NAT'L INST. OF STAND & TECH R.I.C.

I I fill I AlllDS fisnoi

NBSIR 80-2184

NBS

PUBLICATIONS

Energy Analysis of a

Prototype Single-Family

Detached Residence:

The Effects of Climate,

House Size, Orientation,

Internal Heat Release,

and Natural Cooling

Center for Building Technology

National Engineering Laboratory

Bureau of Standards

DEPARTMENT OF COMMERCE

100

.U56

80-2184

1981

c. 2

January 1981

ENERGY

EFFiaENCY
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NBSIR 80-2184

mATlOmAL UUUAD

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APR 3 M)

ENERGY ANALYSIS OF A PROTOTYPE SINGLE-FAMILY

DETACHED RESIDENCE: THE EFFECTS OF CLIMATE,

HOUSE SIZE, ORIENTATION, INTERNAL HEAT RELEASE,

AND NATURAL COOLING

James P. Barnett

Building Thermal Performance Division

Center for Building Technology

National Engineering Laboratory

U S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

Prepared for: Building Codes and Standards Branch

Office of Building and Community Systems

U.S. Department of Energy

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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ABSTRACT

A computer study was done to determine how the annual

heating and cooling requirements of a prototypical ranch

style house are affected by changes in four energy use

parameters: climate (13 locations), floor area (nominal 800

ft2, 1200 ft2 and 1800 ft2 ) , orientation (north, south, and

east/west), and internal heat generation (two different

levels in the 1200 ft2 house). In addition, the effects of

natural cooling on the annual cooling requirement were

investigated.

The results are quantified such that the effects

attributable to each variation are easily identified. Also,

the heating and cooling requirements of the various sized

houses are correlated to degree days.

Some of the more important findings regarding the

prototypical house (as simulated in this study) are: (a)

annual cooling requirements/unit area decreased with

increasing floor area, while (b) annual heating

requirements/unit area remained relatively constant

regardless of floor area; (c) rotation of a house (with

windows on only two facades) significantly affected the

annual energy requirements (approximate range 20-50

percent); (d) house internal heat generation significantly

affected the annual energy requirements (approximate range

10-50 percent); and (e) annual cooling requirements were

significantly reduced (by as much as 48 percent) by the use

of natural cooling.

Key words: Building design; building energy performance

standards; computer simulation of house energy

requirements; degree days; single- family

detached residence; energy analysis — variation

of house energy requirements with climate, floor

area, orientation, internal heat gains, and

natural cooling.

Ill
PREFACE

This report is one of a series documenting NBS research

and analysis efforts in developing energy and cost data

to support the Department of Energy/National Bureau of

Standards Building Energy Conservation Criteria

Program. The work reported in this document was

supported by DOE/NBS Task Order A008-BCS under

Interagency Agreement No. EA 77A 01 6010.

The author wishes to acknowledge that this report

benefitted greatly from the comments of Paul R. Achenbach,

Jim L. Heldenbrand, and Stephen R. Petersen. Special

thanks is also given to Dr. T. Kusuda for suggesting the

concept of "modified" cooling degree days.

IV
TABLE OF CONTENTS

Page

ABSTRACT iii

PREFACE iv

LIST OF TABLES vi

LIST OF FIGURES vi

1. INTRODUCTION 1

2. INVESTIGATIVE PROCEDURE 2

3. VARIATIONS OF SELECTED HOUSE PARAMETERS AFFECTING

ENERGY REQUIREMENTS 7

3.1 Effect of Climate and House Size

on Heating and Cooling Requirements 7

3.2 Variation of Heating and Cooling

Requirements Due to House Rotation 11

3.3 Variation of Heating and Cooling

Requirements Due to Increase in

House Internal Heat Gains 16

3.4 Reduction in Cooling Requirements

Due to Use of Natural Cooling 20

4. CORRELATION OF ANNUAL HEATING AND COOLING REQUIREMENTS

TO DEGREE DAYS 23

5. CONCLUSIONS 32

REFERENCES 34

APPENDIX A. BASIC HOUSE DATA SET USED IN THIS STUDY 35

APPENDIX B. SI CONVERSION 36

V
List of Tables

Table Page

1. Thermal Physical Properties of Base 1176 ft2 House 4

2. Schedules of Operation for House 5

3. Annual Heating and Cooling Requirements of Houses 9

4. Annual Heating and Cooling Requirements/ft2 of Houses 10

5. Annual Heating and Cooling Requirements of Rotated Houses 12

6. Annual Heating and Cooling Requirements/ft2 of Rotated Houses .... 13

7

.

Percent Change in the Annual Heating and Cooling Requirements

of Houses Due to Rotation from South-Facing Direction 15

8. Annual Heating and Cooling Requirements of South-Facing

1176 ft2 House with 50% Increase in the Internal Heat Gains 17

9. Percent Change in the Annual Heating and Cooling Requirements

of 1176 ft2 House due to 50% Increase in the Internal Heat Gains . 18

10. Annual Heating and Cooling Requirements/ft2 of 1176 ft2 House

with 50% Increase in the Internal Heat Gains 19

11. Percent Reduction in the Annual Cooling Requirements of Houses

Due to Natural Cooling 21

List of Figures

Figure Page

1. Design of 1176 ft2 House (Nominal 1200 ft2 ) 3

2 . Annual House Heating Requirements as a Function of

Heating Degree Days (Base 54 °F) 24

3 . Annual House Cooling Requirements as a Function of

Cooling Degree Days (Base 58 °F) 25

4. Annual House Heating Requirements/ft2 as a Function of

Heating Degree Days (Base 54 °F) 26

5. Annual House Cooling Requirements as a Function of

Cooling Degree Days (Base 58 °F) 28

6. House Cooling Requirements for South-Facing 1176 ft2 House as

a Function of Cooling Degree Days (Base 78 ®F) 30

7 . Annual House Cooling Requirements as a Function of

Modified Cooling Degree Days (Base 58/78 °F) 31

VI
1.

INTRODUCTION

Over the past several years, the increasing cost of energy

coupled with the national emphasis on energy conservation

has caused building designers to carefully consider ways of

designing more energy efficient buildings. Most of this

work has relied heavily on computer programs that predict

energy requirements of buildings. In addition, regulatory

agencies faced with the task of promulgating and enforcing

standards have extensively used computer programs to aid

them in drafting standards and/or guidelines.

Generally, only a limited number of computer runs are made

in selecting an energy-efficient building design. This is

because both the computer time and the data preparation and

analysis are expensive. Thus, a user can make the most

effective use of his computer runs if he has some prior

general knowledge of how a change in a particular variable

will affect the energy requirements of a building. This

report looks at several parameters affecting the energy

requirements of a detached single-family house.

The prototype base house design used for the computer

simulation was a single-family ranch house. The variation

of annual heating and cooling requirements is presented as a

function of:

1. Climate - The houses were located in Minneapolis,

Chicago, Boston, Albuquerque, Portland (Oregon),

Washington, DC, Atlanta, Fort Worth, Fresno,

Houston, Phoenix, Los Angeles, and Tampa.

2.

House size - The floor areas of the houses were

816, 1176, and 1792 square feet.

3House orientation - The houses were rotated through

three compass directions (south, west, and north).

4.

Internal heat release - The 1176 square foot house

was simulated with two different levels of internal

heat generation.

5.

Natural Cooling - The annual cooling requirements

of all houses were calculated for both the "windows

closed" and "windows open" cases.

1
2 . INVESTIGATIVE PROCEDURE

The base prototype house is shown in Figure 1. The house

design was taken from Reference [1] and has been used

previously as the house design in several energy analysis

studies [2,3,4]. The house is a 1176 ft2 (nominal 1200 ft2

)

slab-on-grade ranch-style house with 16.4 percent of the

total north- facing wall area and 21.4 percent (including a

sliding glass door) of the south-facing wall area consisting

of double-pane glass. The east and west walls had no

windows. The wood- frame walls were assumed to have R-19

insulation in the stud space and the ceiling to have R-30

insulation. A 1.5 ft roof overhang was assumed. Table 1

lists the areas of the various house surfaces as well as

their respective air-to-air thermal conductances,

absorptivities and/or shading coefficients. The wall

constructions used are meant to be cost-effective (though

not necessarily optimal) for a cold climate such as

Minneapolis. The values for the thermal physical properties

of the wall constructions were taken from Reference [5] and

were meant to be typical values. Appendix A lists the

materials used in the various envelope components as well as

their thermal physical properties.

The internal heat gains modeled for the 1176 ft2 house were

as follows:

1. Lights = 8000 Btu/day

2 . Appliances/equipment = 34600 Btu/day

3. People (sensible) = 10800 Btu/day

These values were selected to give a close approximation to

the internal heat generation level for residential housing

found in the Department of Energy's (DOE) proposed energy

performance standards (BEPS) for new buildings [4].

Appendix A gives the 24-hour lighting, appliance/equipment

and occupant schedule.

The base house was operated according to the schedules given

in Table 2 . The temperature and humidity setpoints were

derived from the American Society of Heating, Refrigerating,

and Air-Conditioning Engineers 's (ASHRAE) Standard 55-74

which is currently under revision. While these conditions

do not represent the widest bands that can be accommodated

by variations in clothing, they are probably appropriate

except during periods of emergency temperature restrictions.

The winter and summer hourly air infiltration rates were

held constant during their respective seasons. This

approach eliminates differences in the final energy

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TABLE 1

THERMAL PHYSICAL PROPERTIES OF 1176 FT2 HOUSE

AREA U-VALUE

(FT2) (BTU/HR-FT2-F) ABSORPTIVITY SHADE COEFF

SOUTH EXPOSURE

INSUL WALL 198.0 0.0489 0.50 — STUD WALL 66.0 0.0736 0.50

WINDOW 32.0 0.5600 — 0.52

S/G-DR 40.0 0.5800 — 0.52

EXPOSURE

INSUL WALL 190.0 0.0489 0.50 — STUD WALL 34.0 0.0736 0.50 —

EXPOSURE

DOOR 20.0 0.4900 0.50 — INSUL WALL 196.0 0.0489 0.50 — STUD WALL 65.0 0.0736 0.50 — WINDOW 55.0 0.5600 — 0.52

EXPOSURE

INSUL WALL 190.0 0.0489 0.50 — STUD WALL 34.0 0.0736 0.50 —

CEILING 1176.0 0.0320 0.50 _ * SLAB 1176.0 0.1061 0.50 —

4
TABLE 2

SCHEDULES OF OPERATION FOR HOUSE

A. Space Temperature Setpoints

1. 68 ®F < T 1 78 °F (from 0700 to 2300)

2. 60 °F X T 78 ®F (from 2400 to 0600)

B. Humidity Control

1. During cooling RH < 65%

2. During heating RH > 20%

3. If not heating or cooling RH floats

C. Design Infiltration Rates (1176 ft2 House)

1. Winter air change rate = 0.63 air changes/hour

2. Summer air change rate = 0.31 air changes/hour

D. Cooling

1. In one mode of operation no natural

cooling was allowed

2 . In the second mode of operation

natural cooling was allowed (all

cooling loads that occured when

the outside temperature was at or

below 78 ®F were eliminated)

5
requirement due to transient air infiltration rates and for

comparative purposes would seem to be a valid assumption.

The hourly air infiltration rate for winter was selected as

being adequate for condensation, combustion, respiration and

odor control, and the summer value was selected as being

adequate for respiration and odor control. While it is well

known that a reduction in the cooling requirement can be

realized by opening windows when the outside temperature is

below the space temperature, it is not clear what conditions

actually cause people to do this. Also, there is a lack of

experimental data on how much air can be brought into a

space by opening windows. For these two reasons, in this

study, the cooling requirements are shown for both the

"windows closed" and "windows open" cases. In the "windows

open" case it was assumed that any cooling requirement that

occured when the outside air temperature was at or below 78

F was eliminated by natural cooling. Thus, the "windows

open" cooling savings shown in this report are near maximum

for the natural cooling of these particular houses.

The National Bureau of Standards Load Determination (NBSLD)

program [6], was used for the dynamic simulation of the

hourly house heating and cooling requirements. The program

uses ambient temperature, wind speed, solar radiation, and

internal heat gains in conjunction with the various envelope

response factors to calculate surface heat flows and

temperatures. A detailed set of simultaneous heat balance

equations are solved at the interior surfaces of the

building envelope to arrive at the hourly heating or cooling

requirement.

Test Reference Year (TRY) weather tapes supplied by the

National Climatic Center and created in accordance with

ASHRAE criteria, were used in the NBSLD simulations. There

are currently TRY tapes available for over 70 cities in the

United States. The tapes contain the following climatic

information for each hour of the year:

1 . Dry-bulb temperature

2 . Wet-bulb temperature

3 . Dew-point temperature

4. Wind direction

5. Wind speed

6 . Barometric pressure

7. Weather conditions (e.g., fog, haze, dust, etc.)

8. Total sky cover

9. Cloud amount for each of four cloud layers

10. Type of cloud for each of four cloud layers

11. Height of base of cloud for each of four cloud layers

The NBSLD computer program uses parameters 1, 2, 3, 5, 6 and

the amount and type of cloud for the lowest cloud layer from

9 and 10.

6
3. VARIATIONS OF SELECTED HOUSE PARAMETERS AFFECTING ENERGY

REQUIREMENTS

The variations to the base house (south facing 1176 ft2

house) were analyzed in the following order:

1. Climate

2. House size

3. House orientation

4. House internal heat gains

3.1 EFFECT OF CLIMATE AND HOUSE SIZE ON HEATING AND COOLING

REQUIREMENTS

The base house was run in 13 different TRY locations. The

cities selected represent a fairly wide range of climates

found in the United States [4] . In addition to those for

the base house, runs were made for an 816 ft2 (nominal 800

ft2) and a 1792 ft2 (nominal 1800 ft2) house in each city.

The aspect ratio of the three sized houses was not kept

constant, since it was felt that builders commonly make the

depth of a house in multliples of 4 ft. Thus, the aspect

ratio of the 816 ft2 house (House S) is 1.42 (length = 34 ft

and depth = 24 ft), the aspect ratio of the 1176 ft2 house

(House M) is 1.5 (length = 42 ft and depth = 28 ft) and the

aspect ratio of the 1792 ft2 house (House L) is 1.75 (length

= 56 ft and depth = 32 ft). For all three sized houses the

ratios of window to floor area were held constant at 0.11.

It is important to note that in these runs the internal heat

gains from people and appliances/equipment were the same for

a given hour in all three house sizes, while the internal

heat gains from lights varied as the square root of the

house floor area. The reason the lighting internal heat

gains were not held constant for the various house sizes was

that this was judged to be unrealistic, and there was no

data to support such an assumption. The same reasoning

applied to having the lighting heat gains vary in direct

proportion to floor area. So, as a more realistc

compromise, the above approach was arbitrarly taken.

Likewise, it was judged to be unrealistic to hold the air

infiltration (in terms of air changes/hour) constant for the

various sized houses, since this would imply that air

leakage (in terms of cfm) is proportional to floor area. In

this study, it was assumed that air leakage is proportional

to exposed wall area; therefore, the air infiltration rates

(in terms of cfm) were made directly proportional to the

perimeter of the various sized houses.

7
The results of this part of the study are shown in Table 3

The 13 cities are listed in descending order of heating

degree days, base 54 °F (see Section 4). Table 3 shows what

one would expect to find for a given house size: the

largest heating requirements are in the colder climates and

the largest cooling requirements in the warmer climates.

Table 3 further reveals the following results:

1.

The heating requirements in all cases increase with

increasing floor area.

2.

The "windows open" cooling requirements (NCOOL)

increase with increasing floor area, but the

"windows closed" cooling requirements (COOL)

decrease for six locations (Minneapolis, Chicago,

Boston, Portland, Washington, and Los Angeles)

3.

For a given city, the relative size of the heating

requirement compared to the cooling requirement

increases with increasing floor area.

In Table 4 the results of Table 3 are normalized by floor

area. Since the internal heat gains/unit area in these runs

actually decreased as house size increased (which is

probably the real world situation), one might expect both

the absolute and the normalized heating requirement to

increase with increasing house size. However, Table 4 shows

the heating requirement/unit area remains almost constant

with increasing house size. The reason for this is that the

air infiltration heat losses were somewhat smaller in Houses

M and L than they would have been if the air infiltration

rates were assumed proportional to floor area rather than

house perimeter (house perimeter does not increase as fast

as floor area). Had the air leakage rate been directly

proportional to the floor area the normalized heating

requirement would have increased with increasing area.

The reason why some locations show the "windows closed"

cooling requirements decreasing with increasing floor area

is because of relatively light envelope loading. For these

locations the house internal heat gains are by far the

largest contributers to the cooling requirements. Since the

internal heat gains/unit area decrease with increasing floor

area, while the volume of space air being heated by these

gains increases in direct proportion to the floor area, then

the number of hours the house has a cooling requirement is

substantially reduced. Thus, the annual cooling requirement

is decreased. Those locations that show the "windows

closed" cooling requirements increasing with increasing

8
TABLE 3

ANNUAL HEATING AND COOLING REQUIREMENTS OF HOUSES, IN MILLIONS OF BTU

CITY ORIENTATION 816 FT2 1176 ]FT2 1792 FT2

MINNEAPOLIS,

HDD (BASE 54

CDD(BASE 58

MN

OF)

°F)

SOUTH

= 5712

= 1769

HEAT

29.8

COOL

3.6

NCOOL

2.6

HEAT

40.3

COOL

3.4

NCOOL

2.7

HEAT

57.2

COOL*

3.2

NCOOL**

2.9

CHICAGO, IL

HDD=3571

CDD=1586

SOUTH 17.6 3.0 1.8 24.4 2.7 1.9 35.6 2.6 2.1

BOSTON, MA

HDD=3244

CDD=1508

SOUTH 16.7 2.3 1.4 23.4 2.0 1.4 34.2 1.7 1.5

ALBUQUERQUE,

HDD=2236

CDD=2411

NM SOUTH 8.0 6.2 4.5 11.3 6.5 5.0 16.4 7.1 5.9

PORTLAND, OR

HDD=2173

CDD=850

1 SOUTH 11.9 1.3 0.9 17.5 1.1 0.9 26.8 0.9 0.9

WASHINGTON, DC

HDD=2018

CDD=2595

SOUTH 10.1 5.5 4.2 14.6 5.4 4.4 21.8 5.4 4.7

ATLANTA, GA

HDD=1248

CDD=2720

SOUTH 4.3 8.1 4.4 6.2 8.3 5.0 9.2 8.8 5.9

FORT WORTH,

HDD=940

CDD=4062

TX SOUTH 3.0 12.7 10.0 4.4 13.9 11.4 6.7 15.9 13.8

FRESNO, CA

HDD=919

CDD=2859

SOUTH 2.9 9.2 7.3 4.5 10.1 8.4 7.0 11.9 10.3

HOUSTON, TX

HDD=532

CDD=4379

SOUTH 1.7 13.8 10.1 2.6 14.8 11.5 4.0 16.6 13.8

PHOENIX, AZ

HDD=339

CDD=5067

SOUTH 1.0 15.7 14.0 1.6 17.6 16.3 2.6 21.0 19.9

LOS ANGELES,

HDD=45

CDD=1488

CA SOUTH 0.2 3.2 0.4 0.3 2.8 0.5 0.7 2.6 0.6

TAMPA, FL

HDD=44

CDD=5239

SOUTH 0.1 15.6 10.0 0.2 16.3 11.2 0.3 17.5 13.2

* COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

** NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE; I.E., THE NATURAL COOLING CASE

9
TABLE 4

I

ANNUAL HEATING AND COOLING REQUIREMENTS/FT2 OF HOUSES AT

13 TRY LOCATIONS, IN THOUSANDS OF BTU/FT2

CITY ORIENTATION 816 FT2 1176 FT2 1792 FT2

MINNEAPOLIS, MN SOUTH

HEAT

36.5

COOL

4.4

NCOOL

3.1

HEAT

34.2

COOL

2.9

NCOOL

2.3

HEAT

31.9

COOL*

1.8

NCOOL**

1.6

CHICAGO, IL SOUTH 21.5 3.6 2.2 20.8 2.3 1.6 19.8 1.4 1.2

BOSTON, MA SOUTH 20.5 2.8 1.7 19.9 1.7 1.2 19.1 1.0 0.8

ALBUQUERQUE

,

NM SOUTH 9.8 7.6 5.5 9.6 5.5 4.3 9.2 4.0 3.3

PORTLAND, OR SOUTH 14.6 1.6 1.1 14.9 0.9 0.7 15.0 0.5 0.5

WASHINGTON, DC SOUTH 12.4 6.8 5.1 12.4 4.6 3.8 12.2 3.0 2.6

ATLANTA, GA SOUTH 5.2 9.9 5.4 5.2 7.0 4.2 5.1 4.9 3.3

FORT WORTH, TX SOUTH 3.7 15.6 12.2 3.8 11.8 9.7 3.8 8.9 7.7

FRESNO, CA SOUTH 3.6 11.2 8.9 3.8 8.6 7.2 3.9 6.6 5.8

HOUSTON, TX SOUTH 2.1 16.9 12.4 2.2 12.6 9.8 2.2 9.3 7.7

PHOENIX, AZ SOUTH 1.3 19.2 17.2 1.4 15.0 13.9 1.5 11.7 11.1

LOS ANGELES, CA SOUTH 0.2 4.0 0.5 0.3 2.4 0.4 0.4 1.5 0.3

TAMPA, FL SOUTH 0.1 19.1 12.2 0.2 13.8 9.6 0.2 9.8 7.3

*COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

I

10
floor area experience heavy envelope loading (high ambient

temperature and solar radiation). Table 4 shows quite

clearly, however, the dramatic decrease in both the "windows

open" and "windows closed" cooling requirements/unit area

with increasing floor area. This decrease in both the

"windows open" and "windows closed" normalized cooling

requirements is caused by the house internal heat gains

increasing much more slowly than the house floor area.

Result (3) above is a consequence of (1) and (2) but is the

more important result as far as its implications for

effective energy analysis of detached single- family housing

is concerned. Using Atlanta as an example, one sees from

Table 3 that the ratio of heating to cooling requirement

goes from approximately 0.53 (for House S) to approximately

1.05 (for House L). These substantial changes in the

relative size of the heating and cooling requirements will

influence the choice of energy conserving modifications made

for each sized house. For example, one might opt for a

whole-house fan or external awnings to reduce the cooling

requirement of House S while the choice for House L might be

increased insulation or storm windows to reduce the heating

requirement which is now as large as the cooling

requirement. Other factors also influence the choice of

house modifications (e.g., seasonal efficiencies of heating

and cooling equipment, cost of heating and cooling energy,

etc.), but the relative size of heating and cooling

requirements for a given house plays an important role in

the choice of energy conserving options. This example also

illustrates the potential error of generalizing the results

of one house size to another house size.

3.2 VARIATION OF HEATING AND COOLING REQUIREMENTS DUE TO

HOUSE ROTATION

The three differently sized houses were originally oriented

as shown in Figure 1. For purposes of reference the base

orientation of the houses is taken to be south (this is the

direction the facade with the most glass area is facing).

The houses were then rotated to face west, and then to

north. The houses were not run with an east- facing

orientation since a sensitivity analysis showed the annual

heating and cooling requirements for the east and west

orientations to be virtually identical (this would not be

necessarily true for peak hourly requirements). The results

of these runs are shown in Tables 5 and 6. An analysis of

those tables show the following:

1. For all locations the lowest heating requirements

are found for a south- facing orientation.

11
TABLE 5

ANNUAL HEATING AND COOLING REQUIREMENTS OF ROTATED HOUSES

IN MILLIONS OF BTU

CITY ORIENTATION 816 FT2 1176 FT2 1792 FT2

HEAT COOL NCOOL HEAT COOL NCOOL HEAT COOL* NCOOL**

MINNEAPOLIS, MN EAST/WEST 30.9 4.9 3.2 41.8 5.1 3.6 59.5 5.6 4.3

NORTH 30.4 3.4 2.5 41.2 3.2 2.6 58.7 3.1 2.8

CHICAGO, IL EAST/WEST 18.5 4.3 2.3 25.7 4.5 2.6 37.4 4.9 3.1

NORTH 18.1 2.9 1.8 25.3 2.6 1.9 36.8 2.5 2.0

BOSTON, MA EAST/WEST 17.8 3.4 1.8 24.8 3.4 2.0 36.3 3.5 2.3

NORTH 17.3 2.2 1.4 24.3 1.9 1.4 35.7 1.6 1.4

ALBUQUERQUE, NM EAST/WEST 8.9 8.2 5.4 12.6 9.2 6.5 18.5 11.0 8.1

NORTH 8.5 6.1 4.4 12.0 6.4 5.0 17.7 7.0 5.8

PORTLAND, OR EAST/WEST 12.6 2.2 1.2 18.5 2.2 1.4 28.2 2.4 1.7

NORTH 12.3 1.3 0.8 18.1 1.0 0.8 27.7 0.9 0.8

WASHINGTON, DC EAST/WEST 10.8 7.1 5.1 15.6 7.6 5.8 23.5 8.4 6.8

NORTH 10.5 5.4 4.1 15.2 5.3 4.4 22.8 5.3 4.7

ATLANTA, GA EAST/WEST 4.8 10.3 5.4 6.9 11.3 6.5 10.4 13.2 8.2

NORTH 4.6 7.9 4.3 6.6 8.1 4.9 10.0 8.6 5.9

FORT WORTH, TX EAST/WEST 3.5 15.1 11.6 5.2 17.3 13.9 7.9 21.0 17.5

NORTH 3.3 12.5 9.8 4.8 13.6 11.3 7.4 15.5 13.6

FRESNO, CA EAST/WEST 3.4 11.8 8.9 5.3 13.8 10.8 8.2 17.3 13.9

NORTH 3.1 8.9 7.1 4.9 9.8 8.3 7.7 11.5 10.1

HOUSTON, TX EAST/WEST 2.0 16.2 11.8 3.1 18.2 14.0 4.8 21.6 17.5

NORTH 1.9 13.6 10.0 2.9 14.5 11.4 4.5 16.3 13.6

PHOENIX, AZ EAST/WEST 1.3 18.4 16.4 2.0 21.5 19.7 3.4 26.9 25.1

NORTH 1.1 15.3 13.9 1.8 17.2 16.1 3.0 20.5 19.6

LOS ANGELES, CA EAST/WEST 0.2 5.0 0.5 0.4 5.1 0.6 1.0 5.7 0.7

NORTH 0.2 3.0 0.4 0.4 2.5 0.5 0.9 2.3 0.6

TAMPA, FL EAST/WEST 0.1 17.6 11.5 0.3 19.2 13.4 0.5 22.0 16.4

NORTH 0.1 15.2 9.8 0.2 15.8 11.1 0.4 17.0 12.9

*COOL = ANNUAL COOLING REQUIREMENT FOR “WINDOWS CLOSED“ CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR “WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE
TABLE 6

ANNUAL HEATING AND COOLING REQUIREMENTS/FT2 OF ROTATED

HOUSES, IN THOUSANDS OF BTU/FT2

CITY ORIENTATION 816 FT2 1176 FT2 1792 FT2

HEAT COOL NCOOL HEAT COOL NCOOL HEAT COOL* NCOOL**

MINNEAPOLIS, MN EAST/WEST 37.8 6.0 3.9 35.6 4.3 3.1 33.2 3.11 2.4

NORTH 37.2 4.2 3.1 35.0 2.7 2.2 32.7 1/7 1.5

CHICAGO, IL EAST/WEST 22.7 5.2 2.8 21.9 3.8 2.2 20.9 2.7 1.7

NORTH 22.2 3.5 2.2 21.5 2.3 1.6 20.6 1.4 1.1

BOSTON, MA EAST/WEST 21.8 4.2 2.2 21.1 2.9 1.7 20.3 2.0 1.3

NORTH 21.2 2.7 1.7 20.7 1.6 1.2 19.9 0.9 0.8

ALBUQUERQUE, NM EAST/WEST 10.9 10.1 6.7 10.7 7.8 5.5 10.3 6.1 4.5

NORTH 10.4 7.5 5.4 10.2 5.4 4.2 9.8 3.9 3.3

PORTLAND, OR EAST/WEST 15.4 2.7 1.5 15.7 1.9 1.2 15.7 1.3 0.9

NORTH 15.0 1.6 1.0 15.4 0.9 0.7 15.4 0.5 0.5

WASHINGTON, DC EAST/WEST 13.3 8.7 6.2 13.3 6.4 4.9 13.1 4.7 3.8

NORTH 12.9 6.7 5.0 12.9 4.5 3.7 12.7 3.0 2.6

ATLANTA, GA EAST/WEST 5.9 12.6 6.6 5.9 9.6 5.5 5.8 7.4 4.6

NORTH 5.6 9.7 5.3 5.6 6.9 4.2 5.6 4.8 3.3

FORT WORTH, TX EAST/WEST 4.3 18.5 14.3 4.4 14.7 11.8 4.4 11.7 9.8

NORTH 4.0 15.3 12.0 4.1 11.5 9.6 4.1 8.7 7.6

FRESNO, CA EAST/WEST 4.2 14.4 10.9 4.5 11.7 9.1 4.6 9.6 7.8

NORTH 3.9 10.9 8.7 4.1 8.4 7.0 4.3 6.4 5.7

HOUSTON, TX EAST/WEST 2.5 19.8 14.5 2.6 15.4 11.9 2.7 12.1 9.8

NORTH 2.3 16.6 12.3 2.4 12.3 9.7 2.5 9.1 7.6

PHOENIX, AZ EAST/WEST 1.6 22.5 20.1 1.7 18.3 16.7 1.9 15.0 14.0

NORTH 1.4 18.8 17.0 1.5 14.6 13.7 1.7 11.4 10.9

LOS ANGELES, CA EAST/WEST 0.2 6.1 0.6 0.4 4.4 0.5 0.5 3.2 0.4

NORTH 0.2 3.6 0.5 0.3 2.2 0.4 0.5 1.3 0.3

TAMPA, FL EAST/WEST 0.2 21.6 14.0 0.2 16.3 11.4 0.3 12.3 9.2

NORTH 0.2 18.6 12.1 0.2 13.4 9.4 0.2 9.5 7.2

*COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

13
2. For all locations the lowest cooling requirements

are found for a north- facing orientation.

3 . For all locations both the heating and cooling

requirements for the east/west facing house are

always larger than for the south- and north-facing

houses

.

It is instructive to look at the results of Tables 5 and 6

as percent changes in the respective heating and cooling

requirements of the three sized houses, as they are rotated

form south to west to north. This is done in Table 7. An

analysis of Table 7 yields the following results:

1.

The percent change in the heating requirement, due

to house rotation, can be significant depending on

the location of the house (the range is 2 to 20

percent, neglecting the large percentage changes

for locations having very small heating

requirements )

.

2

.

The percent change in the cooling requirement can

be very significant depending on the location and

house size (the range is -1 to 56 percent for the

"windows closed" case, and -1 to 45 percent for the

"windows open" case, neglecting the large

percentage changes for locations having very small

cooling requirements).

3

.

For a given location the rotational effect on the

heating requirement remains relatively constant,

regardless of house size (neglecting abrupt changes

for locations having very small heating

requirements )

.

4. For a given location the north-facing orientation

has little effect on the cooling requirements (the

average decrease is about 3 percent).

The above points deserve some additional discussion. House

orientation effects on the annual heating and cooling

requirements are totally due to the changes in solar loading

on the house (primarily the solar heat gain through glazed

surfaces). Therefore, the direction in which the glazed

surfaces face has a great impact on the house energy

requirements. One can see from the results presented in

this study that reorienting window area from the south and

14
TABLE 7

PERCENT CHANGE IN THE ANNUAL HEATING AND COOLING REQUIREMENTS

OF HOUSES DUE TO ROTATION FROM SOUTH-FACING DIRECTION

CITY ORIENTATION 816 FT2 1176 FT2 1792 FT2

HEAT COOL NCOOL HEAT COOL NCOOL HEAT COOL* NCOO

MINNEAPOLIS, MN EAST/WEST 4 36 23 4 50 33 4 75 48

NORTH 2 -6 -4 2 -6 -4 3 -3 -3

CHICAGO, IL EAST/WEST 5 43 28 5 67 37 5 88 48

NORTH 3 -3 0 4 -4 0 3 -4 -5

BOSTON, MA EAST/WEST 7 48 29 6 70 43 6 106 53

NORTH 4 -4 0 4 -5 0 4 -6 -7

ALBUQUERQUENM EAST/WEST 11 32 20 12 42 30 13 55 37

NORTH 6 -2 -2 6 -2 0 8 -1 -2

PORTLAND, OR EAST/WEST 6 69 33 6 100 56 5 167 89

NORTH 3 0 -11 3 -9 -11 3 0 -11

WASHINGTON, DC EAST/WEST 7 29 21 7 41 32 8 56 45

NORTH 4 -2 -2 4 -2 0 5 -2 0

ATLANTA, GA EAST/WEST 12 27 23 11 36 30 13 50 39

NORTH 7 -2 -2 6 -2 -2 9 -2 0

FORT WORTH, TX EAST/WEST 17 19 16 18 24 22 18 32 27

NORTH 10 -2 -2 9 -2 -1 10 -3 -1

FRESNO, CA EAST/WEST 17 28 22 18 37 29 17 45 35

NORTH 7 -3 -3 9 -3 -1 10 -3 -2

HOUSTON, TX EAST/WEST 18 17 17 19 23 22 20 30 27

NORTH 12 -1 -1 12 -2 -1 13 -2 -1

PHOENIX, AZ EAST/WEST 30 17 17 25 22 21 31 28 26

NORTH 10 -3 -1 13 -2 -1 15 -2 -2

LOS ANGELES, CA EAST/WEST 0 56 25 33 82 20 43 119 17

NORTH 0 -6 0 33 -11 0 29 -12 0

TAMPA, FL EAST/WEST 0 13 15 50 18 20 67 26 24

NORTH 0 -3 -2 0 -3 -1 33 -3 -2

*COOL

**NCOOL

ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

15
north to the east and west increases both the annual heating

and cooling requirements. The annual heating requirements

are increased because the winter window solar heat gains

through the southern exposure have been removed, and the

annual cooling requirements have been increased due to

substantial summer window solar heat gains through east/west

exposures. The rotational effect on the annual heating

requirements remains almost constant because the ratio of

window area to floor area, for all sized houses, was held

constant.

A south to north rotation of these houses resulted in a

relatively small decrease in the cooling requirements. This

seems to indicate that only a small part of the annual

cooling requirements were due to solar heat gains through

south- facing windows. However, a shift in window area from

south to north might be considered as an energy saving

option in very warm climates.

3.3 VARIATION OF HEATING AND COOLING REQUIREMENTS DUE TO

INCREASES IN HOUSE INTERNAL HEAT GAINS

In this part of the study, the internal heat gains for House

M (1176 ft2, south facing)) were increased by 50 percent.

This was thought to be a reasonable upper limit for house

internal heat gains [7]. These results are shown in Table

8. It is useful to show the results of Table 8 as

percentage changes in comparison with the corresponding

results of Table 3. This is done in Table 9. For all

locations the percent change in the heating and cooling

requirements is very significant. For those locations

having substantial heating requirements the decreases in the

heating requirements vary from 13 to 46 percent. For those

locations having substantial cooling requirements, the

increases in the cooling requirements vary from 30 to 57

percent for the "windows closed" case and from 21 to 36

percent for the "windows open" case. The reason the

"windows open" increases are less than the corresponding

"windows closed" increases is that internal heat gains are

less important in the "windows open" case where conduction,

air infiltration, and solar heat gains dominate because of

the high ambient temperature (always above 78 °F).

It is interesting to normalize the results of Table 8 to

floor area. This is done in Table 10. Comparing the

results of Table 10 to the House S results in Table 4, one

sees that the cooling requirements for a given location are

now nearly proportional to floor area. This result is not

surprising since the internal heat gains/unit area for House

M in Table 10 are about equal to the internal heat

gains/unit area for House S in Table 4. The fact that the

16
TABLE 8

ANNUAL HEATING AND COOLING REQUIREMENTS OF SOUTH-FACING

1176 FT2 HOUSE WITH 50% INCREASE IN INTERNAL HEAT GAINS

IN MILLIONS OF BTU NCOOL* CITY HEAT COOL’' **

MINNEAPOLIS, MN 35.1 5.6 3.7

CHICAGO, IL 19.8 4.8 2.6

BOSTON, MA 18.7 3.8 2.1

ALBUQUERQUENM 7.7 9.7 6.4

PORTLAND, OR 12.8 2.3 1.3

WASHINGTON, DC 10.7 8.5 6.0

ATLANTA, GA 3.9 12.7 6.2

FORT WORTH, TX 2.6 18.9 14.1

FRESNO, CA 2.3 13.9 10.4

HOUSTON, TX 1.4 20.5 14.3

PHOENIX, AZ 0.7 22.8 19.7

LOS ANGELES, CA 0.1 6.1 0.6

TAMPA, FL 0.1 23.4 14.1

*COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

17
TABLE 9

PERCENT CHANGE IN THE ANNUAL HEATING AND COOLING REQUIREMENTS

OF 1176 FT2 HOUSE DUE TO 50% INCREASE IN INTERNAL HEAT GAINS

CITY HEAT COOL* NCO«

MINNEAPOLIS, MN -13 65 37

CHICAGO, IL -19 78 37

BOSTON, MA -20 90 50

ALBUQUERQUE, NM -32 49 28

PORTLAND, OR -27 109 44

WASHINGTON, DC -27 57 36

ATLANTA, GA -37 53 24

FORT WORTH, TX -41 36 24

FRESNO, CA -49 38 24

HOUSTON, TX -46 39 24

PHOENIX, AZ -56 30 21

LOS ANGELES, CA -67 118 20

TAMPA, FL -50 44 26

*COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

18
TABLE 10

ANNUAL HEATING AND COOLING REQUIREMENTS/FT2 OF 1176 FT2 HOUSE

WITH 50% INCREASE IN INTERNAL HEAT GAINS, IN THOUSANDS OF BTU/FT2

CITY HEAT COOL* NCOOL**

MINNEAPOLIS, MN 29.8 4.7 3.2

CHICAGO, IL 16.8 4.1 2.2

BOSTON, MA 15.9 3.2 1.7

ALBUQUERQUE

NM 6.5 8.3 5.4

PORTLAND, OR 10.9 2.0 1.1

WASHINGTON, DC 9.1 7.1 5.1

ATLANTA, GA 3.4 10.8 5.3

FORT WORTH, TX 2.1 16.1 11.9

FRESNO, CA 2.0 11.8 8.8

HOUSTON, TX 1.2 17.4 12.1

PHOENIX, AZ 0.6 19.4 16.7

LOS ANGELES, CA 0.0 5.2 0.5

TAMPA, FL 0.0 19.9 12.0

*COOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS CLOSED" CASE

**NCOOL = ANNUAL COOLING REQUIREMENT FOR "WINDOWS OPEN" CASE, I. E.,

THE NATURAL COOLING CASE

19
annual cooling requirements in Table 9 are almost directly

proportional to floor area serves to illustrate the fact

that the internal heat gains can be the prime drivers for

the cooling requirements of a well-insulated house with

glazing on the east and west facades only. However, one

sees by comparing the heating requirements/unit area of

Table 10 with the heating requirements/unit area for House S

of Table 4 that the heating requirements are no longer

almost directly proportional to floor area. This result was

caused by the air infiltration rate varying with house

perimeter rather than area. This fact illustrates the point

that air infiltration is a major contributor to well

insulated house heating requirements.

The results in Tables 8-10 point out the need for having

standard internal heat gains. If researchers use different

internal heat gains it will be difficult, if not impossible,

to compare the results of their work. Heating and cooling

variations in the 5 to 10 percent range can be partially or

totally masked by an inappropriate choice for house internal

heat gains

.

3.4 REDUCTION IN COOLING REQUIREMENTS DUE TO USE OF NATURAL

COOLING

Tables 3 and 5 show both the annual cooling requirements for

the "windows closed" and "windows open" conditions. Table

11 displays these results as a percent decrease in the

respective "windows closed" annual cooling requirement. In

other words. Table 11 shows the potential savings in the

annual cooling requirement attainable through the use of

natural cooling (the introduction into the space of cool

outside air by non-mechanical means). One sees that the

percent savings possible in almost all cases are quite

significant. The savings range from 4 percent (for the 1792

ft2 north-facing Phoenix house) to 48 percent (for the 816

ft2 south- facing Atlanta house), for houses having an

appreciable annual cooling requirement.

The percent savings are least for the Phoenix houses since

much of the time the ambient temperature there is very high

and, thus, much of the cooling requirement occurs when the

ambient temperature is above 78 °F. The percent savings are

greatest for House S in all cities because the internal heat

gains per unit floor area are greatest. This causes a

larger portion of the cooling requirement to occur when the

ambient temperature is at or below 78 °F, resulting in a larger percent savings than in the larger houses. This same

reasoning applies in comparing the percent savings of House

M to House L.

20
TABLE 11

PERCENT REDUCTION IN THE ANNUAL COOLING REQUIREMENTS

OF HOUSES DUE TO NATURAL COOLING

CITY ORIENTATION 816 FT2 1176 FT2 1792 FT2

MINNEAPOLIS, MN SOUTH 28 21 9

EAST/WEST 35 29 23

NORTH 26 19 10

CHICAGO, IL SOUTH 40 30 19

EAST/WEST 47 42 37

NORTH 38 27 20

BOSTON, MA SOUTH 39 30 12

EAST/WEST 47 41 34

NORTH 36 26 13

ALBUQUERQUE, NM SOUTH 27 23 17

EAST/WEST 34 29 26

NORTH 28 22 17

PORTLAND, OR SOUTH 31 18 0

EAST/WEST 45 36 29

NORTH 38 20 11

WASHINGTON, DC SOUTH 24 19 13

EAST/WEST 28 24 19

NORTH 24 17 11

ATLANTA, GA SOUTH 46 40 33

EAST/WEST 48 42 38

NORTH 46 40 31

FORT WORTH, TX SOUTH 21 18 13

EAST/WEST 23 20 17

NORTH 22 17 12

FRESNO, CA SOUTH 21 17 13

EAST/WEST 25 22 20

NORTH 20 15 12

HOUSTON, TX SOUTH 27 22 17

EAST/WEST 27 23 19

NORTH 26 21 17

PHOENIX, AZ SOUTH 11 7 5

EAST/WEST 11 8 7

NORTH 9 6 4

LOS ANGELES, CA SOUTH 88 82 77

EAST/WEST 90 88 88

NORTH 87 80 74

TAMPA, FL SOUTH 36 31 25

EAST/WEST 35 30 25

NORTH 36 30 24

21
The percent savings for the east/west orientation, for a

given location, are always greatest because the increased

window solar heat gains cause more of the cooling

requirement to occur when the ambient temperature is at or

below 78 °F. Generally, the south orientation gives a larger

percent savings than the north for this same reason.

It should be remembered that the percent savings shown in

Table 11 do not reflect the effect natural cooling would

have on the cooling requirement if substantial amounts of

"thermal mass" were present in the house and nighttime

flushing were used. In certain locations (e.g., Phoenix)

because of the large diurnal temperature cycle it would be

expected that "thermal mass" and nighttime flushing would

substantially increase the percent savings in Table 11.

However, the percent savings in Table 11 are probably very

near the maximum possible for the houses modeled here for

natural cooling. The reason is because of the natural

cooling mode of operation assumed in this study. It was

assumed that the entire cooling requirement could be

eliminated when the ambient temperature was at or below 78

°F. This would not be possible when there is a large cooling

requirement and the ambient temperature is very close to 78

°F. Thus savings due to natural cooling were overestimated.

This overestimate was partially compensated for by not

allowing the house to be cooled below 78 °F and therefore

neglecting these potential savings. If, in addition to

natural cooling a whole-house fan were employed, the savings

in Table 11 would be increased because a higher ambient air

temperature could be used to cool with (due to higher air

velocities) and the house could be more frequently cooled

below 78 °F (because larger amounts of cool air could be

brought into the house )

.

22
4. CORRELATION OF ANNUAL HEATING AND COOLING

REQUIREMENTS TO DEGREE DAYS

The heating and cooling requirements (Table 3, "windows

closed" case) were plotted as a function of heating degree

days (base 54 °F) and cooling degree 4ays (base 58 °F) in

Figures 2 and 3 (the straight lines in the figures are least

square fits to the points ) . As can be seen from the

figures, both the total annual heating and cooling

requirements of the three sized houses lie roughly on

straight lines, with the heating requirements showing a better correlation with degree days than the cooling

requirements. This fact is not surprising since cooling

degree days only indirectly reflect solar radiation

intensity and do not reflect latent requirements at all.

Taking this into account, it is interesting that the

correlation between cooling requirements and cooling degree

days is as good as it appears to be. Another point that

should be made is that the heating and cooling degree day

bases actually change when the house size changes. This is

because the "balance point" temperatures of the house are

affected' by changes in air leakage rate, internal heat

gains, and to some extent envelope area. The "balance

point" temperatures of the house are taken here to mean

those ambient temperatures above which or below which no

heating or cooling is required, respectively. Since in

theory the degree day base temperatures should equal the

"balance point" temperatures of the house, then a change in

one should produce an identical change in the other [2].

Thus, there is an inherent error in the correlation that is

caused by using the same degree day base for all three sized

houses. But this shortcoming may be acceptable (providing

the error remains small) if it avoids the awkwardness of

having different degree day bases for different sized

houses. The reason for choosing the heating degree day base

of 54 °F and the cooling degree day base of 58 °F is that

these two degree day bases gave the best fits for all the

points for the three different sized houses. Figures 2 and

3 show that Albuquerque and Portland have a poor fit for

heating degree days,- and that Washington, DC and Fresno have

a poor fit for cooling degree days. Albuquerque most

probably shows a poor correlation to heating degree days

because of its large diurnal temperature cycle (daylight

hours considerably warmer than nighttime hours) and a high

level of incident solar radiation. The reasons why Portland

does not have better correlataion to heating degree days and

Washington, DC and Fresno to cooling degree days are unclear

at this time. :

In Figure 4 the annual heating requirements of Figure 2 have

been divided by their respective floor areas and plotted

against heating degree days. The solid line is a least

23
HEATING

REQUIREMENTS

ASA

FUNCTION

OF

DEGREE

DAY

ANNUAL

HEATING

DEGREE

DAYS

(BASE

54

F)

PLOT

SYMBOLS!

X»816

FT2,o=117e

FTa,*=179a

FT2

FIGURE

a.
COOLING

REQUIRENENTS

ASA

FUNCTION

OF

DEGREE

DAY

Ob. ^

ANNUAL

COOLING

DEGREE

DAYS

(BASE

58

F)

PLOT

SYMBOLS I

X-816

FT2,o*1176

FT2,/i-179E

FTE

FIGURE

3.
CLUIQ^ U.HCU
squares fit of the points. One can see from the figure that

the fit is fairly good for all locations with the exception

of Albuquerque and Portland (this result was to be expected

from the poor fits these two cities showed in Figure 2 )

.

Figures 2 and 4 tend to suggest that it may be possible to

develop an equation to predict heating requirements, for a

particular style of house with standard operating

conditions, as a function of heating degree days for a

fairly wide range of floor areas. This possibility is a

consequence of the fact that the heating requirement of a

house is assumed nearly proportional to the floor area. For

a large number of locations with heating degree days above

1000 (base 54 °F) it may be possible to predict the heating

requirements to within 10 percent. There will, of course,

be certain locations that need to be handled individually

(e.g., Albuquerque) or in small groups because of poor

correlation to heating degree days. For locations with

heating degree days above 2500 it is likely the error in the

predicted heating requirement will be less than 10 percent,

with the error decreasing as the heating degree days

increase. For locations with heating degree days between 0

and 1000 (base 54 °F) it will probably be best to handle them

as a subset- and fit these points separately.

The prediction of cooling requirements is more difficult

than the prediction of heating requirements because of the

behavior of the cooling requirements as calculated in this

study. In some locations the cooling requirement decreases

as the floor area increases, in other locations, the cooling

requirement increases as the floor area increases. This

problem is shown graphically in Figure 3 . One can see the

cooling requirement lines for the three different sized

houses cross over each other at about 1700 cooling degree

days. It does no good to normalize the cooling requirement

to floor area since the cooling requirement/unit area

decreases with increasing floor area (see Table 4). One

simple solution to this problem is to perform a regression

analysis on all the cooling requirement points regardless of

house size, since the cooling requirement for a given

location does not vary significantly with floor area. This

was done and the results are shown in Figure 5

.

The major problem with this method for predicting cooling

requirements is that the difference between the predicted

and calculated values becomes larger at the high end of the

cooling degree day scale where the cooling requirements are

most important. It’ is suggested, though not attempted in

this analysis, that a more accurate method for predicting

cooling requirements would take into account solar

radiation, latent requirement, and internal heat generation

as well as cooling degree days.

27
COOLING

REQUIREMENTS

AS

A

FUNCTION

OF

DEGREE

DAV

I r in

03

o

03

O




O

IS

LO

O

<S

<S

n

o

o

03

<S> OO

—Jo<r Q Ob. CQI-Z)

ANNUAL

COOLING

DEGREE

DAYS

(BASE

58

F)

PLOT

SVnBOLSI

X=816

FTa,o=1176

FTa,A.i

79

a

FTa

FIGURE

5.
In Figure 6 the annual cooling requirements for the "windows

open" condition are plotted against cooling degree days

(base 78 °F). At first, one might think this should yield a

"good" fit since the "windows open" requirements are only

made up of cooling requirements that occured when the

ambient temperature was above 78 °F. However, a glance at

Figure 6 shows this is not the case. The reason for this

"poor" fit is that the cooling requirements for the "windows

open" case actually occur when the windows are closed (the

windows are opened to eliminate the cooling requirements

that occur when the ambient temperature is at or below 78

°F). As mentioned previously, a regression analysis showed

that the cooling "balance point" temperature for these

houses was 58 °F with the windows closed. However, one would

not expect a good correlation between the "windows open"

case cooling requirements and cooling degree days (base 58

°F) since the cooling requirements that occured when the

ambient temperature was between 58 °F and 78 °F have been

eliminated by natural cooling. What one needs to calculate

are "modified" cooling degree days defined as follows:

MCDDT//T2L (TAVG^- Tl) for TAVG^> T2

where TAVG = average daily temperaure

Tl = cooling "balance point" temperature

T2 = ambient temperature above which no

natural cooling is used

For our case Tl equals 58 ®F and T2 equals 78 ®F. In Fi^re 7

the "windows open" cooling requirements are plotted against

"modified" cooling degree days (base 58 °F / 78 ®F). The line

is a least squares fit of all the points. The fit in Figure

7 is a vast improvement over the fit found in Figure 6.

This suggests that in the future with more and more homes

using some form of natural cooling that "modified" cooling

degree days may prove useful in predicting annual cooling

requirements

29
COOLING

REQUIREMENTS

ASA

FUNCTION

OF

DEGREE

DAY

<s
COOLING

REQUIRENENTS

AS

A

FUNCTION

OF

DEGREE

DAV

o

o

ocn

9

o

OJ

9

9

ru ^0^0 s:i-4^^i-iOZ OLi. caiMODIFIED

COOLING

DEGREE

DAYS

(BASE

58/78

F)
5. CONCLUSIONS

The annual heating and cooling requirement trends, due to

parametric variations made to a base house, provide a useful

guide from which to select the initial energy options that

are to be made to a detached single- family house. The major

findings of this report are as follows:

1.

It appears that annual cooling requirements for a

well-insulated house show only a small variation

with house floor area, and therefore annual cooling

requirements/unit area decrease as house floor area

increases

.

2

.

House annual heating requirements are almost

directly proportional to house floor area, and

therefore annual heating requirements/unit area

tend to remain relatively constant with varying

house floor area.

3.

Orientation of a house can have a significant

effect on both the annual heating and cooling

requirements and should be considered as a energy

conserving option.

4.

The internal heat gains of a house can have a

significant effect on both the annual heating and

cooling requirements of a house and thus careful

attention should be given to making realistic

assumptions regarding them.

5.

Natural cooling can significantly reduce house

cooling requirements in most locations in the

country, therefore the use of screened windows and

doors as well as whole house fans should be

considered as viable energy conserving options.

Conclusion 1 above, if confirmed, is very important because

of its potential impact on building codes and standards. If

these regulations allocate house energy consumption on the

basis of it being directly proportional to floor area,

smaller houses will be unfairly penalized (especially in

warm climates). This is because the cooling

requirement/unit area decreases as house size increases.

Therefore, larger houses would receive an implicit energy

"credit" that would allow them to meet their required energy

levels more easily than smaller houses.

32
As mentioned earlier in this report air infiltration rates

and internal heat generation are major contributors to the

annual heating and cooling requirements. In this report

what were thought to be real istc assumptions were made

concerning these two variables. It should be stressed that

if these assumptions should prove to be partially or totally

incorrect then many of our results would be substantially

changed. The assumptions one makes regarding these two

paramenters are of prime importance in determing house

energy requirements. Currently there exists a pressing need

to get measured data for these two parameters, especially

data on how these parameters vary with house size.

A note of caution should be made about applying these

results. The results of this report apply in detail only to

the houses modeled here along with their operating

assumptions (e.g., air infiltration rates , internal heat gain

schedules, etc.) and while the results should apply in

general to similar houses, discretion is advised. For

example, it is obvious that the rotation effects on the

heating and cooling requirements would not apply to a house

with equal window areas on all exposures. What might be

overlooked is that the rotation findings would have to be

modified somewhat if applied to houses where the window area

varied as the exposed wall area (in this study, window area

varied as the floor area).
REFERENCES

1. Hastings, R.S., Three Proposed Typical House Designs for

Energy Conservation Research . NBSIR 77-1309, National

Bureau of Standards, Washington, D.C., 1977.

2. Arens, E.A., Carroll, W.L., Geographical Variation in the

Heating and, _CoQling . Reguirements of a Typical SingleFamily House, and Correlation of these Reguirements to

Degree Days . BSS 116, U.S. Department of Commerce,

National Bureau of Standards, Washington, D.C., 1978.

3. Peterson, S.R., Barnes, K.A.

,

Peavy, B.A., Determining

Cost-Effective Insulation Levels for Masonry and WoodFrame Walls in a Single-Family Residence . BSS (in

review), U.S. Department of Commerce, National

Bureau of Standards, Washington, D.C., 1980.

4. "Energy Performance Standards for New Buildings;

Proposed Rulemaking and Public Hearings", Federal

Register, Vol. 44, No. 250, November 28, 1979.

5. American Society of Heating, Refrigerating and AirConditioning Engineers, ASHRAE Handbook of Fundamentals . ASHRAE, N.Y., N.Y.

,

1977.

6. Kusuda, T., NBSLD, The Computer Program for Heating

and Cooling Loads in Buildings . BSS 69, U.S. Department

of Commerce, National Bureau of Standards, Washington,

D.C., 1976.

7. Clark, R.E., Hastings, R.S., Quantified Occupant-Use

Factors Affecting Energy Consumption in Residences , NBSIR 78-1501, U.S. Department of Commerce, National

Bureau of Standards, Washington, D.C., 1979.

8. Department of Housing and Urban Development, Residential

Energy Consumption, Single-Family Housing , Report No.

HUD-HA2-2, Prepared by Hittman Associates, Columbia, Md.,

1973.

9. Peavy, B.A., Burch, D.M., Powell, F.J., and Hunt, C.M.,

Comparison of Measured and Computer-Predicted Thermal

Performance of a Four Bedroom Wood-Frame Townhouse . BSS 57, U.S. Department of Commerce, National Bureau

of Standards, Washington, D.C., 1975.

34
APPENDIX A - Basic House Data Set Used In This Study

This appendix is intended to describe the computer input data set used in

this study. The appendix is divided into three sections:

1. House Envelope Data

2 . House Internal Heat Gain Data

3 . House Operational Data

Within each section the value of the appropriate parameters are listed along

with the reasons for the selection of these values.

House Envelope Data

T=THICKNESS(FT), C=CONDUCTIVITY(BTU/HOUR-FT-F) , D=DENSITY(LB/FT3 )

SH=SPECIFIC HEAT(BTU/LB-F), R=RESI STANCE ( HOUR-FT2 - F/BTU )

,

U=THERMAL CONDUCTANCE ( BTU/H0UR-FT2 -F

)

ROOF COMPOSITION

LAYER T C D SH R DESCRIPTION

— — — — — —

1 0.000 0.000 0.00 0.000 0.60 INSIDE SURF. RES. (ROOF)

2 0.042 0.070 34.00 0.290 0.00 1/2 IN. PLYWOOD

3 0.000 0.000 0.00 0.000

THERMAL CONDUCTANCE

WALL (INSULATED PORTION) COMPOSITION

0.50 BUILD. PAP.+ASPH. SHIG.

U= 0.590

LAYER T C D SH R DESCRIPTION

— — — — — —

1 0.042 0.094 50.00 0.200 0.00 1/2 IN. GYPBOARD

2 0.292 0.026 2.00 0.200 0.00 3 1/2 IN. INSUL Rll

3 0.125 0.017 2.20 0.290 0.00 1 1/2 IN RIGID INSUL R7

4 0.031

WALL (STUD PORTION)

0.050 37.00 0.290

THERMAL CONDUCTANCE

COMPOSITION

0.00 3/8 IN. WOOD SIDING

U= 0.051

LAYER T C D SH R DESCRIPTION

— — — — — —

1 0.042 0.094 50.00 0.200 0.00 1/2 IN. GYPBOARD

2 0.292 0.070 32.00 0.330 0.00 2X4 STUD

3 0.125 0.017 2.20 0.290 0.00 1 1/2 IN RIGID INSUL

4 0.031 0.050 37.00

THERMAL

0.290

CONDUCTANCE

0.00 3/8 IN. WOOD SIDING

U= 0.078

35
FLOOR COMPOSITION

LAYER T C D SH R DESCRIPTION

“ — — —

1 0.000 0.000 0.00 0.000 1.50 CARPET&PADDING

2 0.333 1.000 140.00 0.200 0.00 4 IN. CONCRETE SLAB

3 0.000 0.000 0.00 0.000 5.00 1 IN. POLYSTYRENE INSUL

4 0.333 0.200

SOUTH EXPOSURE

100.00

THERMAL

BASE 1176

AREA

0.400 0.00

CONDUCTANCE

FT2 HOUSE

U-VALUE ABS

.

4 IN. EARTH

U= 0.118

S.C.

INSUL WALL 198.0 0.0489 0.50 — STUD WALL 66.0 0.0736 0.50 — WINDOW 32.0 0.5600 — 0.52

S/G-DR

WEST EXPOSURE

40.0 0.5800 0.52

INSUL WALL 190.0 0.0489 0.50 -

STUD WALL

NORTH EXPOSURE

34.0 0.0736 0.50

DOOR 20.0 0.4900 0.50 — INSUL WALL 196.0 0.0489 0.50 — STUD WALL 65.0 0.0736 0.50 — WINDOW

EAST EXPOSURE

55.0 0.5600 0.52

INSUL WALL 190.0 0.0489 0.50 — STUD WALL 34.0 0.0736 0.50 —

CEILING 1176.0 0.0320 0.50 . _ . SLAB 1176.0 0.1061 0.50 —

The design of this house is taken from Reference [1]. This style of ranch

house represents 30% of current U.S. detached single-family housing. The

actual wall constructions shown are meant to be cost effective (though

not necessarily optimal) for a cold climate (such as Minneapolis, MN).

The values for the thermal physical properties were taken from the ASHRAE

Guide and Data Handbook of Fundamentals (1977). The values selected for

absorptivity of wall/roof and shading coefficient of double pane glass

are meant to be typical values. An off-white paint would have an abso^-

tivity of approximately 0.5 while a double pane window with average weight

draperies has a shading coefficient of about 0.52. The windows and wall

were assumed to be shaded by a 1.5 ft over hang.

36
Internal Heat Gain Data

Lighting:

24-Hour Profile

1. 0.0 7. 1.0 13. 0.023 19. 0.023

2. 0.0 8. 1.0 14. 0.023 20. 0.5

3. 0.0 9. 0.023 15. 0.023 21. 0.5

4. 0.0 10. 0.023 16. 0.023 22. 1.0

5. 0.0 11. 0.023 17. 0.023 23. 0.0

6. 0.0 12. 0.023 18. 0.023 24. 0.0

Normalization Factor = 0.38 watts/sq. ft

In the absence of any published studies of actual hourly use of lighting

in single-family detached housing as well as a lack of established guidelines, the hourly lighting profile found in Reference [8] was used. While

this profile may not necessarily reflect actual lighting schedules, it still

seems to be a reasonable representation of hourly lighting use.

Equipment/Appliance

:

24-Hour Profile

1. 0.25 7. 0.33

2. 0.25 8. 0.46

3. 0.25 9. 0.63

4. 0.25 10. 0.88

5. 0.25 11. 0.35

6. 0.25 12. 0.88

13. 0.42 19. 0.92

14. 0.25 20. 0.68

15. 0.25 21. 0.68

16. 0.58 22. 0.68

17. 0.58 23. 0.51

18. 1.0 24. 0.23

Normalization Factor = 0.75 watts/sq. ft

As with lighting use there are no published data or guidelines for

hourly equipment/appliance use in detached housing. This schedule

was taken from Reference [9]. It was chosen because there is an

hourly breakdown given of the appliance being used and it correlated

well with annual values given in Reference [7].

37
Occupants

24-Hour Profile

1. 1.0 7. 1.0 13. 0.4 19. 1.0

2. 1.0 8. 1.0 14. 0.4 20. 1.0

3. 1.0 9. 0.4 15. 0.4 21. 1.0

4. 1.0 10. 0.4 16. 0.69 22. 1.0

5. 1.0 11. 0.4 17. 0.69 23. 1.0

6. 1.0 12. 0.4 18. 1.0 24. 1.0

Normalization Factor = 2 adults and 2 children

This profile was taken from Reference [8] since there is at present

inadequate data for determining a typical profile. This profile

appears to be reasonable.

House Operational Data

A. Space Temperature Setpoints

1. 68 ®F < T < 78 ®F (from 0700 to 2300)

2. 60 ®F < T < 78 °F (from 2400 to 0600)

B. Humidity Control

1. During cooling RH < 65%

2 . During heating RH > 20%

3. If not heating or cooling RH floats

C. Design Infiltration Rates (1176 ft2 House)

1. Winter air change rate = 0.63 air changes per hour

2. Summer air change rate = 0.31 air changes per hour

D. Cooling

1. In one mode of operation no natural

cooling was allowed

2 . In the second mode of operation

natural cooling was allowed (all

cooling loads that occured when

the outside temperature was at or

below 78 ®F were eliminated)

38
APPENDIX B. SI Conversion

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

United States and the structure of the NBS Load Determination computer

program used in this report, common U.S. units of measurement have been used

throughout this report. In recognition of the position of the United States

as a signatory to the General Conference of Weights and Measures, which gave

official status to the metric SI system of units in 1960, appropriate conversion factors have been provided in the table below. The reader interested in

making further use of the coherent system of SI units is referred to:

NBS SP330, 1972 Edition, "The International System of Units"

E380-72 ASTM Metric Practice Guide (American National Standard

2210 . 1 )

.

Metric Conversion Factors

Length

:

1 inch (in) = 25.4 millimeters (mm)

1 foot (ft) = 0.3048 meter (m)

Area: 1 ft2 = 0.092903 m2

Volume: 1 ft3 = 0.028317 m3

Temperature

:

]? = 9/5 C + 32

Temperature

Interval

:

1 F = 5/9 C or K

Mass

:

1 pound (lb) = 0.453592 kilogram (kg)

Mass per unit

volume: 1 lb/ft3 = 16.0185 kg/m3

Energy: 1 Btu = 1.05506 kilojoules (kJ)

Specific heat: 1 Btu/(lb)( F) = 4.1868 kJ/(kg)(K)

U-value: 1 Btu/(ft2)(h)( F) = 5.67826 W/(m2)(K)

R-value

:

l(ft2)(h)( F)/Btu = 0.176110(m2)(K)/W

39
NBS-114A (REV. »-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2184

4, TITLE AND SUBTITLE

ENERGY ANALYSIS OF A PROTOTYPE SINGLE-FAMILY DETACHED RES II Effects of Climate, House Size, Orientation, Internal Heat

Release, and Natural Cooling

5. Publication Date JANUARY 1981

7, AUTHOR(S) "s. ^e^orming Organ R^ep'ort No!'' James P. Barnett

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. C^ntract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS city. State, ZIP) 13. Type of Report & Period Covered

I

i

I

! Buil(iing Codes and Standards Branch

Office of Buildings and Community Systems

Department of Energy , Washington, DC 20545

15, SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of moat eignffleant Information. If document Includes a aignllleant bibliography or

literature survey, mention it here*) A computer study was done to determine how the annual heating and cooling requirements

of a prototypical ranch-style house are affected by changes ^

four energy

2 use parameteist

climate (13 locations), floor area (nominal 800 ft , 1200 ft , and 1800 ft ), orientation

(north,

2 South, and east/west), and internal heat generation (two different levels in tho

1200 ft house). In addition, the effects of natural cooling on the annual coolin<r re- quirement were investigated.

The results are quantified such that the effects attributable to each variation are eas- ily identified. Also, the heating and cooling requirements of the various sized houses

are correlated to degree days.

Some of the more important findings regarding the prototypical house (as simulated in

this study) are: (a) annual cooling requirements/unit area decreased with increasing

floor area, while (b) annual heating requirements/unit area remained relatively constant

regardless of floor area; (c) rotation of a house (with windows on only two facades)

significantly affected the annual energy requirements (approximate range 20-50 percent);

and (e) annual cooling requirements were significantly reduced (by as much as 48 percent)

by the use of natural cooling.

12. KEY_ WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) building design; building energy performance standards; computer simulation of house

eriergy requirements; degree days; energy analysis—variation of house energy requirements with climate, floor area, orientation, internal heat gain, and natural cooling; single13. AVAILABILITY ramiiy aetacneu : residence.

XKS Unlimited For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office. Washington, D.C.

20402.

XXX; Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

45

15. Price

USCOMM-DC 6043-P80
i

n

4