window facing south and

utilizing it for daylight, but not equipping it with

Venetian blinds or thermal shutters.

2. Regardless of window size, orientation, and glazing type,

the least-cost mode of window use is the third case examined, i.e., using the window for daylighting but not

investing in accessories. In descending order of cost

effectiveness, the other cases rank as follows: the fourth

i.e., using the window for daylight and accessorizing it;

the first, i.e., neither using the window for daylight nor accessorizing it; and, lastly, the second, i.e., accessorizing the window but not using it for daylight.

3. When Venetian blinds and thermal shutters are used, it does

not appear to pay to invest additionally in double glazing,

and vice-versa.

4. Even when Venetian blinds and thermal shutters are not used

double glazing appears to be cost effective only for window

areas with northern exposures, and even then it is only

slightly preferable to single glazing.

5. Small windows appear to be less costly than large windows;

very large windows tend to raise life-cycle building costs even with the most favorable method of window use.

When Energy Prices Rise Rapidly at 12 Percent, Compounded

Annually (All Four Cases of Use Considered )

1. Life-cycle building costs are minimized by adding a mediumsized (18 ft^), single-glazed window facing south, by

equipping it with Venetian blinds and thermal shutters,

and by using it for daylighting.

2. Regardless of window size, orientation, and glazing type,

the least-cost case of window use is the fourth examined,

i.e., using the window for daylighting and managing it.

In descending order of cost effectiveness, the other cases

rank as follows: the third, i.e., using the window for

daylight but not investing in accessories; the second.

Here specific reference is made to the accessories as opposed to the more general term "window management" because "window management"

includes thermostat adjustment, an action which generally is cost effec

tive since it can save energy at little or no cost.

62
i.e., using accessories but not daylight; and lastly, the

first, i.e., neither using the window for daylight nor accessorizing it.

3. Double glazing pays for all of the window sizes and orientations examined whenever they are not accessorized.

4. For most window sizes, single glazing is more cost effective on southerly exposures and double glazing is more cost

effective on northerly exposures, if the windows are accessorized

.

5. Double glazing and the use of Venetian blinds and thermal

shutters appear to have closely comparable impacts on the

life-cycle costs of windows even though double glazing has

the advantage, accounted for in the evaluation model, of

not requiring daily attention for effective use.

6. Small- to moderate-sized windows tend to be less costly

than large ones in all cases examined.

7. If used for daylighting and managed for energy conservation, even a relatively large window (60 ft^) will tend to reduce

life-cycle building costs below what they would be without

a window.

8. If neither managed nor utilized for daylighting, large

windows can greatly increase life-cycle building costs.

The key results that can be drawn from this case study are summarized in

Figure 3.13 for the condition of rapidly rising energy prices. One major

finding is that life-cycle costs of the room, thus the building, tend

to be raised by the windows examined if they are not used for daylighting,

particularly if they are large in size, oriented to the north, single

glazed, and unmanaged. A second major finding is that life-cycle costs

tend to be lowered by the windows examined if they are used for daylighting, particularly if they are small to medium in size, oriented to the

south, double glazed,^ and managed. The negative cost impact of windows

not used for daylighting can be lessened by reducing their size as much

as possible, orienting them to the south, and managing them. (As was indicated by the more detailed listing of conclusions, the cost-effective

rules for double glazing and window management become somewhat more complex when the uncertainty in energy price escalation is taken into

account.

)

Note the exceptions in footnote b of Figure 3.13.

Note the exceptions in footnote a of Figure 3.13.

63
Apart from the consideration of psychological or other factors, these

conclusions suggest that a homeowner, builder, or designer in the

Washington, D.C. area could reduce life-cycle building costs by keeping

window areas as small as possible in those rooms which are not used much

during the day or for which for some other reason cannot be used substantially to reduce electric lighting requirements. But if a large savings

potential from daylighting exists and is to be utilized,^ it is better

from a life-cycle cost standpoint to have windows — even relatively

large ones, but preferably small- to moderate-sized ones — than to have

a windowless exterior wall.

It should be recognized that an individual occupant's use of windows

may not closely fit any of the four cases of use described here. In

fact, individual use will likely shift over time. If this is the case,

a homeowner may seek to minimize the maximum losses that could be

expected from windows. This behavior, which can be explained in part

by game theory, would lead to the choice of smaller windows even though

the potential for significant savings might exist from larger windows.

Such an unstable equilibrium might persist for some time if the public

were not made aware of the potential that daylight utilization has for

permitting homeowners to achieve significant dollar savings in lifetime building costs.

Facing page: Tm majo^ chcLttzng2J>

-in LLi>Zng LoZndom mofiz 2,{i{i^d2,ntty

o^ij^ce biLitdinQi, aj> enxSu/txng pn.oP2A managmdYit OfJ -IvitQAlon. ikadlng

dovZcu, and Integrating tkd oie 0|J

dayLigkting and oAtil-ioA^at Lighting.

64
4. ILLUSTRATION OF THE METHODOLOGY; COMMERCIAL CASE STUDY

The application of the life-cycle cost model in this section is different

from the preceding residential case study in two ways: (1) it includes

income tax considerations and depreciation allowances which are currently

accorded commercial structures; and (2) it is based on a different set of

data and assumptions.

4.1 Basic Assumptions

The wall construction of the office building used in the case study is

representative of curtain wall construction which has become increasingly

important in recent years. The office building under consideration is

assumed to be between 5 and 10 stories in height. However, for purposes

of the economic analysis, we shall focus on one room, an office module,

ii
to evaluate the alternative window systems. The floor area of the office

module is 180 square feet and the volume of the module is 1800 cubic feet.

It is assumed that two people are assigned to the office during the normal

working hours of 9:00 AM to 5:00 PM. Their occupancy is averaged over the

day and the resultant figure of 1.8 persons is used in the computer model

to calculate heat loads generated by the occupants. The calculation of

expected heat gains also takes into account the effects of office equipment, such as typewriters, electric adding machines/desk calculators, and

computer time-sharing terminals as well as lighting.

The exterior wall of the office building consists of a dark glass

spandrel panel with 1 inch of rigid fiberglass insulation applied with

a mastic. The mullions which hold the spandrel panels are dark anodized

aluminum with a thermal break. The interior surface of the exterior

wall is cut out and installed. The size of the cutouts is determined

by the size and type of glazing. The U value of the non-windowed wall

is approximately 0.15.

The windows used in the economic analysis are fixed. The framing is

dark anodized aluminum with a thermal break. No tints or low emissivity

coatings are assumed to be used. All glazing is erected from the inside

of the building as a stick wall; elastomeric gaskets'*" (dryset) are used

on all windowed areas. Both single and double glazing are examined.

Window sizes range from 0 to 90 square feet. Figure 4.1 illustrates the

shape of the office module and the window sizes in relationship to the

size of the exterior wall. Additional assuptions, such as lighting

requirements, heating and cooling system efficiencies, and occupancy

patterns are given in Table 4.1.

2 As in the residential case studies, the thermal model which is used to

provide input to the economic model assumes that all heat flows entering

or leaving the office module are through the exterior wall which contains

the window. It is therefore explicitly assumed that no heat flows occur between the office module under examination and adjacent office modules.

(Such a system is usually referred to as adiabatic.)

Furthermore, the thermal model does not account for the interdependencies

between building components whereby a change in one factor affects the

thermal performance of other factors. To account for all possible heat

flows and thermal interdependencies would require a considerably more

sophisticated thermal model than that which was used in the case studies.

The use of elastomeric gaskets reduces the likelihood of repairs which

would probably result from chemical sealants applied at the site. The

elastomeric gaskets are assumed to perform satisfactorily throughout

the 25 year life cycle.

Tamani Kusuda and Belinda L. Collins, Simplified Analysis of

Thermal and Lighting Characteristics of Windows: Two Case Studies , February 1978.

66

TABLE 4.1

Assumptions for Commercial Case Study^

Building Specifications Assumptions

Dimensions of Module

Floor Area

Volume of Module

Type of Construction

Exterior Wall Area

Window Size

Window Construction

12' wide X 15' long x 10' high

180 ft^

1800 ft^

Curtain Wall

120 ft^

0, 12, 30, 60, 90 ft^- Anodized aluminum with thermal

break

Internal Loads

Lights

Equipment

Air Leakage

Occupancy

Heat Load/Person

J . z 3 wa cts/sq. rt,

0.50 watts/sq. ft.

0.25 air changes/hour

1.8 persons

ZbU otu/ hour/person

QT^or* 1 'T"! r*£i l"i r\no

Gas Furnace Efficiency

Cooling COP"^

Electric Heating (Heat Pump)

0.65

3.0

2.0

Fuel Type Cost

Electricity

Gas

$0.03 per kl^Hi $0.30 per therm

Operation Conditions

Thermostat Adjustment 72° to 62° F winter nights

78° to 84° F summer nights

Window Management Times

Thermal Shutters

Venetian Blinds

Winter nights

Summer days

^ The model assumes no heat transfer to adjacent surfaces such as walls.

floors, or ceilings.

COP = Coefficient of Performance.

68
As was the case for the residential case example, the simplifying assumptions employed in the thermal model result in specific limitations for

the life-cycle cost measures produced by the economic model.

The thermal model's limitations, however, are less serious for the com- mercial case than they were for the residential cas«. Since most office

buildings have a large number of office modules which have only one exterior wall, the majority of these office modules are surrounded mostly

by other modules which are maintained at approximately the same temperature such that an adiabatic system is probably typical in most cases.

Again the thermal model's assumptions about daylighting are tenuous.

However, the problem is somewhat less severe in the commercial case study

than in the residential case study. For the office module the assumption

of a fixed level of illumination appears more reasonable than for the

residential module. (We have followed the GSA guidelines and used 50 fc as the reference level.) The fixed level of illumination could be maintained by the use of automated controls, a much more feasible approach

for the office module than for the residential module. (The cost of auto

mated controls is, however, not included in this life-cycle cost analysis

The method of presentation of this case study is similar to the preceding

residential case study. First, the life-cycle envelope costs, including

the costs of purchasing and installing the windows, equipping them with

management devices and maintaining and repairing them, are presented.

Second, the life-cycle energy costs of the office module with alternative

window sizes, orientations, and glazing types are presented based on four

different cases of window use: (1) unmanaged windows^ not used for daylighting; (2) managed windows not used for daylighting; (3) unmanaged

windows used for daylighting; and (4) managed windows used for daylighting. Third, the total life-cycle costs are presented, again for

each of the four different cases of window use. Conclusions and the

implications of these conclusions for the building community are then

summarized.

There would, of course, be some office modules which are not adiabatic

even though they have only one exterior wall. For example, offices

adjacent to unconditioned areas (e.g., equipment rooms and/or garages)

would generally not be adiabatic.

As explained earlier, "unmanaged windows" means those windows that are used bare; "managed windows" means those windows that are equipped, in

this case study, with Venetian blinds and thermal shutters which are used to reduce undesired daytime heat gain in the summer and nighttime

heat loss in the winter, and, in addition are used in a room in which

nighttime adjustment of the thermostat is practiced for energy conservation.

69
4 . 2 Envelope Costs

In commercial buildings, unlike private residences, the cost of windowed

areas may either increase or decrease initial construction costs, that

is, a wall with windows may cost more or less to purchase and install

than a windowless wall. For this reason the costs presented in Table 4. are the acquisition costs of the 10' x 12' exterior wall (bay) and not

the net costs of the windowed area above or below those of the exterior

wall. These costs include the purchase and installation prices for all

glazing (spandrel panel and window), mullions, and the interior curtain

wall. A 25 percent markup for contractor overhead and profit is include

TABLE 4.2

Commercial Case Study:

Acquisition Costs^

Dollar Cost by Size of Window

Glazing Type 0 ft^ 12 ft^ 30 ft^ 60 ft^ 90 ft^

Single 1238 1229 1204 1257 1009

Double 1238 1301 1323 1446 1255

a Cost figures are for the entire exterior wall (bay) and include

materials, installation, and markup costs. The cost differential

associated with a given window size/type is thus equal to the difference between the cost of the bay with that window size/type and

the windowless bay. The area of the exterior wall is 120 square feet,

Variations in cost among window sizes are due both to differences

in framing costs and to the costs of glazing. Lower framing costs

are particularly evident in the case where the window area is 90

square feet.

Source: A leading manufacturer and distributor of building materials

provided the cost estimates.

70
The exterior wall used in the case studies was chosen for two reasons.

First, it is typical of those currently used in commerical buildings.

Second, it is relatively less expensive than the alternative exterior

walls which are sometimes used in commercial buildings. The lower cost

reflects the factory fabrication and the design for inside glazing and

stick wall erection. The building envelope used in the economic analysis

is on the low end of the cost range for envelopes currently being used

in commercial applications. The costs of the glazed portions of the

envelope, unlike the rest of the envelope, should remain about constant

even if more expensive opaque portions of the envelope are substituted

for those used in the economic analysis. Consequently, if the window

system is found to be economically viable (cost effective) in this case

it will also be cost effective for those cases where more expensive

exterior walls are used, other things being equal.

If management devices are used, additional acquisition costs are incurred.

Costs of Venetian blinds and insulated thermal shutters based upon large

volume sales are given in Table 4.3. All costs reflect purchase, installation, and contractor markup. If management devices are used it is

assumed that their costs are added to the capital costs of the exterior

wall system and then depreciated along with the rest of the exterior wall

system.

TABLE 4.3

Commercial Case Study:

Cost of Window Accessories

Dollar Cost By Size of Window

Type of Accessory 12 ft^ 30 ft^ 60 ft^ 90 ft^

Venetian Blinds 47 47 64 95

Thermal Shutter 232 412 750 1050

Source: A leading manufacturer and distributor of building materials

provided the cost estimates.

71
Most commercial buildings usually have a schedule for window washing

which may be done by contract. The cost-of -cleaning data presented in

Table 4,4 reflect estimated contract prices for annual washing of all

windows of a specified size in the building. The annual insurance premium is used as a proxy for repair/replacement costs. These figures are quoted in Table 4.4 as a function of the size of the window and the

glazing type,

TABLE 4.4

Commercial Case Study:

Cleaning and Insurance^ Costs

Current Dollar Cost By Size of Window

Type of Cost 12 ft^ 30 ft^ 60 ft^ 90 ft

Annual Cleaning 3,10 4.10 5.80 7»50

Annual Insurance

Premium^

Single 0,60 1,70 4,10 5.90

Double 3,80 10,40 24,50 35.60

Source: Insurance costs were provided by a major insurance company,

based on company rate manuals.

Insurance costs are used as a proxy for repair and replacement costs,

^ Premiums are unadjusted for regional rate differentials.

4 . 3 Energy Costs

4,3.1 Energy Costs for Unmanaged Windows Not Used for Daylighting

Base-year energy costs for the office module heated by natural gas

start at $53 for a windowless wall and go as high as $70 for a 90 ft^

single-glazed window with a southern orientation, with energy costs

rising steadily in relationship to window size. For a northern

72
orientation, costs range from $53 to $91. With heating by electricity,

base-year energy costs range from $52 for the windowless module, to $70

for the module equipped with a 90 ft'^ single-glazed windovy facing south.

For a northern orientation, costs range from $52 to $89. Rase-year

energy costs are increased by as much as $18 by a large, south-facing,

single-glazed window when either gas or electric resistance heating is

used.

Figure 4.2 shows the base-year energy costs converted to a life-cycle

basis, both for constant future energy prices and for 12 percent escalation. The lower portion of Figure A. 2, Part A, shows that even at

fixed energy prices, life-cycle energy costs for the office module

may be raised by about $160 (from $480 to $640) by using 90 ft^ singleglazed windows for southern orientations, and by about $350 (from $480

to $830) for northern orientations. Part R of Figure 4.2 shows that

the increases in life-cycle energy costs are limited to between $70 and

$110 by the use of double glazing.

The upper portion of Figure 4.2 shows that with energy prices escalating

at a rate of 12 percent, the energy costs for the reference point (i.e.,

the windowless office module) are $1670, as compared with $480 with constant energy prices. With large, single-glazed windows facing north,

the life-cycle energy costs for the office module now approach $2900,

indicating an increase in life-cycle energy costs of approximately $1200

attributable to the windows. Part B of Table 4.2 indicates that the

savings potential of double glazing becomes more important the higher the

rate of energy price escalation (net losses attributable to windows are now limited to $400).

4,3.2 Energy Costs for Managed Windows Not Used for Daylighting

The addition of window management causes base-year energy costs to remain

approximately level as the window area is increased, with the exception

of single-glazed windows on the north side. Energy costs are reduced

dramatically below those found in the preceding case.

The coefficient of performance (COP) for electric heating is 2.0;

therefore the equivalent energy cost per therm is

(100,000 Btu) X $/kwh ^ (3413 Btu/kwh) x COP

The equivalent energy cost per therm for gas heating is $0.46. Thus

electric resistance heating is, in this case, less expensive than gas

heating by virtue of the relatively high coefficient of performance

for electric heating. Since the energy costs are nearly equal, all

future references in the text to the figures plotting energy costs will

refer only to the orientations and glazing type and ignore energy type.

73
2800

^ 2600 h

w

I-V)O 2 400

>

o

tr. z 2 200

_l> 2000 - o

I Ul

_i 1800 h /

leooJ^

PART a: single GLAZED

2900-

N

/'(|2%PE)

PART b: double glazed

S

y

y '''(I2%PE)

,-'fl2%PE)

-1lT2%PE)

850 I-

_ 775

•<J> w

CO 700 O

o

>

o

5 625

z

UJ

UJ 550

>-o

I UJ

u- 475

400

/lO'/oPE)

/

,--^(0%PE)

^^-"'(O'/.PE)

^^^---'(oVoPE)

0 12 30 60 90 0 12 30 60 90

WINDOW AREA IN FT^ WINDOW AREA IN FT^

Figure 4.2 Life-Cycle Energy Costs for the Office Module for North (N)

and South (S) Facing Unmanaged Windows Not Used for Daylighting

(Gas Heating and Electric Cooling Evaluated for both 0 and

12% Price Escalation)

74
With constant fuel prices, there is, therefore, little difference in the

life-cycle energy costs of the room with and without windows. The exception is for north-facing, single-glazed windows which raise costs hy ahout

$120.

When energy prices rise at a rate of 12 percent per year, life-cycle

energy costs are about quadrupled. The windowless office module now requires an energy expenditure of $1,620, as shown by Figure 4.3A, and

the office with a single-glazed 90 ft^ window facing north now has an energy cost of about $2,050. Part B of Figure 4.3 shows that double

glazing used in conjunction with management tends to reduce energy consumption, but not by as much as in the preceding case when the windows were unmanaged. (By comparing Figure 4.3 with Figure 4.2, it may be seen that the nighttime temperature adjustment reduces life-cycle costs

by approximately $50.)

4.3.3 Energy Costs for Unmanaged Windows Used for Daylighting

VJhen the use of daylight is considered, but window management is not,

base-year energy costs start at $53 and decline as window area is initially increased. However, as window area is increased past 30 ft^

with single glazing or past 60 ft^ with double glazing, energy costs

begin to rise. As in the residential case, this pattern of initially

falling and then rising energy prices reflects the fact that once the

reference illumination level of 50 fc is met, additional daylight provided by increasing window size is not treated as energy savings in the

evaluation model, while undesirable heat flows that tend to increase

as window area increases continues to be treated as costs.

The life-cycle energy costs for this third mode of operation are particularly interesting since a large potential for savings exists. With

constant energy prices, the life-cycle energy costs of the windowless

office module are approximately $480. Even with a 90 ft" window area,

costs remain below the initial energy cost provided the window area is

either facing south or double glazed.

The utilization of daylight becomes even more important with a rise in

energy costs. This relationship is demonstrated by Figure 4.4 which

assumes that energy prices increase at the rate of 12 percent per year.

The energy costs for the windowless room are now about $1670. (They are about $50 more than in the preceding case because there is no thermostat

adjustment in this case.) If used for daylighting, south-facing,

single-glazed windows may reduce energy costs to as low as $965, and

south-facing, double-glazed windows to as low as $625. By comparing

Figure 4.4 with Figure 4.2 (the first case), we can see the considerable

potential that daylight utilization has to conserve scarce energy resources.

By comparing Parts A and B of Figure 4.4, the large potential savings for

double glazing may be seen. For example, the energy costs associated

with a 90 ft^ double-glazed window facing north are nearly $900 less

75
^ CO

1 \

o

UJN

<

o

-I

ffi O

o

ffi <

Q.

<

-JO

UJo

(O

<

I-s

1

1

ii

I I

I I

1

1

1

1

II

"lHI

CO

I

_1L

o

o

00CM

o

o

o

o

cvi

O

CNJ

CVJ

O

O

o

O

00 O

o

<0

o

cn

(M

CD ±

<

U

<

K) 9

CVJ

o

CJ

2 s

<

UJ

<

lo 9

C\i

•H

4-1

" ' cua r-"

I-*-! •H •H

1 > iH

J_l P-i Q

(—1 5^

CN

ll?-< H 1—

1

Pi O

1

1 ( M-l W

o

Tj

<D

1—

Tiw CU r\ 4-1 5^ P cd O 3

QJ

CO P>

1

1 1

1

1 1

•H •HiHO

r) O

f-l 0) CJ

t>0 CO a

d •H U u

o

o cu u

•H M

>^ o

cC

0) n3 C

w bcd

<u •H iH 4-1 O •u CO

(U a o cc

1cu CO

cC •H a

1-1

cuu

•H

($) SISOD A9d3N3 310A0-3dn

76
CO

o

UJN

<

_l

o

UJ

ffi o

o

ffl

2

o

0>

o

CO

O

HIN

<

o

z

I-2

\

5±

O

O

00

O

o

O o

o

CNJ O

o

o

o

00

o

o

<0

o

CSi

o

0)

o

o

lO

CM

(Vi

<

UJ

<

o

o

<

UJ

a:<

o

o

z

a) •H (JHM

o •H

1-1 6^

>. CN

1—

1

o O

i-t

t-i O

D O M-i

,—

j

U~i *^ dJ Q QJ U

)£; W cfl t3

i-HCO>

i-l-i o W 1

1

o c t>C •H C

12 •H

^ r; iH

1 1 TD O

0) O

O 03

M-i C CJ •H

Cfi V-i U c U

(/} U

Q 0) C w H

&c O

CO

cti cw bCC

(U •H C

iH U o

O CO •H

CU U

o X CO

1 o rH

(1) CO CO

CO u

•H !-i O

i-J O

0)

S-i 3

&C •H

($) SISOO A9d3N3 310A0-3dn

77
than the energy costs associated with a single-glazed window of the same size also facing north.

4.3.4 Energy Costs for Managed Windows Used for Daylighting

When both window management and daylighting are practiced, base-year

energy costs for the module with either single or double glazing are lower

than in the preceding case. Except for single-glazed windows facing north,

costs now continue to decline for window sizes up to 90 ft^. However,

the decline in costs slows down as window size is enlarged.

With constant energy prices, energy requirements over the 25 year period

for the wlndowless room are approximately $460, $20 less with the thermostat adjustment than without it. For southern exposures, the life-cycle

costs fall to $250 when a single-glazed window of 40 ft^ is added, and

fall further to $150 when the window area is expanded to 60 ft^. Lifecycle energy savings may range as high as $340 by having a window.

With energy price escalation at 12 percent, the life-cycle costs of the

windowless module again almost quadruple, and the potential energy benefit from using windows becomes quite large. As is shown by Figure 4.5,

energy costs for the module decline for all the sizes of single- and

double-glazed windows facing south, as well as for double-glazed windows

facing north. Costs are shown to increase if single-glazed windows larger

than 60 ft^ are used facing north. Using a combination of daylighting and

window management results in a potential (before tax) savings from windows

as high as $1,200.

4.4 Life-Cycle Costs

4.4.1 Life-Cycle Costs for Unmanaged Windows Not Used for Daylighting

Let us now examine the effects of combining life-cycle energy costs

for the office module with life-cycle envelope costs for the window.

Table 4.5 shows the combined life-cycle costs associated with unmanaged windows not used for daylighting. For all cases, life-cycle costs

are higher with windows than without, although single-glazed windows add

relatively little to total costs if energy prices are constant. In every

For the life-cycle cost calculations, the deductibility from taxable

income of energy costs and depreciation for capital investment costs

are taken into account in the model.

The depreciation allowance D(n) in year n is given as

D(n) = hil X BV(n-l)

N

(continued on page 81)

78
o

UJN<

(9UJ

-J

CD3OO

ffl I-

(E

to o / y o / o

uiN<

_Jo

UJ

_loz<

cr<

Q. ^ — oo

00 oo

<0 oo oo

(M ooo oo

00 oo

<0 o

rO

CMO

0>Oo

(VJ

N

Iu.z<

UJ

cr<ooz

CSJ<

UJq:<oo

($)S1SOO A9d3N3 310A0-3dn

anoo

•H

M-l O

QJ

l-l uo

14-1 ou

>^u

0)Cw ^ Ph •H 6-^ ^^ CMrH Q ^(O u ^ o

T3mmo

•H -HoO

OJ

u

w

u3

CO>cU

bC QJ C rH

•H

W

U

C

to CO oc ^ c ^o

to -H

13 ^ -U O ffi CO

Cfi CO

to CJ

C c; cn

to ^ w I QJ

S-i 3

txC

ft,

79
TABLE 4.5

Life-Cycle Costs When the Window System

is Unmanaged and Not Used for Daylighting

(Gas Heating and Electric Cooling)

Life-Cycle Costs in Dollars

Window

Area

Single Glazed Doubled Glazed

(Ft2)

Orientation

SW/SE E/W NW/NE N OrientationS SW/SE E/W NW/NE N

12A8 1250 1252 1251 1249 1248 1250 1252 1251 1249

u

<uc

12

30

1270 1276 1282 1284 1282

1284 1295 1308 1317 1313

1364 1368 1371 1370 1368

1473 1479 1486 1487 1482

60 1392 1412 1439 1458 1452 1767 1777 1789 1793 1785

90^ 1249 1279 1320 1349 1343 1769 1783 1800 1806 1795

1846 1853 1857 1854 1848 1846 1853 1857 1854 1848

12 1887 1905 1927 1936 1927 1970 1983 1993 1992 1983

30 1939 1979 2024 2055 2043 2093 2115 2138 2143 2126

60 2119 2189 2283 2351 2331 2415 2453 2494 2506 2478

90^ 2049 2153 2296 2398 2377 2450 2500 2560 2580 2541

^ Life-cycle costs are lower for 90 ft^ of single-glazing than 60 ft^ due to reductions

in framing costs.

80
case, the fuel savings of double glazing are more than offset by its

extra acquisition and repair^ costs, causing overall life-cycle costs

for double glazing to be significantly higher than for single glazing.

4.4.2 Life-Cycle Costs for Managed Windows Not Used for Daylighting

Table 4.6 shows that the total life-cycle costs are higher with windows

that are managed but not used for daylighting than with no windows,

regardless of window size, orientation, glazing type, and energy price

escalation. It can also be seen that single glazing is Inore cost effective than double glazing for all sizes and orientations examined.

By comparing Table 4,6 with Table 4.5, it can be seen that the energy

savings from the blinds and shutters are not sufficient to offset their

extra capital costs, especially for the larger window areas. Life-cycle

(continued from page 78)

where N is the period over which the capital asset is depreciated and

BV(n-l) is the book value of the capital asset in the previous

period. BV(0) is assumed to be the acquistion cost of the system.

The present value, Pj.V(n), of the depreciation allowance in year

n is then given as

PtV(n) = ^

(1 + DISCOUNT RATE)'^

The present value, PV(n), of the reduction in income taxes due to the

depreciation allowance is then given as

PV(n) = Pj.V(n)(l-TAX RATE) .

The present value of all tax reductions due to depreciation allowances

PV is given as

N

PV = 2 PV(n) . n=l

^ Recall that insurance costs are used in this case study as a proxy

for repair costs.

^ At this time there do not appear to be any commercially available

insulated shutters that would be appropriate for office buildings.

Those used in the case study are custom and have an acquisition cost

of approximately $13 per square foot on the average. This figure is

thought to be prohibitively expensive. However, in the fourth case

of window use, two separate analyses are conducted. The first uses

the custom made shutter and the second assumes an in place cost of $5

per square foot. Although the $5 per square foot figure is an assumed

value, price reduction through competition may result if building

owners demand insulated shutters.

81
TABLE 4.6

Life-Cycle Costs When the Window System is Managed and Not Used for Daylighting

(Gas Heating and Electric Cooling)

Life-Cycle Costs in Dollars

Single Glazed Double Glazed Window Area

Orientation Orientation

(Ft2) S SW/SE E/W NW/NE N S SW/SE E/W NW/NE N

0 1241 1242 1244 1243 1242 1241 1242 1244 1243 1242

Prices 12 1482 1486 1489 1488 1487 1579 1583 1585 1585 1583

Energy 30 1626 1631 1638 1643 1642 1828 1833 1838 1838 1835

Constant 60 1995 2006 2018 2035 2033 2402 2407 2414 2414 2410

90 2099 2111 2131 2157 2158 2667 2673 2680 2681 2675

0 1818 1825 1829 1827 1822 1818 1825 1829 1827 1822

12 2058 2070 2080 2080 2074 2153 2165 2175 2173 2166

30 2200 2219 2240 2260 2254 2399 2416 2433 2433 2433

60 2568 2606 2649 2709 2701 2977 2996 3021 3021 3006

90 2683 2728 2797 2890 2891 3254 3276 3229 3305 3284

82
costs are actually higher for the second case than for the first, despite

the beneficial results of thermostat adjustment in the second case. However, management does become relatively more worthwhile with the escalation in energy prices.

4.4.3 Life-Cycle Costs for Unmanaged Windows Used for Daylightlng

Perhaps the most striking result of Table 4.7, which shows the life-cycle

costs when the window system is unmanaged but used for daylighting, is

that double glazing is in all cases uneconomical. Nevertheless, daylighting, as in the residential case, is sufficiently beneficial to cause

the life-cycle costs with a window system, whether double or single glazed,

to be less than the costs without a window system despite the fact that

winter heat losses and summer heat gains are not being mitigated by window

management. However, this does not hold for all combinations of window

sizes, glazing types, and orientations.

When energy prices are assumed to remain constant, the least-cost window,

for all cases except northerly exposures, is 90 ft and single glazed.

This condition is shown in Table 4.8. Notice also from Table 4.8 that

due to reduced capital costs, the use of this window system results in an immediate payback indicated by an "I" entry in the column "Years to

Payback .

Single glazing is also indicated as the least-cost glazing when energy

prices escalate at 12 percent. The least-cost window size is smaller

(30 ft^) with price escalation. Throvigh energy savings and reduced

capital costs the use of these windows again results in an immediate

payback. They can pay for themselves within the 25 year period even

if energy prices fall somewhat.

4.4.4 Life-Cycle Costs for Managed Windows Used for Daylighting

Table 4.9 gives total life-cycle costs for the fourth case when windows

are both managed and used for daylighting. Several facts are apparent

from this table. First, the single-glazed windows are more economical

over the life-cycle than the double-glazed windows for all window sizes

and orientations, and for both constant and escalating energy prices.

Second, the large savings that were found in the residential example when

windows were both managed and used for daylighting do not materialize in

this example; neither single- nor double-glazed windows are more economical than the windowless wall. In fact, large windows raise life-cycle

costs between $700 and $1,300, depending on orientation and glazing type.

The fact that real reductions in life-cycle costs resulted in the third case when daylight alone was used indicates that daylighting is very

beneficial. The increase in costs in the fourth case indicates tliat the

added costs of the management devices are not offset by energy savings

through reduced window heat loss and summer heat gain.

Figure 4.6 uses the polar coordinate chart to compare the life-cycle

costs of the 30 ft^ single-glazed, managed window used for daylighting

83
TABLE 4.7

Life-Cycle Costs When the Window System is Unmanaged and Used for Daylighting

(Gas Heating and Electric Cooling)

Life-Cycle Costs in Dollars

Single Glazed Double Glazed

^ Window Area

o

r] Orientation Orientation > (FT^) S SW/SE E/W NW/NE N S SW/SE E/W NW/NE N

0 1248 1250 1251 1251 1249 1248 1250 1251 1251 1249 u

C

m 12 1215 1221 1227 1231 1229 1306 1310 1315 1316 1313

30 1160 1173 1189 1201 1200 1339 1348 1360 1365 1362

CJ 60 1247 1268 1299 1324 1322 1599 1616 1638 1648 1643

90 1103 1134 1177 1214 1211 1593 1617 1646 1659 1651

rice 0 1846 1852 1857 1854 1848 1846 1852 1857 1854 1848

cu o

•H

hfl rrt 12 1692 1713 1735 1749 1742 1765 1779 1798 1800 1791

Eneri scali 30 1505 1550 1607 1650 1645 1620 1652 1695 1713 1702 w

&^ 60 1609 1684 1792 1881 1872 1825 1886 1963 1998 1981

90 1535 1642 1795 1926 1915 1833 1915 2017 2065 2036

TABLE 4.8

Least-Cost Window Systems When Windows Are Evaluated Without Window Management But With Daylighting

Orientation

Size

(ft2) Glazing

Type Total LifeCycle Cost

Total

Cycle

Saving ($) Years to^ Payback MinimumFPE (%)

rice S 90 Single 1103 145 I < 0

CL,

^>^

tan SW-SE 90 Single 1134 117 I < 0

Ener E-W 90 Single 1177 74 I < 0

ant NW-NE 30 Single 1201 50 I < 0

;;onst N 30 Single 1200 49 I < 0

QJU

S 30 Single 1505 340 I < 0 H

u Co SW-SE 30 Single 1550 302 I < 0

ergy

alati E-W 30 Single 1607 250 I < 0 c uw NW-NE 30 Single 1650 204 I < 0

N 30 Single 1645 203 I < 0

a "I" indicates

90 ft^ window

an immediate payback reflecting

system is used.

the initial reduction in capital costs when the

84
TABLE 4.9

Life-Cycle Costs When the Window System Is Managed and Used for Daylighting

(Gas Heating and Electric Cooling)

Life-Cycle Costs In Dollars

Single Glazed Double Glazed Window Area

Orientation Orientation

(FT^) S SW/SE E/W NW/NE N S SW/SE E/W NW/NE N

<u

0 1240 1242 1244 1243 1241 1240 1242 1244 1243 1241

y

Prlc 12 1444 1447 1450 1451 1450 1541 1544 1547 1546 1544

Energ 30 1529 1536 1548 1560 1560 1732 1737 1742 1746 1744

nstant 60 1841 1855 1881 1906 1908 2241 2246 2261 2271 2270

90 1933 1948 1986 2023 2027 2491 2499 2514 2528 2526

g 0 1818 1824 1829 1826 1821 1818 1824 1829 1826 1821

12 1923 1935 1945 1949 1944 2018 2030 2040 2038 2031

30 1861 1884 1927 1967 1968 2060 2077 2096 2110 2103

60 2029 2079 2167 2256 2265 2413 2432 2484 2517 2516

90 2102 2156 2287 2419 2432 2639 2663 2719 2767 2761

85
Figure 4.6 Graphical Determination of Least-Cost Window System for the

Commercial Office Module

86
(the curve labeled "C") with two^alternatives : (1) the windowless wall

(labeled "A"), and (2) the 30 ft single-glazed, unmanaged window used

for daylighting (labeled "B"). The fact that curves A and B lie everywhere on a lower cost line than curve C indicates that the 30 ft^

single-glazed, managed window used for daylighting is not the most

economical of the three choices.

It was noted earlier that the cost figures for the thermal shutter used

in the commercial case study were quite high due to its small market size

at present. In order to test the cost impact of a lower price for the

shutter, the analysis was repeated using a price of $5 per ft rather

than $13 per ft that was based in all of the preceding commercial window

management calculations. The results are shown in Table 4.10.

The thermal performance of the shutter is assumed to remain constant.

Because the $5/ft cost is purely hypothetical, the results presented in

Table 4.10 should be viewed as an illustration of sensitivity analysis

rather than as a guide for window sizing.

With the lower priced shutters, the life-cycle costs associated with the

windowed wall that is both managed and used for daylighting are lower

than for the non-windowed wall, provided energy prices escalate rapidly,

and the windows are single glazed and kept to 30 ft when used for northerly exposures. However, by comparing Table 4.10 with Table 4.7, the

third case, we can see that the unmanaged windowed wall used for daylighting continues to be more cost effective than the managed windowed

wall used for daylighting.

4.5 Implications of Results

This section summarizes the major conclusions that can be drawn from the

analyses of the life-cycle costs of the alternative window systems in the

office module. The two cases of window operation which do not make use

of daylighting are grouped together.

When Windows Are Not Used for Daylighting

1. In all situations examined, life-cycle costs were higher

with windows than without windows.

2. If management devices are used, the larger the window, the

larger the life-cycle cost of the office module.

3. If windows are used, their costs are lowest if they are located with southern exposures, and highest if located

with northern exposures.

4. Rising fuel prices raise the costs of having windows of

all sizes as compared to the windowless wall.

87
i

4-1 o

•He »-l 0) (U

V

4-1u 60 Cfl c u C V4 0 •H CO

4-1 <H iH

J=O iH

60 o o

•HCSI o Q

iH 4J

C >^ '4-{ o C

•H to •H •H Q )-l </> 4J CO

<U )-lO 4J

O 0 (U CO

4-1u .-H o W u c •o

0) 0) 4-1 Xl CD

05 •H c iH

:=! > CO CJ •H

(0 •a 60 c •H c 1 cn rt 0) •H <u O C 4-1 M-( u •a <11 CO •r-l 0) <u J

(U 60 33

1-1o c CO

CO 4-1 CO

u S O 1 60

(U c

U-{ •H

•H 4J hJ U]

(U H

T3Ooa

<u

00c

•H

C/l

CO

cuu<o

T3c O o CO o 00 ON O cn o CO CM as 00 z CM <f in o 00 00 00 CM

F-H CN CM CM

w v£l CN CO 00 vO as

<tO c^ o CO CM as as -*

CNJ LO ON o 00 00 00 CM

f-H CN CM CM

00 <f ^ as CN ON

O CN CJN CN CM as 00 CN

CSI •<r LTl 00

O 00 00 00 CM

u F-H CM 1—1

CN CN

w

CN ro CN C3N ON <f CTN CM

m

-JO o Cvl 00 NO NO

cs <!• ir\ 00 o 00 00 00 o t/5 1—1

CM <-H CM CM

OO Cvl 00 sD

m ON

o- o O <t -* ~*

CM 00 o oo 00 00 o F-H

C/3 CM F-H CM CN

CO >^ 00 CN

•<rO CO CM

O

m ON

OJ CO in in 00 00 00 ON z

ro

O in cn vO 00 CM OS 00

<f CO CO CN o m 00 CM

CN cn CO in in 00 00 00 OS

<! a\ <r CO vO OS CO o t-~ 3 <f o CO CJN CM o F-H o OS

CN cn CO in 00 00 r~. 00

Ww CN vD CN 00 CT> -d- ON ^ NO o og 00 m CM as vO NO

CO nO NO

CO

O CO in CO 00 CM NO CM CM

<r o 00 <r NO

CN CO -<r 00 NO NO nO

CNH o CN

O o O o CM o o o b CO CJv CO NO CJN •—

88
5, Even if fuel prices escalate rapidly, double glazing, due

to its higher capital and insurance costs, is less economical than single glazing regardless of window size and

orientation.

When Windows Are Unmanaged , But Used for Daylighting

1, The life-cycle cost of the office can be reduced by

adding a window if care is taken in its sizing and

orientation.

2. For some window areas, there may be savings relative to

the windowless wall, as well as energy savings from using

the window for daylighting.

3. When the value of trading daylight for electric lighting

is considered, rising energy costs tend to improve the cost

effectiveness of the window.

4, If fuel prices remain about constant, life-cycle costs of

the office are lowest if a large, single-glazed, southfacing window is added.

5, If fuel prices rise rapidly, life-cycle costs of the

office are lowest if a medium-to-large, single-glazed,

south-facing window is added,

6. Even if energy prices are expected to increase rapidly,

single glazing is more cost effective than double glazing.

When Windows Are Both Managed and Used for Daylighting

1. At the current market price for the thermal shutter

examined, none of the window systems examined was found

to reduce life-cycle costs below those for the windowless

module.

2. Double glazing was less cost effective than single glazing

for all window sizes and orientations examined.

3. Even at a moderate cost of $5 per square foot for the

thermal shutter, the managed window is not as cost effective as the unmanaged window.

4. If a thermal shutter could be purchased and installed for

a cost of $5 per square foot and fuel prices rise rapidly,

life-cycle costs of the office module can be lowered by

adding a window, particularly a large, single-glazed window

facing south.

89
When Energy Prices Are Constant

1. Life-cycle building costs are minimized by adding a large

(90 ft ), single-glazed window facing south and utilizing

it for daylight, but not equipping it with management

devices .

2. Regardless of window size, orientation, and glazing type,

the least-cost case of window use is the third examined,

i.e., using the window for daylighting but not investing

in management devices. In descending order of cost effectiveness, the other cases rank as follows: the first case,

i.e., neither using the window for daylight nor using

management devices; the fourth case, i.e., using the

window for daylight and using management devices; and,

lastly, the second case, i.e., using management devices

with the window but not using it for daylight.

3. Regardless of whether or not window management is practiced,

it does not appear to pay to invest additionally in double

glazing.

4. Small areas are preferred if management devices are used,

whereas large window areas are preferred if management

devices are not used.

When Energy Prices Escalate at a Rate of 12 Percent

1. Life-cycle costs are minimized by placing a medium-sized

(30 ft ), single-glazed window facing south and using it

bare for daylighting.

2. Regardless of window size, orientation, and glazing type,

the least-cost case of window use is the third examined,

i.e., using the window for daylighting but not investing

in management devices. In descending order of cost effectiveness, the other cases rank as follows: the first,

i.e., neither using the window for daylight nor using

management devices; the fourth, i.e., using the window with

management devices for daylight; and lastly, the second.

I.e., equipping the window with management devices but not

using it for daylight.

Recall that the term window management includes not only the use of

selective management devices such as thermal shutters and Venetian

blinds but also nighttime thermostat adjustments. Although selective

management devices may not be cost effective, a nighttime thermostat

adjustment would generally be cost effective since it produced energy

savings at little or no cost.

90
3. Single glazing is preferred for all of the window sizes and

orientations examined.

4. The thermal shutter is not cost effective due to its high

current market price.

5. If the thermal shutter examined could be installed at a cost of $5 per square foot and large windows were used,

it would be more cost effective to use the shutter than

double glazing, but it would be still more cost effective

to use neither.

Apart from the consideration of psychological or other factors, these

results suggest that a building owner, builder, or designer in the

Washington, D.C. area could reduce life-cycle costs by keeping windovj areas as small as possible or eliminating them altogether in those office

modules which are not used much during the day or which for some other

reason cannot be used substantially to reduce electric lighting requirements. The results, however, suggest that it is better from a life-cycle

cost standpoint to have windows — even relatively large ones — than

to have a windowless exterior wall if the windows can be used successfully

to reduce electric lighting requirements.

If energy prices continue to rise rapidly, some consideration might be

given to the use of either window accessories like those described or double glazing but not both. However, in all cases, even for northern

exposures, single glazing should be given precedence.

The results further suggest that double glazing is generally not cost

effective for the type of office module examined, nor is the use of

management devices unless the thermal shutters examined could be acquired

at very low cost or unless energy prices rise much more rapidly than the

12 percent rate examined here.

91
Facing page: T^e p^OV.U>-loyi o{) a V-lOiO thAough u)Zndom can biz. an important

{^acXon. -in w-indoi^j do^-lgn but aj> dillli.c.uZt

to a^hoj^b tn dolZoA toAmi,

.

92
5. SUMMARY, CONCLUSIONS, AND SUGGESTIONS FOR FURTHER RESEARCH

This report has developed a life-cycle costing model and computer program for evaluating the dollar costs of acquisition, maintenance, and

repair for windows of alternative design, size, orientation, geographical location, and with various accessories and modes of use, as well

as the energy costs for the interior space with the alternative windows.

It has described the importance of windows to energy costs, delineated

the costs and benefits of windows, presented the economic evalution mode

verbally, algebraically, and in BASIC computer language, provided stepby-step illustrations of the application of the model to the windows in

a residence and in an office building, and has drawn tentative conclusions based on the illustrations.

93
The results of the illustrations have indicated that window size,

orientation, thermal resistance, accessories, and use —particularly

the substituting of daylight for electric lighting—can affect significantly the energy consumption and life-cycle costs of windows. By

examining the impact of windows on the life-cycle costs for four different levels of thermal analysis, the illustrations have shown how energy

and overall building costs are influenced by the use of selected window

accessories, thermostat adjustment, and daylighting. By examining

energy and other costs separately and in combination, the approach has

demonstrated the relationship between the energy efficiency and the cost

effectiveness of different window designs. If energy costs can be reduced

with little or no additional investment cost, as by changing window orientations, the least-cost energy decision is also the least-cost building

decision. However, if capital and labor costs are increased by choosing

one window design or accessory over another, it is important to weigh

these costs against the value of energy savings to determine the costeffective decision.

Considerable work remains to be done in the economic evaluation of

windows. One task is to apply this or other evaluation models to develop

guidelines for cost-effective window decisions for many types of window

features, buildings, geographical locations, and uses. The companion to

this report"^ takes a step in this direction by analyzing selected window

features for two types of buildings in nine locations under alternative

conditions. Much remains to be done, however, to develop a comprehensive

set of guidelines for the nation.

Another Important task is to extend and improve the thermal model to

incorporate important effects now omitted. The inclusion in the model

of the effects of natural ventilation from operable windows is a prime

example of a needed improvement. An additional critical task is to

raise the quality and reliability of the data with which the model must

be used. For example, there is notable uncertainty about the utilization

of excess heat gain in the winter and the available daylight under various

sky conditions, room configurations, and window designs. Further research

in both the laboratory and in the field is needed to provide reliable

estimates of the energy impacts of windows.

With these suggested improvements in the evaluation model and in the

input data to the model, a high payoff could be expected from the

intensive application of the model to window alternatives and the

analysis of the results. Development of easy-to-use, reliable guides

to energy efficient and cost-effective windows could result in better

decisions by the building community and in energy and dollar savings

to the nation.

Rosalie T. Ruegg and Robert E. Chapman, Regional Economic Assessment

of Selected Window Systems .

94
SELECTED REFERENCES

Architectural Aluminum Industry Statistical Review , Architectural Aluminum Manufacturers Association, Publication No. AAMA-MIR-1-1975 , 1975.

ASHRAE Handbook of Fundamentals , (New York: American Society of Heating,

Refrigerating and Air-Condi tioning Engineers, Inc., 1972).

BASIC LANGUAGE , Honeywell Software Series 400, Honeywell Information

Systems, Inc., August 1971.

Beckett, H. E. and Godfrey, J. A., Windows: Performance, Design, and

Installation (New York: Van Nostrand Reinhold Company, 1974).

Berman, S. M. and Silverstein, S. D. , "Energy Conservation and Window

Systems," Efficient Use of Energy; the APS Studies on the Technical

Aspects of the More Efficient Use of Energy , Part III, LC 75-18224,

1975.

Booz-Allen and Hamilton, Inc. , Life-Cycle Costing in the Public Building

Service , Volume 1, (A Report Prepared for the General Services Administration, Public Building Service), undated.

Chapman, Robert, Economic Analysis of Experimental Lead Paint-Abatement

Methods: Phase I , National Bureau of Standards, Technical Note 922,

September 1976.

Collins, Belinda L. , Ruegg, Rosalie T., Chapman, Robert, and Kusuda,

Tamami, A New Look at Windows , National Bureau of Standards, NBSIR

77-1388, January 1978.

Collins, Belinda L. , Windows and People , National Bureau of Standards,

Building Science Series (BSS) 70, June 1975.

The Economy of Energy Conservation in Educational Facilities , (New

York, New York: Education Facilities Laboratories, Inc.,

November 1973).

Engineering News-Record , April 21, 1977.

Hastings, Robert and Crenshaw, Richard, Window Design Strategies to

Conserve Energy , National Bureau of Standards, BSS 104, June 1977.

Kusuda, Tamami and Collins, Belinda L. , Simplified Analysis of Thermal

and Lighting Characteristics of Windows: Two Case Studies , National Bureau of Standards BSS 109, February 1978.

Kusuda, Tamami and Ishii, K. , Hourly Solar Radiation Data for Vertical

and Horizontal Surfaces on Average Days in the United States and

Canada, National Bureau of Standards, BSS 96, April 1977.

95
National Association of Home builders , "Economic News Notes," Housing

Starts Bulletin , Vol. XXIIl, No. 1, January 20, 1977.

Reistnan, A. and Rao, A., "Stochastic Cash Flow Formulae Under Conditions

of Inflation," The Engineering Economist , Volume 18, Number 1,

Fall 1972.

Report on Building Life Costs (a report prepared by the Council of

Ontario Universities), Toronto, Ontario, November 1973.

Ruegg, Rosalie T. and Chapman, Robert E., A Regional Economic Assessment

of Selected Window Systems , National Bureau of Standards, NBSIR

(in press).

"Science and the Citizen," Scientific American , April, 1977.

Smith, Gerald W., Engineering Economy: Analysis of Capital Expenditures , 2nd ed. (Ames, Iowa: The Iowa State University Press, 1973),

U.S. Department of Labor, Bureau of Labor Statistics, Monthly Labor

Review , Volume 99, Number 2, February 1976.

Young, D. and Contreras, L. , "Expected Present Worths of Cash Flows

Under Uncertain Timing," The Engineering Economist , Volume 20,

Number 4, Summer 1975.

96
Appendix - Computer Program for

Life-Cycle Cost Evaluation of

Windows (BASIC Language)

100 DIM R(5 ,5) , S(5 ,5) , T(5 ,5)

110 DIM U(5,5) , V(5,5) , W(5,5)

120 DIM X(5 ,5) , Y(5 ,5) . Z(5 ,5)

130 DIM C(5 , 3) , F(5 , 3) , H(5 , 3)

140 DIM A(5), B(5), D(5), E(5), G(5), 1(5), J(5)

150 DIM K(5), L(5), M(5), N(5), 0(5), P(5), Q(5)

160 PRINT "F1 FUEL P R I C

E

" , " ES C A L A T 1 0 N RATE"

170 PRINT "D1 DISCOUNT R","ATE"

130 INPUT F1,D1

190 READ CI , C2 , C3

200 READ L9

210 PRINT "COMMERCIAL" , "NO 0 YES 1"

220 INPUT CO

230 IF COrOTHEN 290

240 PRINT "TAX FACTOR"

250 INPUT FO

260 REM

270 REM - calculate annual energy costs for each glazing type

280 REM

290 MAT READ R 300 MAT READ S

310 MAT READ T 320 MAT R=(C1)»R

330 MAT S=(C2)»S

340 MAT T= ( C3 ) *T

350 MAT U=R+S

360 MAT U=U+T

370 MAT R=ZER

380 MAT S=ZER

390 PRINT "TRIPLE GLAZING", "NO 0 YES 1"

400 INPUT TO

410 MAT READ R 420 MAT READ S

430 MAT R= ( CI ) *R

440 MAT S=(C2)*S

J'50 MAT V = R + S

460 MAT V=V+T

470 IF C0=0 THEN 500

480 PRINT " CALCULATIONS" ,' BASED ON TAX", "RATE 0F",F0

490 LET F0=1-F0

500MATR=U

510 MAT U=TRN(R)

520 MAT S=V

530 MAT V=TRN(S)

540 REM

550 REM - calculate life cycle energy costs for each glazing type

560 REM

570 IF F1=D1 THEN .6 20

580 LET A1 = ( ( 1+F1 )/( D1-F1 ) )*( 1-( ( 1+F1 )/( 1+D1 ) ) "L9 ) 590 MAT X= ( A1 )»U

97
600 MAT Y=(A1)*V

610 GO TO 640

620MATXr(L9)*U

630 MAT Y=(L9)*V

640 PRINT "LIFE CYCLE £NE","HGY COSTS FOR", "SINGLE GLAZING"

650 IF C0=0 THEN 680

660 MAT X=(FO)*X

670 MAT Y=(FO)»Y

680 MAT PRINT X 690 IF C0=0 THEN 720

700 MAT U=(F0)*U

710 MAT V=(F0)*V

720 PRINT "LIFE CYCLE ENE",''RGY COSTS FOR "."DOUBLE GLAZING"

730 MAT PRINT Y 740 IF T0=0 THEN 960

7?0 MAT READ R

7 60 MAT READ S 770 MAT R=(C1)»R

780 MAT S=(C2)*S

790 MAT W=F+S

800 MAT W=W+T

810 MAT T=W

820 MAT W=TPN(T)

830 IF F1=D1 THEN 860

840 MAT Zr(A1)*W

850 GO TO 870

860 MAT Z=(L9)*W

870 PRINT "LIFE CYCLE ENE","RGY COSTS F 0 R " . T R I P L E GLAZING"

880 IF C0=0 THEN 910

890 HAT w=(F0)*W

900MATZr(F0)»Z

910MATPRINTZ

920 FOR 1=1 TO 5 930 READ 0(1)

940 NEXT I 950 REM

96c REM - life cycle hardware costs

970 REM

980 FOR 1=1 TO 5 990 READ A(I),B(I),D(I),E(I)

1000 IF C0=0 THEN 1020

10 10 LET D( I ) =FC*D( I ) 1020 NEXT I 1030 MAT READ C 1040 IF C0=0 THEN 1060

1050 MAT C=(FO)*C

1060 LET D2=( 1/D1 )*( 1-( 1/( 1+D1 ) )'L9)

1070 IF C0=0 THEN 1170

1080 LET EC=0

1090 LET E9=1

1100 FOR K=1 TO L9

1110 LET E8= ( 1 . 5/L9 ) *E9

1120 LET E9=E9-E8

1 1 30 LET E1 = E8/ ( 1+D1 ) "K

1140LETE0 = E1*F0-t-E0

1150 NEXT K 1160 LET E0= 1 -EO

1170 LET Q=L9/5

98
1180 MAT F=(D2)*C

1190 FOR 1=1 TO 5 1200 LET D3=0

1210 LET K=1

1220 LET M=5*K

1230 LET D3=D3+( 1/(1+D1 )"M)

1240 LET K=K+1

1250 IF K<rQ THEN 1220

•1260 LET G(I) = (D3)*D(I)

1270 FOR J=1 TO 3 1280 LET H(I,J)=F(I,J)+G(I)

1290 NEXT J

1 300 IF COrO THEN UIO

1310 LET R( 1 ,I) = A(I)

1320 LET R( 2 , I) =B( I

)

1330 LET R(3,I)=E(I)

13^0 IF T0=0 THEN 1360

1350 LET R ( 4 , I ) =0( I

)

1360 LET A(I)=EO«R( 1,1)

1370 LET E(I)=E0*K(2,I)

1380 LET E( I ) =EO»R( 3,1)

1 390 IF T0 = 0 THEN 1410

1400 LET 0( I ) =EO*R ( 4,1)

1410 LET K( I ) =B( I )+E( I )+H( I , 2

)

1420 LET J( I) =A( I )+E( I)+H( I , 1

)

1430 IF T0=0 THEN 1450

1440 LET P(I)=0(I)+E(I)+H(I,3)

1450 NEXT I 1460 PRINT "LIFE C Y C L E " , H A h D W A h E COSTS"'

1470 IF T0=0 THEN 1610

1480 PRINT "SINGLE" , "DOUBLE" , "TRIPLE" , "ACCESSORIES"

1490 FOR 1=1 TO 5 1500 PRINT A(I) ,B(I) ,0(1) ,E(I)

1510 NEXT I 1520 PRINT "MAINTENANCE" ," COSTS"

1530 PRINT "SINGLE" , "DOUBLE" , "TRIPLE"

154 0 MAT PRINT H

1550 FOR 1=1 TO 5 1560 FOR J=1 TO 5 1570 LET Z(I,J) = Z(I, J)+P(I)

1580 NEXT J 1590 NEXT I 1600 GO TO 1650

1610 PRINT "SINGLE' ."DOUBLE" . "ACCESSORIES" , "MAINT SING", MAINT DOUB

1620 FOR 1=1 TO 5 1630 PRINT A ( I ) , B( I ) , E( I ) , H ( I , 1 ) , H ( I , 2 ) 1640 NEXT I 1650 FOR 1=1 TO 5 1660 FOR J=1 TO 5 1670 LET X(I, J)=X(I , J)+J(I)

1680 LET Y ( I , J ) =y ( I , J ) + K(I

)

1690 NEXT J 1700 NEXT I 1710 PRINT

99
1720

1730

1740

1750

1760

1770

1780

1790

1 SOO

1810

1820

1830

18 4 0 1850

1860

1870

1880

1890

1900

1910

1920

1930

1940

1950

I960

1970

1980

1990

2000

2010

2020

2030

2040

2050

2060

2070

2080

2090

2 100

2110

2 120

2130

2140

2150

2160

2170

2180

2190

2200

2210

2220

2230

2240

2250

PRINT "TOTAL

MAT PRINT X PRINT "TOTAL

HAT PRINT Y IF T0=0 THEN

PRINT TOTAL

MAT PRINT Z GO TO 1950

LET Q( J) =Z(

1

FOR L=1 TO 5 IF Q(J)<Z(L,J) THEN

LET Q( J ) =Z(L . J)

LET S4=L

NEXT L REM

COSTS" ." SINGLE GLAZING"

COSTS" , "DOUBLE GLAZING"

1950

COSTS"

J)

"TRIPLE GLAZING"

1870

REM - minimization of life cycle costs

REM

GO TO 2200

IF M(J)<Q(J) THEN 2280

LET S=S4

LET Z=3

LET N(J)=Q(J)

GO TO 2310

PRINT "MINIMIZATION" , "OF LIFE CY CLE COSTS

REM

REM

REM

REM

REM

REM

REM

REM

REM

FOR J =

- identify least cost window system for each orientation

- j correspondes to the orientation as follows:

j

= 1 implies south

j

= 2 im. plies southeast/southwest

j = 3 implies east/west

j

= 4 implies nor t hea s t / no r t h we s

t

j = 5 implies north

LET Z = LET L(

FOR 1 = IF L(J

LET L(

LET SI

NEXT I LET M(

FOR K = IF M ( J LET M(

LET S2

NEXT K IF T0 = IF M(J

IF T0 = IF Q(J

LET N(

LET SLET Z =

1 TO 5 0J) =X( 1 , J )

1 TO 5 )<X( I , J ) THEN 2 120

J) =X( I , J)

= I

J)=Y( 1 , J)

1 TO 5 X Y ( K , J ) THEN 2 180

J ) = Y ( K , J ) = K

1 THEN 1800

)<=L(J) THEN 2270

0 THEN 2230

)<=L(J) THEN 1910

J)=L( J)

SI

1

100
2260 GO TO 2310

2270 IF T0=1 THEN 1900

2280 LET N(J)=M(J)

2290 LET S=S2

2300 LET Z=2

2310 LET I( J)=X( 1 , J)-N( J)

2320 IF S=1 THEN 2370

2330 IF S=2 THEN 2390

2340 IF S=3 THEN 2^110

2350 IF S = i4 THEN 2430

2350 GO TO 2450

2370 LET S=0

2380 GO TO 2460

2390 LET S=12

2400 GO TO 2460

2410 LET S=30

2420 GO TO 2460

2430 LET S=60

2440 GO TO 2460

2450 LET S=90

2460 IF Z=2 THEN 2500

2470 IF Z=3 THEN 2520

2480 PRINT "ORIENTATION" , J

, "AREA IN SQ FT" , S SINGLE GLAZING"

2490 GO TO 2530

2500 PRINT "ORIENTATION" , J , ''AREA IN SQ FT S DOUBLE GLAZING"

2510 GO TO 2530

2520 PRINT "ORIENTATION" , J

, "AREA IN SQ FT " , S, " T R I PL E GLAZING"

2530 PRINT "TOTAL COST " , N ( J ) , " TOT AL SAVINGS" , I ( J ) 2540 IF C0=0 THEN 2620

255 0 FOR 1=1 TO 5

2560 LET A(I)=R( 1 ,1)

2570 LET B(I)=R(2,I)

2580 LET E(I)=R(3,I)

2590 IF T0=0 THEN 2610

2600 LET 0( I ) =R( 4 , 1

)

2510 NEXT I 2620 IF S=0 THEN 3850

2530 LET E9=1

2640 LET E2=0

2650 LET E0=0

2650 IF Z=1 THEN 2710

2670 LET X1=B(S2)+E(S2)

2680 LET H( J,Z)=V(S2, J)

2690 LET S3=S2

2700 GO TO 2750

2710 IF Z=3 THEN 2760

2720 LET X1rA(S1 )+E(S1 ) 2730 LET H( J ,Z)=U(S1 , J)

2740 LET S3=S1

2750 GO TO 2790

2750 LET X1=0(S4)+E(S4)

101
2770 LET H(J,Z) = W(Si<,J)

2780 LET S3=S4

2790 LET D9=0

2800 LET X4=A(1)

2810 REM

2820 REM - calculate payback period on least cost window system

2830 REM

28'JO LET P = 5 2850 LET M=1

2860 LET L=1

2870 IF D1=F1 THEN 3100

2880 LET D4=((1+F1)/(D1-F1))»(1-((1+F1)/(1+D1))'M)

2890 LET D6=( 1/( 1+D1 ) )«( 1-( 1/( 1+D1 ) )'M)

2900 IF C0=0 THEN 3010

2910 LET X0=D4*U( 1 , J)

2920 LET X5 = XJ4 2930 LET X9=X1

2940 LET E8 = ( 1 . 5/L9 )*E9

2950 LET E9=E9-Ee

2960 LET El =E8/ ( 1+D1 ) "M

2970LETE2=E1*F0+E2

2980 LET E0=1-E2

299 0 LET X1=X1*E0

3000 LET X4=X4*E0

3010 LET X2=X1+D4»H( J,Z)+D6*C(S3,Z)

3020 LET X0 = D'4*U( 1 , J)+X4

3030 IF M=P THEN 31^0

3040 LET X3=X2+D9*D(S3)

3050 IF X3<=X0 THEN 3170

3060 LET Mr M+1

3070 LET L=L+1

3080 IF L>L9 THEN 3120

3090 GO TO 2870

3100 LET D4=M

3110 GO TO 2890

3120 PRINT 'PAYBACK 0VER",L9

3130 GO TO 3420

3140 LET D9 = D9 + 1 / ( 1 +D1

) "

P

2150 LET P=P+5

3160 GO TO 3040

3170 IF H=P THEN 3350

3180 LET Y=M-1

3190 IF C0=0 THEN 3210

3200 LET E0=E0+E1*F0

3210 LET N=1

3220 IF D1=F1 THEN 3380

3230 LET D5=( ( 1+F1 )/(D1-F1 ) )»( 1-( ( 1+F1 )/( 1+D1 ) )"¥)

3240 LET D6= ( 1 / ( 1 + D 1 )

) * ( 1 - ( 1 / ( 1 +D 1 ) ) " Y ) 3250 IF C0=0 THEN 3280

3260 LET X1=EC»X9

3270 LET X4=E0*X5

3280 LET X3 = X1 +D5*H( J , Z ) +D9*D( S3 ) +D6» C ( S3 , Z

)

3290 LET X0 = D5*U( 1 , J )+X4

3300 IF X3<=X0 THEN 3410

3310 LET Y=Y+0.1

3 3 20 LET N=N+1

3330 IF N>10 THEN 3410

3340 GO TO 3220

102
3350 IF X3=X0 THEN 3^00

3360 LET D9=D9-1/( 1+D1 )"P

3370 GO TO 3180

3380 LET D5=Y

3390 GO TO 32i)0

3400 LET Y=M

3410 PRINT "YRS TO PAYBACK ",Y

2420 REM

3430 REM - calculate the minimum rate of fuel price escalation at

3440 REM - which least cost window system can still pay for itself

3450 REM - duing the study period,

3460 REM

3470 IF510

3480 LET G(J)=K(S2)

3490 LET H(J,1)=V(S2,J)

3500 GO TO 3570

3510 IF Z=3 THEN 3550

3520 LET G( J)=J(S1

)

3530 LET H(J,1)=U(S,J)

3540 GO TO 3570

3550 LET G(J)=P(S4)

3560 LET H( J , 1

) =W( S4 , J

)

3570 LET F2=0

3580 IF F2=D1 THEN 3660

3590 LET D7=( ( 1+F2)/(D1-F2) )•( 1-{ ( 1+F2)/( 1+D1 ) )"L9)

3600 LET B0=(D7)*U( 1 , J)+J( 1 ) 3610 LET B1=G( J)+(D7)*H( J, 1

)

3620 IF B1<=B0 THEN 3680

3630 LET F2=F2+0.01

3640 IF F2>F1 THEN 3840

3650 GO TO 3580

3660 LET D7=L9

3670 GO TO 3600

3680 LET F2=F2-0.01

3690 LET P=1

3700 IF F2=D1 THEN 3790

3710 LET D8=( ( 1+F2)/(D1-F2) )*( 1-( ( 1+F2)/( 1+D1 ) )"L9)

3720 LET B0=(D8)«U( 1

, J)+J( 1

)

3730 LET B1=G( J)+(D8)*H( J , 1

)

3740 IF B1<=B0 THEN 38IO

3750 LET F2=F2+0.001

3760 LET P=P+1

3770 IF P>10 THEN 38IO

3780 GO TO 3700

3790 LET D8=L9

3800 GO TO 3720

3810 LET F2=F2»100

3820 PRINT "PER CENT FPE",F2

3830 GO TO 386O

3840 LET F2=F1

3850 GO TO 3820

3860 NEXT J 3870 REM

3880 REM - repeat for next orientation (j)

3890 REM

3900 REM data section

3910 REM - itemize inputs as follows:

3920 REM - input data for cost per 100,000 btu delivered to the

3930 REM - condition space for heating, for cooling, and the cost

3940 REM - per kilowatt hour of electricity as line 3950.

3950 REM data -- , --

,

1 03
2950

3970

3980

3990

4000

M010

i4020

4030

4040

4050

4060

4070

4080

4090

4100

4110

4 120

4 1 30

4 140

4150

4160

4170

4180

4 190

4200

4210

4220

"230

4240

4250

4260

4270

4280

4290

4300

4310

4320

4330

4340

4350

4360

4370

4 380

4390

4400

4410

4420

4430

4440

4450

4460

4470

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

REM

- input data for length of study period in years as line 3970 - data --data for heating loads with single glazing

- for each orientation input the number of therms of heating-

- energy for each of five window sizes. begin each line

- with the number of therms associated with a windowless wall

- and orientation of: line 4040 for south; 4050 southeast/southwest

- 4060 east/west; 4070 northwest/northeast; 4080 north.

data - data [ data - data - data - data for cooling loads with single glazing

orientation key is the same as that used for heating

using lines 4140 4150, 4160, 4170, and 4180 for the

corresponding orientation: south, southeast/southwest,

east/west, northeast/northwest, and north,

data - data - data - data - data - data for illumination requirements in kilowatt hours -

- (note - this data is assumed the same for double and triple -

- glazing also) for each orientation input the number of

- kilowatt hours for lighting for each of five window sizes-

- if daylight is not used as a light source, this figure

- should remain constant for all window sizes. begin

- with the number of kilowatt hours associated with a - windowless wall. orientation key is same order as before-

- using lines 4280, 4290, 4300, 4310, and 4320.

- data -

- data -

- data -

- data -

- data -

- data for heating loads with double glazing

- input data will follow the same format and the same key

- as that of single glazing. use lines 4370, 4380, 4390

- 4400, and 4410 for data.

- data -

- data -

- data -

- data -

- data -

- data for cooling loads with double glazing

- input data will follow the same format and the same key

- as that of single glazing. use lines 4460, 4470, 4480,

- 4490, and 4500.

- data --, --, --

- data --, --, -- , --, --

104
ji Ji P rv n c f . — H ;a f a

4 ^4 y U

D r ?wi n fc ri -

/I R n n J u u nun R R M _•

llH

c; 1 n J 1 U

R F M _ dsta for heating loads with triple glazing

il c; 5 0 H D b nR C/F nM — inpuL aaca wiii loixow trie same lormat. and the same key -

a n RET. - Xiri*ro ^ J J \J

i "

4 5 4 0 REM - 4^60 4S70 4'^80 and 4SQn if nppdprl

4^ SJ SJ 0\j REM - 4560 REM - 4570 REM - 4580 REM - data -- . --, --, --, --

4^ SJ Qy 0\J R E M - data -- , --, -- , --, -- 4600 REM - data for cooling loads with triple glazing - if used

4 510 REM - input data will follow the same format and the same key

ii fi ? n REM - as that of single and double glazing. use lines 4640,

H U J U R F M - 4650, 4660, 4670, and 4680 if needed.

4H 6U 4*T 0U R F M - data --, --, --, --, --

4 65 0 REM - data --, -- , --, --, --

4 6 6 0 REM - data -- , — , -- , - -

, --

4 6 7 0 null R F M - data -- , -- , -- , , --

4T 6u u nu nR iLF 1M -

1

data -- , --, --, --, --

ij A Q n M D y uR F Ivl input the cost of single glazing, cost of double glazing -

ii 7 n n R F M five year painting cost, cost of window accessories as a -

ii 7 1 n nR Fc, riM — function of window size. statement 4750 should correspond

ii 7 ? n R F M _ to the sm. allest window size, statement 4790 should

ii 7 n R F correspond to the largest window size exam ined and 4760 -

ii 7 ii n R F M — through 4770 represent intermediate sizes.

il 7 R fl R F M - data -- , -- , -- , --

ii 7 fi n R F M - data — , — , — , —

7 7 n Rn CjR 1Mi - data — , — , — , —

ij 7 ft n RXl CjR 1M1 -

4 7 Q 0 REM - the first column of data in statements 4890 through

iit RU nVJ 0u Rn CjR M1 -

1

statement 4930 corresponds to the annual cl eaning and

iiT RU 11 nU nR PCj M1 -

J

insurance costs of a single glazed window. statement 4890-

4820 REM - is the smallest window size, statement 4930 is the largest

4830 REM - window size examined. the second column of state m. ents

4840 REM - 4890 through 4930 corresponds to the annua 1 cleaning and4850 REM - insurance costs of a double glazed window. the third

4860 REM - column of statements 4690 through 4930 corr espondS to

4870 REM - the annual cleaning and insurance costs of a triple glazed

4880 REM - window

.

4890 REM - 4900 REM - data -- 4910 REM - data , --

,

4920 REM - 4930 REM - 4940 END

105
NBS-114A tREV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS BSS 1192. (aov't. Accession No. 3. Rectptent' * Accession No

4. TITLE AND SUBTITLE

Economic Ev^iluation of Windows in Buildings: Me thodology

5. Publication Date April 1979

6. Performing Organization Code

7. AUTHOR(S)

Rosalie T. Ruegg and Robert E. Chapman

8. Performin g Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

18. Project/Task/WorK linit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rS(ree(, City. St^te, ZIP) 13. Type of Report & Period Covered

Final

14. ^jonsofing Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 79-600042

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of most sigm ficant information. If document includes a significant bibliography or

literature survey, mention it here.) This study, which is one part of a National Bureau of Standards interdisciplinary project on windows, is aimed at improving the cost-effectiveness of

window selection and use in buildings. It develops and illustrates a life-cycle

costing evaluation model and computer program for assessing for alternative window

systems the net dollar impact of acquisition, maintenance and repair, heating and

cooling energy gains and losses, and artificial lighting and daylighting tradeoffs. The method is applicable to the evaluation of many different window sizes,

designs, accessories, and tises, both for new and existing residential and commercial

buildings. Two step-by-step examples of evaluating selected window alternatives

in a residence and in an office building in Washington, D.C. serve to illustrate the

application of the method.

A companion report, A Regional Econom ic Assessment of Selected Window Systems , presents the results of eight additional residential case studies and eight

additional commercial case studies. While the emphasis of this report is on the

method of evaluation, the companion report focuses on summarizing the results of a regional analysis in a form that will be convenient for use by building owners,

operators, designers, financiers, and builders, those whose interest centers on

the actual implementation of research results.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Building economics; daylighting; economic analysis; energy conservation; engineering

economics; life-cycle costs; solar heat gain; thermal efficiency; window; window

management

18. AVAILABILITY [X^Unlimited

1

1 For Official Distribution. Do Not Release to NTIS

pXt Order From Sup. of Doc. U.S. Government Printing Office, Washington, DC

LJ 20402, SD Stock No. SN003-003-02042-7

1 1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

118

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$3.00

USCOMM-DC

ir U.S. GOVERNMENT PRINTING OFFICE : 1979 O—289-260
For a review copy, write Journal of

Researcii, National Bureau of Standards,

U.S. DEPARTMENT OF COMMERCE

Washington, D.C. 20234

Subscribe now -

The new

HationalBniean

of Standaids

Jonnial

The expanded Journal of Research of the National

Bureau of Standards reports NBS research and

development in those disciplines of the physical

and engineering sciences in which the Bureau is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers

cover a broad range of subjects, with major

emphasis on measurement methodology, and the

basic technology underlying standardization. Also

included from time to time are survey articles on

topics closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete citations

to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single

copy, $3.00 domestic; $3.75 foreign.

• Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Sectioi. B "Mathematical Sciences."

NBS Board of Editors

Churchill Eisenhart,

Executive Editor (Mathematics)

John W. Cooper (Physics)

Donald D. Wagman (Chemistry)

Andrew J. Fowell (Engineering)

Joseph 0. Harrison (Computer Science) Helmut W. Hellwig (Boulder Labs.)

Subscription Order Form

I Enter my subscription to NBS Journal of Research

j at $17.00. Add $4.25 for foreign mailing. No additional postage is

j required for mailing within the United States or its possessions.

I (SJR—File Code 2Q)

Send Subscription to: Name-First, Last

I I I I I I I I I I I I III I I I I I I I I I I I I

I Company Name or Additional Address Line

Street Address

i II 1 1 1 1 1 1 1 1 1 1 II

City

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

State

1

Zip Code

1 1 1 1

MAIL ORDER FORM TO:

Superintendent of Documents

Government Printing Office Washington, D.C. 20402

Remittance Enclosed (Make checks payable

to Superintendent of Documents)

Charge to my Deposit Account No.

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research

of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and

engineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and

computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology, and

the basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely

related to the Bureau's technical and scientific programs. As

a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

$3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." DIMENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and consumers of the latest advances in science and technology,

with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $1 1. 00; Foreign $13.75

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers,

chemists, biologists, mathematicians, computer programmers,

and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference Data (JPCRD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest

to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR s and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau: Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D,C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, S300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

SPECIAL FOURTH-CLASS RATE BOOK