NAT'L INST. OF STAND & TECH

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NBSIR 77-1388

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PUBLICATIONS

REFERENCE

A NEW LOOK AT WINDOWS

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Belinda L Collins

Rosalie T. Ruegg

Robert Chapman

Tamami Kusuda

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

JANUARY 1978

Center for Building Technology

Institute for Applied Technology

Washington, D.C. 20234

NBSIR 77-1388

A NEW LOOK AT WINDOWS

Belinda L Collins

Rosalie T. Ruegg

Robert Chapman

Tamami Kusuda

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

January 1978

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

1.0 Introduction 1

2.0 Key Factors Affecting Window Thermal Performance 3

2.1 Size 3

2.2 Orlentatlon/Shadlng 4

2.3 Thermal Resistance and Air Leakage 6

2.4 Management of Interior Window Coverings 7

2.5 Daylight Ing 8

3.0 Case Example 8

3.1 Approach 9

3.2 Energy Costs 12

3.2.1 First Level of Analysis - Thermal Loads 12

3.2.2 Second Level of Analysis - Management of V71ndows 15

3.2.3 Third Level of Analysis - Dayllghtlng and

Management 16

3.3 Acquisition Costs 20

3.4 Maintenance and Repair Costs 20

3.5 Life-Cycle Costs 24

4.0 Conclusions 30

References ' 32

111

LIST OF FIGURES

Page

Fig. 1. Effect of Changing Sun Position on the Shading of

South-Facing Windows 5

Fig. 2. Schenatic Diagram of House with Residential Module

Shown in Shaded Areas 10

Fig. 3. Estimated Yearly Energy Costs with North (N) or South

(S) Facing Window with Gas or Electric (Elec) Heat:

Analysis of Internal and External Thermal Loads Only 14

Fig. 4. Estimated Yearly Costs with North (N) or South (S)

Facing Window with Gas or Electric (Elec) Heat:

Analysis of Internal and External Loads, Window

Management and Nighttime Thermostat Adjustment 17

Fig. 5. Estimated Yearly Energy Costs with North (N) or South

(S) Facing Window with Gas or Electric (Elec) Heat:

Analysis of Internal and External Loads, Management,

Adjustment, and Daylight 19

Fig. 6. Savings or Losses Due to a Single-Glazed, South-Facing

Window over 25 Years for a Residential Module with Gas

Heating and Electric Cooling - Washington, D.C. Case

Example 25

Fig. 7. Savings or Losses Due to a Double-Glazed^ South-Facing

Window over 25 Years for a Residential Module with

Gas Heating and Electric Cooling - Washington, D.C.

Case Example 26

Fig. 8. Savings or Losses Due to a Single-Glazed, North-Facing

Window over 25 Years for a Residential Module with Gas

Heating and Electric Cooling - Washington, D.C. Case

Example 27

Fig. 9. Savings or Losses Due to a Double-Glazed, North-Facing

Window over 25 Years for a Residential Module with

Gas Heating and Electric Cooling - Washington, D.C.

Case Example 28

V
I.IST OF TABLES

Page

1. Assumptions for Case Example 11

2. Acquisition Costs of a Window in Excess of the Cost of a Non-Windowed Wall 21

3. Cost of Window Accessories 22

4. Maintenance and Repair Costs 23

VI
ABSTRACT

Recent design recommendations have called for reduced window area

In buildings to conserve energy. This report presents new information on thermal loads, daylighting, management, and life-cycle

costs which indicates that such recommendations may neglect important design and operational aspects of windows which can conserve

energy resources and reduce life-cycle building costs. A case example is described in which energy consumption and life-cycle

costs are given for windows in a typical house in the Washington,

D.C. area. Npticeable reductions in overall energy consumption and

life-cycle costs are found if daylight is used, and if the window

is managed. It is suggested that builders and lending institutions

consider the long-term effects of window design and operation

decisions.

Key Words: Daylighting; energy conservation; life-cycle costs;

residential; solar heat gain; window; window management

VII
VIII
A NEW LOOK AT WINDOWS

1 . 0 INTRODUCTION

With rising fuel prices driving homeowners' utility bills to ever

higher levels, builders, homeowners, and mortgage lenders are giving

greater attention to overall monthly housing costs. Unexpectedly large

utility bills can hurt the family budget and jeopardize repayment of

the mortgage loan.

A considerable amount of energy waste in buildings has been

attributed to windows. Their thermal resistance is typically lower

than that of the surrounding wall; they leak air; and they let in

the hot summer sun. Simple calculations of the thermal gains and

losses attributed to windows have led to recommendations that window

area be reduced drastically or that windows be eliminated where possible.

Such recommendations neglect, however, the many possible benefits

that can be provided by windows — benefits not only of view and

aesthetics — but also, potentially, of energy conservation. When

windows are utilized as passive solar collectors, light sources, and

ventilators, a different picture of their impact upon energy conservation emerges.

Depending upon critical decisions about design and use, windows

can either greatly increase, decrease, or have little impact on energy

consumption and total life-time building costs. The range of impacts is

illustrated by the following highlights of a case study which measures

the energy consequences associated with windows of different design,

orientation, and use in a "typical" Washington, D.C. home.

1
° Poorly designed and operated windows often account for as much

as 25 percent of typical energy use.

° Yearly energy costs can skyrocket as window sizes increase

if the windows are poorly designed and operated.

° When carefully located and operated, windows — even large

windows — can be energy efficient.

° For a given level of annual energy use, a south-facing window

can be almost twice as large as a north-facing window.

° Double glazing will reduce energy costs, but may not always

be cost effective.

° Well designed windows and good operating practices can result

in net savings to homeowners over the life of their house.

This article is aimed at helping the building community and

mortgage lenders determine the likely energy effects of windows in new

housing and thereby encourage the choice of cost-effective windows. By

choosing energy-efficient and cost-effective window designs, the

building community can reduce the impact of rising utility costs while

serving the national interest for conserving nonrenewable energy

resources.

The information in this article is drawn from an investigation of

window performance in residences and in office buildings at the National

Bureau of Standards (NBS) . The NBS research team first identified

specific window design strategies with the potential for saving energy (1)

.

A computer model was then developed to estimate the impact of selected

window strategies on energy conservation (2). Life-cycle costing

techniques were used to account for the costs of acquisition,

2
maintenance, repair, and energy, to determine the overall costs of

alternative window designs over the life of the building (3). The

benefits of windows to people in buildings (in terms of view and

aesthetics) were also investigated (4).

The NBS research effort was sponsored jointly by NBS, the Energy

Research and Development Administration (ERDA) , and the Department of

Housing and Urban Development (HUD) . The study was undertaken in

response to the needs of the building community for reliable information

on window performance. It is a step in the process of formulating new

building standards and design recommendations for windows.

Some of the key factors affecting the thermal performance of

windows are first described briefly. Then the energy and cost impacts

of particular window designs in a typical residence in the Washington,

D.C. area are shown.

2.0 KEY FACTORS AFFECTING WINDOW THERMAL PERFORMANCE

In addition to climate, there are a number of basic design and

operational factors which are critical to a window's performance.

These include: size, orientation and exterior shading, thermal resistance, air leakage, use of internal and external window coverings, and

use of daylight.

2.1 SIZE

Decisions about the size of a window can often pose a dilemma for

designers, builders and owners. Calculations of the increase in thermal

loads associated with increasing window area have led to recommendations

that window area be as small as possible, often as small as 5 to 8

percent of the window wall. Calculations of only the thermal loads may
may not consider potential benefits which also tend to increase as

window size increases. These benefits include usable solar collection

during the heating season, ventilation during the cooling season, and

daylight. In addition, thermal calculations do not always include the

potential for reducing negative thermal effects by shading and insulating the window area. Furtheirmore, the varied thermal effects associated

with different window orientations and glazing types are often overlooked. Thus, to determine the optimal window size for conserving

energy, both negative and positive thermal effects must be considered.

2 . 2 ORIENTATION/ SHADING

A window's orientation plays a critical role in determining the

balance between desirable and undesirable thermal effects. In moderate

to cold climate zones, the annual energy costs of south-facing windows

are generally lower than those for northern, eastern or western

exposures. Their energy costs are reduced because the elevation of the

sun is lowest during the winter so that south-facing windows receive

more direct sunlight. During the summer, the elevation of the sun is

higher, so that south-facing windows receive less direct sunlight.

Furthermore, as Figure 1 indicates, the changing elevation of the sun

permits permanent overhangs to be installed which shade south-facing

windows during the summer, but not during the winter.

4
SUMMER WINTER

Figure 1- Effect of Changing Sun Position on the

Shading of South-Facing Windows

On the other hand, because windows oriented in a northerly direction receive little solar heat gain, their net heat loss is greater

than that of other exposures during the winter. This heat loss is

sometimes increased by prevailing northerly winds which further decrease

a window's thermal resistance.

Windows with either eastern or western exposures also have considerable winter heat loss. In addition, they have greater heat gains in

the summer because they are difficult to shade against the lower summer

sun. Thus, the energy consumption due to east or west-facing windows

is often greater than that for south-facing windows.

Given a fixed, but undesirable, window orientation, unwanted solar

heat gains can be reduced by the use of shading devices which block the

direct rays of the sun. Shading devices are most effective if they are

located outside of the window, because they will block the sun's heat

before it reaches the interior of the house. Roof overhangs, awnings.
sun screens, vegetation, and operable shutters are examples of effective

exterior shading devices.

Orientation influences the type of shading device selected. For a

southern exposure, horizontal devices are most effective because the

sun is high in the sky during the summer. For eastern or western exposures, vertical fins or lattice-type devices are most effective because

the sun is lower in the sky and moves diagonally across the window.

2.3 THERMAL RESISTANCE AND AIR LEAKAGE

Probably the most common method for improving the thermal performance of windows is to increase their resistance to heat loss. Thermal

resistance may be increased either by adding storm windows, plastic

sheets, draperies, shutters, or by installing special glazing products

such as sealed insulating glass. Storm windows are now marketed as

permanent, operable fixtures, which can either be installed on new

houses during construction or added to existing houses. Sealed insulating glass, on the other hand, is used mainly in new residential construction. In addition to reducing heat loss, storm windows or

insulating glass can reduce water vapor condensation, radiant heat loss,

and air leakage.

The cost effectiveness of double glazing depends largely on the

severity of the climate, the cost and availability of heating fuel, and

the size and orientation of the window. For example, in a moderate

climate, double glazing usually will reduce energy costs only slightly

for south-facing windows, and may, therefore, not be cost effective.

On the other hand, double glazing is usually an effective treatment for

north-facing windows (in a moderate climate) because it reduces energy

6
costs substantially. Yet, in a severe climate, double or triple glazing

may be cost effective for all orientations.

Upgrading the thermal resistance of a window can save significant

amounts of energy through reduced conductance, but energy can still be

wasted through air leakage (infiltration) around the window. Infiltration may occur around the operable portions of the window or between the

window frame and the wall. Air leakage increases with the external wind

speed and the inside-outside temperature difference. These losses can

be reduced by the installation of weatherstripping and good latching

hardware or by caulking and insulating the frame-wall gap. Caulking and

weatherstripping are particularly cost effective since they are relatively inexpensive, entail low labor cost, and usually provide a

noticeable reduction in heating costs.

2.4 MANAGEMENT OF INTERIOR WINDOW COVERINGS

Although double glazing and fixed shading can reduce winter heating

and summer cooling costs substantially, they are not readily adaptable

to changing environmental conditions. Window performance can be improved

further by the selective adjustment of shading devices and thermal

barriers — hereafter referred to as "window management". For example,

Venetian blinds, draperies, or shutters may be closed by the homeowner

to block solar radiation during the siimmer, but opened to admit solar

heat during the winter. Thermal barriers may also be used to reduce

the heat flow from the house on cold winter nights. At the same time,

these window coverings can be used by the homeowner to provide additional benefits unrelated to energy use — e.g., to control glare,

reduce outside distractions, provide greater privacy, and to complement
the Interior decor of the home.

2.5 DAYLIGHTING

One of the best means for improving the performance of windows is

to use them to provide natural light instead of electric light during

the day. Most private residences in the United States are lighted by

incandescent lights, a relatively inefficient light source which generates a considerable amount of heat. The substitution of daylight for

incandescent lighting will not only reduce the cooling requirements

within the room but will also cut down on the amount of electricity used

for lighting. Potential savings from daylight may not be as great in

the winter because the heat from the electric lights can help to heat

the room. Nevertheless, because of the potential for saving energy by

using daylight, it may not be energy efficient to exclude a window from

a room which will receive substantial use during the day.

A moderately-sized window can provide adequate daylight up to about

10 to 15 feet into the room under all but the cloudiest conditions (2)

.

At distances beyond this, supplementary task lighting is generally

required. Careful consideration of both window location and size can

improve the effectiveness of daylighting as the primary light source.

For instance, tall vertical windows permit daylight to penetrate a

greater distance into a room than narrow horizontal windows. Similarly,

windows placed high on the wall permit more daylight to enter. Skylights

can also be effective for daylighting (5).

3.0 CASE EXAMPLE

Now let us examine in dollar terms the impact that each of these

factors can have upon yearly energy costs and total life-cycle costs.

8
We will concentrate upon one example in a single location to illustrate

the range of possibilities for altering the energy balance at the window.

This case study is intended to show, for a typical residence in the

Washington, D.C. area, how various window sizes, orientations, thermostat adjustment, daylight, and window management affect not only annual

energy costs but also lifetime building ownership costs. The costs of

purchase, installation, maintenance, and repair are taken into account,

as well as energy costs. Our focus is on the window of an 18' x 15'

room with one exterior wall in a typical brick veneer house. The room

could be envisioned as the family room-kitchen of the house given in

Figure 2. Additional assumptions are presented in Table 1.

3.1 APPROACH

In the case example, the yearly and lifetime energy costs associated with changes in size, orientation, thermal resistance, window

management, and daylighting are calculated first. Next, the costs of

purchase, installation, maintenance, and repair are added to the energy

costs in a life-cycle cost analysis.

The life-cycle cost analysis includes all the relevant costs or

savings, past, present, and future, and takes into account changing

prices due both to inflation and to changing supply and demand factors.

It also incorporates the concept of the "time value of money," i.e.,

a dollar today is worth more than a dollar in the future, apart from

inflation. The life-cycle cost model enables different window designs

to be considered in terms of their net impact on building and energy

costs over a number of years.

9

TABLE 1. ASSUMPTIONS FOR CASE EXAMPLE^

Rn 1 1 H'fTiO' ^npp i "F ^ pa t* "I on q A QQiiTnn'h "f onQ

I. DiniensT_orm of ModiiT&

Type of Construction

Exterior Wall Area

Window Size

Window Construction

18 ' wide X 15 ' lone x 8 ' hich

Block with brick veneer; u - 0.07; 3 1/2" insul.

144 sq. ft.

0, 12, 18, 30, 60 sq. ft.

Wood; double hung; weather stripped

Internal Loads Snecification*?

Lights

Equipment

Air Leakage

Occupancy

Btu/person

0.65 watts/sq. ft.

0.52 watts/sq. ft.

0.5 air changes /hour

0.5 persons

260 Btu/hour

System Efficiency Specif ications

Boiler Efficiency

Cooling CO .P.

Electric Heating

0.65

2.0

1.0

Fuel Type Costs

Electricity

Gas

30 per KwH

30c 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.

bDue to the difficulty of modeling the thermal exchange between rooms,

only a single room was modeled. Study of a single room within a larger

house may not necessarily reflect the performance of the whole house.

All loads are averaged over the 16 hour period from 7:00 AM to 11:00 PM.

11
Each of the cost factors will be examined first separately. Then

they will be combined into an overall life-cycle cost analysis.

3.2 ENERGY COSTS

A thermal model with three levels of analysis was developed to

examine the effects on energy costs of varying window size, orientation,

thermal resistance, management, and daylight utilization. The three

levels of thermal analysis are presented here for north and south-facing

windows of several sizes, and with single and double glazing.

In the first level of analysis, energy costs associated with a bare

window are calculated, taking into account only the thermal loads. In

the second, the calculation of energy costs includes thermal loads, but

also allows for modifications in thermal loads by the selective use of

thermostat adjustment and internal window coverings for shading and

insulation ("window management"). In the third level, the computation

of energy costs adds daylight utilization to window management and

thermostat adjustment. Although the three levels of analysis simulate

most of the important energy effects of windows, they are not fully comprehensive because they omit the potential savings or losses from

natural ventilation. '

For the three levels of analysis, energy costs are estimated for

both gas and electric heating and for electric cooling. Energy costs

also include the cost of electricity to operate the household equipment

and interior lighting.

3.2.1 First Level of Analysis - Thermal Loads ; First, we will consider

how windows affect estimated yearly energy costs when only internal and

external thermal loads are taken into account and the bare window is

12
considered. This Is the traditional approach taken in analyzing the

influence of windows on energy consumption. It is the appropriate

approach when neither daylight ing nor management is feasible. In this

analysis, the thermal gains and losses through the window wall are calculated for different window areas. In addition, thermal loads generated

inside the room due to incandescent lighting, equipment, and human

occupancy are estimated, along with the potential losses due to air

infiltration*

Figure 3 shows the estimated yearly energy costs for the room as a

function of the area of the window. Note that on the vertical axis, or

zero window area, energy costs are given for a windowless room. To

determine the operating costs attributable to just the window, the

yearly operating costs for a room with a given window area must be subtracted from those for a windowless room with zero window area. Part A

of Figure 3 demonstrates that, when only thermal loads are considered,

estimated yearly energy costs increase for both northern and southern

exposures as the size of the single-glazed window increases. A window

with a northern exposure has greater energy costs, however, than one

with a southern exposure, particularly when the more expensive electric

heating is used. The added energy costs for the room are as much as

$20 to $25 more per year for large window areas on the north wall with

electric heat than for large windows on the south with gas heat.

Part B plots similar yearly energy costs for a room with a doubleglazed window. When double glazing is used, energy costs are lowered

for both orientations. For example, double glazing lowers energy costs

by about $30 per year for the largest north-facing window in the

13

electrically heated room. The reduction is somewhat lower for the southfacing window.

To estimate the additional energy costs over the assumed 25 year

life, it is necessary to take into account the cost of money (assumed

here to be 8 percent compounded annually) and the expected future rise

in energy prices (assumed here to be in the range of 0 to 12 percent

compounded annually). The adjusted yearly costs are then summed over

the 25 years to estimate present value life-cycle energy costs. For

example, the large area of single-glazed window on the north side, which

adds about $24 a year to the costs for gas heating and electric cooling,

is expected to add between $250 and $980 to energy costs over 25 years,

depending on the rate of increase in energy prices. The large area of

double-glazed window on the south side, which adds about $6 a year to

costs for gas heating and electric cooling, is expected to add between

$60 and $250 to energy costs over 25 years, depending on future prices.

In summary, the first level of analysis has shown that increasing

the size of a bare, unmanaged window in a room in a Washington, D.C.

home will increase energy costs over those for a windowless room, even

on the south side. The use of double glazing along with careful orientation of the window will reduce these costs, but they will still tend

to rise with increasing window size.

3.2.2 Second Level of Analysis - Management of Windows : In the second

level of analysis, the energy exchange at the window is assumed to be

managed. In this example, wooden thermal shutters are used during

winter nights and Venetian blinds are used during summer days. In

addition, the thermostat setting is reduced from 72°F to 62°F on winter

15
nights and increased on summer nights from 78°F to 84°F (see Table 1).

Figure 4, Parts A and B, shows that management of the window and

thermostat adjustment almost eliminate the negative thermal effect of

increasing window area shown in Figure 3. (Thermostat adjustment reduces

annual energy costs by about $5, regardless of window area). For a

managed, south-facing window, energy costs remain about level even if

the window has single glazing. For the north-facing window, doubleglazing is needed to achieve the same effect.

Given that the yearly energy costs of a managed, south-facing

window are negligible, the lifetime energy costs are also small. On the

north side, the extra lifetime energy costs for a room with a singleglazed, managed window range from an insignificant amount to as high as

$820, depending upon both the size of the window and the present and

future price of energy. With double glazing, the extra life-time energy

costs of a north-facing window are held to no more than $200.

3.2.3 Third Level of Analysis - Daylighting and Management ; In the

third level of analysis, the effects of substituting daylight for electric light are added to the window management and load calculations.

For this case example, it was assumed that the lights were turned off

whenever daylight exceeded a specified level of illumination (see

Table 1) . Energy use was reduced for both lighting and cooling.

The results of the daylighting analysis are more uncertain than

those from the preceding two analyses. They are based upon a computer

model which calculates the energy consequences of substituting natural

light for electric light, and which has not been adequately verified by

laboratory and field testing. Furthermore, typical residential lighting

16
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suggest that there is a large potential for some reduction of energy use.

Experimental validation of the model is needed to reduce the present

uncertainty regarding the amount of energy savings when daylight is used

for illumination.

With the above qualifications in mind, let us examine the estimated

energy costs of windows when daylight, window management, and thermostat

adjustment are added to the thermal load calculations. Figure 5 shows

that, when daylight is used, energy costs decline initially with increasing window area. These results contrast with those given in Figure 3 which

shows energy costs continuously rising as window size is increased, and

in Figure 4 which shows energy costs remaining constant with management.

Part A of Figure 5 demonstrates that, for a gas-heated house, estimated

yearly energy costs for a room with large areas of single glazing can be

lower than those for a windowless room. Similar results occur when electric heating is used, provided the window is oriented to the south. Only

when a window is oriented to the north and electric heating is used, are

energy costs increased by the window, and then only if the window area

is quite large.

As shown in Part B of Figure 5, double glazing is most effective

when used on a north-facing window in a room with electric heat. For

south-facing windows, double glazing has little effect on energy costs

when gas heat is used, and only reduces costs slightly for electric heat.

Translating the cost effects for the current year into lifetime

savings and losses, we find that with rapidly rising energy prices,

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energy savings with a moderately-sized window are expected to be more

than $1000 over a 25-year period if the window is properly managed and

daylight is used. The only exception to this finding occurs with a

large, single-glazed window on the north side of an electrically heated

home

.

3.3 ACQUISITION COSTS \

Having calculated the energy costs associated with different

window configurations, let us now turn to the costs of acquisition,

maintenance, and repair. The purchase and installation of windows in

a new home are generally more expensive than the costs of an equivalent

area of nonwindowed wall. To estimate the additional acquisition costs,

window costs are compared with wall costs in Table 2. This table shows

that the costs of the 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 $48 to $216 for double-glazed

windows, depending on their size. (Because windows displace portions of

the wall, they raise initial building costs by substantially less than

their full purchase and installation costs.)

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.

3.4 MAINTENANCE AND REPAIR COSTS

Window areas usually require somewhat more maintenance than nonwindowed areas. Estimated homeowner window cleaning costs, based on a

current cost of $.10/ft^, are shown in the first row of Table 4.
Table 2

Acquisition Costs of a Window in Excess of the

Cost of a Non-Windowed Wall

Dollar Costs, By Size of Area

Component

2 12 ft

2

18 ft

2

30 ft

2

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

c 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. Prices are for

single and multiple units of windows of a size which comes close to

providing_the designated percentages of the exterior wall in glazing.

The 12 ft area is provided by2a 3' x 3 '11" window; the 18 ft area,

by a ^' 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 windows is used, based on an estimate given by

a home builder in the Washington, D.C. area.

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

cThe additional costs incurred for windowed areas of the building, are the difference between the costs of windows and the costs of walls

for the same wall area. i

21
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Estimates of maintenance and repair costs based on scraping, repainting,

and recaulking once every five years at a current cost of $1.50/ft2, are

shown in the second row of Table 4 (6). Both costs are shown first in

current dollars, and then, in the right hand portion of the table, in

equivalent life-time present values. Present value estimates are based

on future price increases in materials and labor at a rate comparable

to the average of other goods and services, and based on an opportunity

cost of money of 8 percent compounded annually. An estimate for window

breakage is not included. The initially installed window is assumed

to last the life of the building (estimated at 25 years)

.

3.5 LIFE-CYCLE COSTS

After considering each major cost element — energy, purchase and

installation of windows and accessories, maintenance, and repair — we

need to combine them over the life of the building for a more complete

picture of the overall cost impact of windows. Life-cycle cost effects

are shown in Figures 6 through 9. Figures 6 and 7 show the life-cycle

costs for single and double-glazed windows oriented to the south.

Similarly, Figures 8 and 9 show the costs for single and double-glazed

windows oriented to the north. Costs given in each figure are based on

the use of gas heating and electric cooling. On each figure the three

bands indicate the three levels of energy analysis: load only, management, and management plus daylight. For each band, a range of potential

net savings or losses is shown depending upon the future rate of fuel

price escalation. This rate is assumed to range from 0 percent to 12

percent, as indicated by the extremes of the bands.

24
FIGURE 6. SAVINGS OR LOSSES DUE TO A SINGLE-GLAZED, SOUTH-FACING WINDOW OVER 25 YEARS FOR

A RESIDENTIAL MODULE WITH GAS HEATING AND ELECTRIC COOLING - WASHINGTON, D . C . CASE EXAMPLE

+2000-

+1500-

LJ

CO

O +1000-

-1500-

-2000-

12 18 30

WINDOW SIZE

THE PERCENTAGES GIVEN AT THE TOES OF THE BANDS REPRESEffT FUEL PRICE

ESCALATION (f.P.E.) RATES OF 0% AND 12%.
FIGURE 7. SAVINGS OR LOSSES DUE TO A DOUBLE-GLAZED, SOUTH-FACING WINDOW OVER 25 YEARS FOR

A RESIDENTIAL MODULE WITH GAS HEATING AND ELECTRIC COOLING - WASHINGTON , D . C . CASE EXAMPLE

+ 2000-

+1500-

hJ

^+1000-

Q

\I2%FI?E.

Daylight a

MonoQ^msfTt

0%

0%

Lood Only

-2000

30

WINDOW SIZE

60

Nolt: storm windows would b« Mmowhat mort cost tfftctivs ttnn doubts gkizsd

wiftdows bscouto thslr initial purchase and instdlotion costs ars Isss.
FIGURE 8. SAVINGS OR LOSSES DUE TO A SINGLE-GLAZED, NORTH-FACING WINDOW OVER 25 YEARS FOR

A RESIDENTIAL MODULE WITH GAS HEATING AND ELECTRIC COOLING - WASHINGTON , D . C . CASE EXAMPLE

+2000-

-2000-

12 18 30

WINDOW SIZE

60
FIGURE 9. SAVINGS OR LOSSES DUE TO A DOUBLE-GLAZED, NORTH-FACING WINDOW OVER 25 YEARS FOR

A RESIDENTIAL MODULE WITH GAS HEATING AND ELECTRIC COOLING - WASHINGTON , D . C . CASE EXAMPLE

+ 2000-

+ 1500-

if)

UJ

§ +1000-

g +500-

CO

-2000-

30

WINDOW SIZE

I2%FPE.

Daylight a

Management

0%

0%

Lood Only

Management

12%

60

Nott: Storm windows would b« sonMwhot mor« cost tffoctive than double glozod

windowt bocouM thoir initial purchoM and Installation costs art lost.
The four figures show that when the potential savings from daylight are excluded, life-cycle costs increase as the window size is

increased. With consideration of daylight, net life-cycle savings first

rise and then decline as the window size increases, but for the most

part remain positive even for large windows.

Beyond this overview, a close examination of the figures is

instructive in indicating ways in which the life-cycle costs of windows

can be lowered. Looking at the dotted band on Figure 6, we can see

that investment in shutters and blinds will often tend to pay off for

single-glazed windows of all sizes. Looking at Figure 7, we can see

that the cost of shutters and blinds will tend not to be recovered in

fuel cost savings when used on double-glazed south-facing windows under

the conditions indicated. (However, other benefits from shades, blinds,

draperies, and shutters, such as privacy, light control, and decorative

appeal may make them desirable, together with the energy savings benefit,

even if life-cycle cost savings alone do not)

.

A comparison of Figures 6 and 7 shows that double glazing tends

to be cost effective for south-facing windows whose thermal resistance

has not already been increased by the use of management devices. The

higher the rate of fuel price escalation, the more likely double glazing

is to pay off.

Looking now at Figured 8 and 9, we can see that, if energy prices

rise sharply, the use of shutters and blinds on north-facing windows

is likely to be cost effective in conjunction with single glazing,

but is unlikely to be cost effective with double glazing. Comparing

Figures 8 and 9, it appears that for moderate to large windowed areas

29
on north walls, double glazing would tend to be the most economical

choice. . , . . .. , ... ,

The more expensive the fuel, the more critical the use of smaller

window sizes or southern orientations becomes in controlling costs. In

addition, higher fuel costs also make management devices or double

glazing more worthwhile for both south and north-facing windows.

4.0 CONCLUSIONS

It has been demonstrated here that window size, orientation,

thermal resistance, management, and daylight can noticeably affect the

energy use and life-cycle costs of windows. By determining the cost

of energy for the three different levels of analysis, the case example

has shown how the energy and overall life-cycle cost effects of windows

are influenced by the use of window management, thermostat adjustment,

and daylight. The first level of analysis demonstrates that increasing

window area, without using either management or daylight, increases

heating and cooling costs in a typical home in Washington, D.C. These

costs can be reduced, but not eliminated, by using double glazing and

orienting windows to the south. The second level of analysis demonstrates that selective window management can be used to reduce further

the energy costs associated with increasing area. The third illustration shows that the selective use of daylight as a substitute for

electric light may change the energy impact of windows from a negative

to a positive one. These findings emphasize the need to verify the

potential energy savings attributable to daylight.

The combination of energy costs with acquisition, maintenance,

and repair costs over a 25 year period, demonstrates the relationship

30
between the energy efficiency and the life-cycle cost efficiency of

different window designs. Where energy costs can be reduced with little

or no additional investment cost, as by changing orientation from north

to south, the least-cost energy decision is also the lowest life-cycle

cost decision. However, when capital and labor costs are increased by

choosing one window design or accessory over another, these cost

increases must be weighed against energy cost savings to determine the

most cost-effective decision.

In summary, it is suggested that the following items be considered

in the effort to promote the construction and operation of more energyefficient and cost-effective homes:

" Note the direction in which the windows face. Orienting the

windows away from the north and away from prevailing winds can

cut energy costs substantially.

° Encourage the use of double glazing or storm windows in moderate

to cold climates particularly on north walls and for large

windows

,

° Encourage the use of designs which provide good natural lighting

particularly in rooms with predominantly daytime use.

° Encourage the use of window coverings such as draperies, blinds,

shades, or shutters, which have good thermal resistance, shading

potential, and complement daylight utilization.

° The additional costs incurred in making windows energy-efficient

may be offset for the purchaser by psychological benefits such

as a view to the outside, sunshine, variety, and enhancement of

the interior decor.
References

1. Hastings, S. R. and Crenshaw, R. Window Design Strategies to

Conserve Energy . BSS 104, U.S. Department of Commerce, NBS,

June 1977.

2. Kusuda, T. and Collins, B. L. Simplified Analysis of Thermal and

Lighting Characteristics of Windows; Two Case Studies . BSS 109,

in press.

^,; 3. Ruegg, R. T. and Chapman, R. Economic Evaluation of Windows in

Buildings . NBS BSS in preparation.

4. Collins, B. L. Windows and People: A Literature Survey . BSS 70,

U.S. Department of Commerce, NBS. June 1975.

5. Holton, J. K. Daylighting of Buildings; A Compendium and Study of

Its Introduction and Control . NBSIR 76-1098, October 1976.

6. Chapman, R. Economic Analysis of Experimental Lead Paint Abatement

Methods; Phase I . NBS Technical Note 922, September 1976.

32
NBS.n4A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-1388

2* Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITI E

A New Look at Windows

5. Publication Date January 1978

6. Performing Organization Code

7. AUTHOR(S) Belinda L. Collins, Rosalie T. Ruegg,

Robert Chapman, Tamaml Kusuda

8. Performing Organ. Report No.

9. PFRFORMINn OH(^ ANI7 ATIOIM NAMF AND Anr)RP<sS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ot moat significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Recent design recommendations have called for reduced window area in buildings

to conserve energy. This article presents new information on thermal loads, daylighting

management, and life-cycle costs which indicates that such recommendations may neglect

important design and operational aspects of windows which can conserve energy resources

and reduce life-cycle building costs. A case exan^le is described in which energy

consumption and life-cycle costs are given for windows in a typical house in the

Washington, D.C. area. Noticeable reductions in overall energy consumption and lifecycle costs are found if daylight is used, and if the window is managed. It is

suggested that lending institutions and builders consider the long-term effects of

window design and operation decisions.

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)

Daylighting; energy conservation; life-cycle costs; residential; solar heat gain;

window; window management

18. AVAILABILITY [T Unlimited

1

' For Official Distribution. Do Not Release to NTIS

1

1 Order From Sup. of Doc, U.S. Government Printing Office Washineton. D.C. 20402. SD Cat. No. C13

1 X! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

39

20. SECURITY CLASS

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22. Price

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USCOMM-OC 29042-P74
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