Sensitivity of residential heating energy to building envelope thermal conductance

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site

LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB. Building Research Note, 1981-11

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=c57868b8-2514-4d75-bcb3-011ec75cffa9 https://publications-cnrc.canada.ca/fra/voir/objet/?id=c57868b8-2514-4d75-bcb3-011ec75cffa9

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/40000535

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Sensitivity of residential heating energy to building envelope thermal

conductance

Ser

m

no.

177

SENS TTTV

X

TY 01: R E S I IIENTIAL

TO BUILDING ENVELOPE THERMAL CONDUCTANCE

by M.R. Bassctt*

INTRODUCTION

The "Measures far Energy Conservation in N e w B u i l d i n g s , 19781q' contains t a b l e s o f minimum thermal resistances for v a r i o u s components of t h e building construction. T h e s e values were derived from a life c y c l e c o s t analysis that endeavoured to determine the m i n i m u m value f o r t h e sum o f t h e c o s t o f s u p p l y i n g and installing t h e i n s u l a t i o n and of t h e present worth o f t h e heating energy consumed wer a given p e r i o d of years. Studies of t h i s type2 have been based upon the best estimates of many factors - t h e p e r i o d of time to be considered, the c o s t of money ( i n t e r e s t r a t e ) , the c o s t of energy and t h e escalation

in the c o s t _{of energy, t h e seasonal efficiency of the heating equipment, t h e}

i n c r e m e n t a l c o s t of i n c r e a s i n g t h e thermal resistance and, o f course, t h e c h a n g e i n t h e q u a n t i t y of h e a t i n g energy required u n d e r v a r i o u s climatic con-

d i t i o n s resulting f r o m a change i n the thermal resistance of t h e component. This s t u d y was c a r r i e d o u t with the o b j e c t i v e of q u a n t i f y i n g t h i s l a s e i t e m more p r e c i s e l y .

COMPUTER SIMULATIONS

3

The ENCORE computer program was used t o simulate t h e energy consumption of a house with f o u r d i f f e r e n t levels of insulation under t e n d i f f e r e n t climatic

conditions. The model building chosen is a common t y p e o f b u i l d i n g found in

2

Canada. It is a two-storey detached house with 1 2 4 m o f above-grade f l o o r a r e a and occupied by a family of three. Detailed thermal and dimensional

details a r e given in Appendix A.

The f a c t o r s a f f e c t i n g space heating demand can be summarized as follows. The occupants gcncrate about 30 kl.h/day d i r e c t l y and t h r o u g h energy-demanding a c t i v i t i e s , and t h e r a t e of below-grade heat l o s s from t h e f u l l y h e a t e d base-

ment i s a constant 1 kW. Window areas were chosen for daylighting and appearance r a t h e r t h a n f o r optimum solar assistance to space h e a t i n g , and

CLIMATIC DATA

Hour-by-hour climatic records f o r t h e locations l i s t e d in t h e following t a b l e were used in t h i s s t u d y , Location Vancouver Summer land T o r o n t o St. John's Montreal O t t a w a S u f f i e I d S w i f t Current Winnipeg Edmonton Year Annual Degree-Days Computed EOT t h e Particular Year (base 1 8 " ~ )

These l o c a t i o n s were selected to include t h e main c e n t r e s of population, the f u l l range o f climatic s e v e r i t y measured in degree-days, t h e maritime and i n l a n d climate types.

r e p r e s e n t 9 . 6 per cent o f t h e floor a r c a . A i r leakage o p e n i n g s i n t h e envelope allow approximately 0.2 a i r changelhour of infilsration in w i n t e r

climatic conditions.

T l ~ c envelope t h e r m a l conductance was increased i n t h ~ c e s t e p s t o c r c a t e

a s e r i e s of f o u r buildings l a b e l l e d A, R , C and D - B u i l d i n g A h a s RSI 2 . 7 5

2

r n 2 ~ / w w a l l s a n d RSI 4 . 1 m K/W roof. It l i e s at fhe low end of recommendations i n t h e Measures f o r Energy Conservation i n New Buildings, 1978. Buildings

B , C and D arc insulated to progressively h i g h e r l e v e l s w i t h an _{upper limit}

2

of triple glazing, RSI 6.5 _{m K/W walls and RSI 11 r o o f .}

SAVINGS IN PURCHASED SPACE HEAT

The results of the 40 s i m u l a t j o n s show t h a t at each location the e n e r g y consumption ( E ) is proportional t o t h e above-grade thermal conductance (C)

e x c l u d i n g infiltration. F i g u r e 1 shows t h i s relationship f o r seven of t h e l o c a t i o n s . It does not matter that ventilation and basement h e a t l o s s a r c

n o t

a l l t e n locations and a l l four b u i l d i n g t y p e s . Hence t h e change in energy consumption (AE) as a result of a change in conductance (AC) as given by t h e s l o p e of t h e l i n e s i n Figure 1 h a s a r e l a t i o n s h i p of the form

where t h e valuc of t h e c o n s t a n t _{k depends upon t h e climate. The o n l y ex-} c c p t i o n i s a small amount of d i m i n i s h e d r e t u r n f o r b u i l d i n g

n

t h a t t h e d a t a may n o t b e extrapolated to more h i g h l y i n s u l a t e d b u i l d i n g s in

1 ocations w i t h fewer degree-days than Vancouver.

The change i n e n e r g y consumption w i t h a change in conductance (dE/dC)

t a k e n from F i g u r e 1 a r e p l o t t e d a g a i n s r annual degree-days t o base 18'~

(Dl&) i n Figure 2 . T h e r e is some scatter around a s t r a i g h t line drawn through

t h e d a t a but this is expected becausc degree-days havc never been c o n s i d e r e d

t o encompass a l l climatic variables. The scatter i s random, however, and the

straight l i n e cannot b e significantly improved with a h i g h e r o r d e r r e l a t i o n - s h i p .

The i n t e r c e p t a t zero degree-days i s dEJdC = (-3*90] k h p K a l t h o u g h we

- -

The s l o p e {[dE/dC)fD) = ZU h with a 95 per c e n t confidence i n t e r v a l of a h o u t 2 h. T h i s uncertainty in thc slope follows from d e s c r i b i n g t h e climate w i t h n s i n g l e variable.

Assembling t h e s e observations i n t o a s i n g l e package g i v e s t h e following relaeionship between a change in t h e purchased space h e a t i n g energy, t h e

change i n envelopc conductance and thc c l i m a t i c s e v e r i t y i n degree-days.

TOTAL ENERGY PREDICTIONS

Having reached the o b j e c t i v e of t h i s study, it i s o f interest to continue f u r t h e r and d e r i v e an equation r e l a t i n g absolute h e a t i n g energy to D and t h e total above-grade conductance i n c l u d i n g i n f i l t r a t i o n , G . When the seasonal f u r n a c e energy is p l o t t e d a g a i n s t annual degree-days [Figure 3) the following p o i n t s emerge.

(a) S t r a i g h t lines drawn through thc d a t a f o r each building

cannot be u s e f u l l y improved by a h i g h e r o r d e r relationship. (h] Intersection of t h e degree-day axis occurs around 500 d - K o r

at about t h e number o f degree-days accumulated while t h e h e a t i n g system i s turned o f f (June, J u l y , August).

(c) The s l o p e s measured i n units k W - h J d - K a r e as follows:

Building A R C D

Slope k ~ * h / d - ~ 2.86 2.19 1 . 8 6 1.58

These s l o p e s a r e t h e product of the degree-day constant and e f f e c t i v e envelope c o n d u c t a n c e . Unravelling t h e two i s greatly simplified by

o b s e r v i n g that t h e mean i n f i l t r a t i o n r a t e can be considered independent

of D and building t y p e (A, B,C,D)

.

changelhour and locality to l o c a l i t y variation of standard d e v i a t i o n 0 . 0 2 a i r

Building A B C D

and t h e seasonal furnace energy can be calculated from:

E = 20.G (D-d) *-. . .

above-grade conductance G, W J K

where d = t h e number o f degree-days accumulated while the furnace i s shut down.

143 109 9 3 79

COblPARISON

WITH

THE ASHRAE

MODIFIED

The modified degree-day procedure given i n t h e 1980 ASHRAE System 4

Handbook can be written as follows:

where E = t h e energy consumed during a h e a t i n g season

HL = t h e design heat l o s s , W

D = _{t h e number of degree-days to base 1 8 % during the}

h e a t i n g season, O C days

AT = t h e design temperature d i f f e r e n c e , "C

n

load performance, full load efficiency, o v e r s i z i n g , and energy conserving devices

V = h e a t i n g value of f u e l , consistent with HL and E

Earlier experience with utility hills showed that the a n n u a l h e a t

requirement f o r a well i n s u l a t e d house falls s h o r t o f

(H

L

.

(hT

m

25 per c e n t and t h a t t h e appropriate value of C was 0.75(*). A number of

D

explanations can b e g i v e n f o r t h e origin o f C a s follows:

n

[I) By meastrring t h e n u m b e ~ of degree-days to base 18'C r a t h e r than

from t h e thermostat s e t point T

STAT and allowance E f is made far

internal heat gains f r o m people, appliances and sunlight:

where -

'

f

-

l a )

AT rlv

If t h e indoor temperature a t which the internal g a i n s balance t h e l o s s e s is less than 1 8 " C , then t h e allowance made in the modified degree-day expression will be inadequate and t h e seasonal energy

h i g h . A value of C u c l would bc appropriate i n this case.

( 2 ) The design conductance H ]AT h a s _L traditionally been c a l c u l a t e d from outdoor temperatures and a i r infiltration rates c l o s e t o the annual extreme values. The mean i n f i l t r a t i o n rate aver a h e a t i n g season w i l l be r a t h e r l e s s than the peak v a l u e , indicating that

HL/AT generally overestimates t h e representative building con-

ductance. Once a g a i n , a value o f C

_a

Modified Degree-day Model compared with Simulations

Equation ( 2 ) can be used to calculate t h e seasonal heating energy consumed by the particular building described in Appendix A. There are, however, t w o important differences hetween t h i s relationship and t h e modified degree-day

expression.

( 4 ) The building conductance

G includes

infiltration rate r a t h e r t h a n the design value.

( 2 ) Below-grade heat loss has been held c o n s t a n t f o r a l l locations.

It has been accounted for in the analysis along with internal

Net internal gain = T o t a l internal g a i n s

-

I n the modified degree-day model, t h e term IIL/AT includes below-grade heat l o s s which implies that it scales with

degree-days.

If t h c conductance G in Equation

12)

H /AT, t h e n a smaller value of CD would he r e q u i r e d to balance the degree-

L

day type expression. T h i s explains, in p a r t , why the numerical value 2 0 for t h e f a c t o r k i n Equation ( 2 3 is h i g h e r t h a n t h e e x p e c t e d C

D

-

A Variable Base Degree-Day Approach

The factor CD is known to depend on t h e comparative s i z e of internal g a i n s

4

and t h c h e a t last from t h e b u i l d i n g . The ASHRAE Handbook i n d i c a t e s ( F f g . 1,

p. 4 3 . 8 ) how CD depends on t h e number of degree-days. The r a n g e of C D

attributed to different building t y p e s and occupancies I s i n d i c a t e d by a span

o f one standard deviation. This means that the 6 3 p e r cent confidence i n t e r v a l f o r C in Canadian climates is approximately 0.25.

D

Various attempts have been made t o eliminate t h e need f o r C by a d j u s t i n g D

the degree-day base temperature to s u i t each particular building. I t is worth considering the possibility f o r t h e buildings described in t h i s r e p o r t . A v a r i a b l e base degree-day equation can t a k e t h e following Eorm.

where T* = The outdoor temperature at which t h e heating

system t u r n s on (the balance temperature) "C

G = The above-grade envelope conductance including air infiltration and excluding below-grade h e a t l o s s (W/K)

The balance t e q e r a t u r e can be estimated u s i n g t h e following equation:

( T s ~ ~ ~

- T*) = S + E - B (41

T~~~~ = thermostat setpoint, ' 6 S = mezn s o l a r h e a t g a i n , W

F = heat gain from occupancy, W

0 = basement heat loss, W

Annual degree-days a r e n o t widely available f o r a variety of base temperatures. For the l o c a t i o n s l i s t e d on page 2 , t h e y were computed from degree-hours f o r

t h e base temperatures 1 2 , 14, 1 6 , 18 and 20°C. A r e l a t i o n s h i p between the i m p o r t a n t variables D and T* h a s been developed which accounts f o r a b o u t

1 8

90 p e r cent o f t h e variance i n D

T*

-

deviation of degree-days [C) for each degree removed from base 1 8 ° C - The v a l u e of DTP can be calculated f r o m D and T* as follows:

1 8

provided t h a t T* f a l l s within t h e range 20 > T* > 1 2

.

E = G - 24 (D + (T* - 1 8 ) ~ 3 0 8

+

-

V V 18

where

Of

₍

₁

Applying t h e s e equations ta building A g i v e s t h e following:

F i g u r e 3 _{shows t h e predicted energy plotted against D as a dashed line.}

18

Agreement w i t h the simulation impravcs in colder climates but t h i s is expected because i n t e r n a l g a i n s a r e more likely t a be utilized. In reality, i n t e r n a l

gains a r e n o t as steady as assumed by t h e base t e m p e r a t u r e selection process, and as the h e a t l o s t through t h e envelope f s r e d u c e d by adding insulation

or

moving to a warmer climate, internal g a i n s mare frequently exceed t h e demand far heat and a r e partly wasted by h i g h e r conduction losses because of h i g h e r i n s i d e temperatures and by d e l i b e r a t e ventilation. In addition, the buildings

s i ~ Il : ~ t c d i n I t f a i s pirl~cr h n v c m u l t i 111 c zorlcs :ln(I t h c ma j n r Jntcrnnl gni n s

rclcasucl on the f i r s t and secorld f l o u r s c;trlnot tjc t r a d c d for h e a t l o s t from

t h e basement.

T h c s e observations h i g h l 5 g h t one of t h e major difficulties e n c o u n t e r e d by simplified energy predictive methods: the problem o f determining how much of t h e h e a t released by occupants, appliances and window s o l a r g a i n s contribute

towards r e d u c i n g demand from t h e f u r n a c e . In t h e degree-day method and its variations, these d i f f i c u l t i e s are c u r r e n t l y accounted f o r by a f a c t o r d e t e r - mined from experience w i t h utility h i 1 1s.

A two-storey housc w i t h basement was modeled u s i n g t h c ERGORE computer program under climatic conditions of 10 Canadian locations and w i f h four d i f f e r e n t l e v e l s o f insulation i n each l o c a t i o n . The r e s u l t s of t h e 4 0

simulations can be expressed in terms of t h e above-grade envelope conductance

C (excluding i n f i l t r a t i o n ) and the c l i m a t i c severity in degree-days (D)

as follows:

(1) Changes to t h e seasonal purchased energy attributed to changes in abovc-grade envelope conductancc follow t h e t r e n d :

( 2 ) T o t a l seasonal purchased energy can be estimated using t h e following expression:

E = 20.G

CD18

'0°1

T h i s carrelation could n o t be reproduced u s i n g a variable base temperature degree-dzy e q u a t i o n unless t h e utilization o f i n t e r n a l gains i s more accurately accounted f o r .

These observations havc simplicitjr i n common with t h e m o d i f i e d degree-

day model b u t t h e r e is an important d i f f e r e n c e . Thc '"design load" canduct-

ance term

i n

a v e r a g e d above-grade conductance u s e d h e r e . Consequently t h e factor

REFERENCES

1) Measures f o r Energy Conservation in New Buildings, 1978. Issued

by t h e Associate Committee on t h e National B u i l d i n g Code. National Research C o u n c i l o f Canada, Ottawa.

NRCC

23 Stephenson, D-G., Determining the Optimum Resistance for Malls

and Roofs. National Research Council of Canada, D i v i s i o n of Building Research, Bldg. Res. Note 105. January 1976-

3 ) Konrad, A . , Description o f t h e Encore-Canada Building Energy Use

Analysis Computer Program. National Research Council of Canada, D i v i s i o n of Building Research, Computer Program No. 46. April 1980.

4) ASHME Handbook; 1980 Systems Volume. American Society of H e a t i n g , R e f r i g e r a t i o n and Air-Conditioning Engineers. New Ysrk, U . S . A .

5 ) Ilanion,

V.S.

ACKNOWLEDGEMENTS

The co-operasive e f f o r t o f the Building Research Association of New Zealand and the D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council of Canada i s acknowledged in supporting t h i s project. h e acknowledgement

must go to J . K . Latta who initiated this study and added many h e l p f u l s u g g e s t i o n s .

APPEND I X A

B U I L D I N G DETAILS

Floor P l a n

A lmi l d i n g of t h e following f l o o r p l ; ~ n has heen modeled :is f i v e e q u : ~ l l ~ '

thermostatted h e a t i n g t o n e s .

B A S E M E N T

G R O U N D

F L O O R

S E C O N D

F L O O R

'A

Building Dimensions

Toral floor area including basement 182.6 m2

Building volume including basement 411.6

m3

Average construction weight approximately 146 kg/m2 of f l o o r area

THERMAL

PROPERTIES

COMPONENTS

Basement above-grade walls

2

thermal resistance, m K/IV 1 , 3 4 E x t e r n a l d o o r s thermal 2 resistance, m K / W T o t a l window conductance,

N/K,

external wall s o l a r absorptivity = 0.4

e x t e r n a l door solar absorptivity = 0.4

SPACE HEATING MEASURES AND TEMPERATURE CONTROL

Adequate electric heating capacity was installed in each zone and controlled to the following s e t p o i n t s with 0.5"C dead band.

Time Thermostat s e t t i n g

Excess heat was s p i l l e d to p r e v e n t t h e indoor temperature. of a n y of t h e zones

exceeding 2 6 . 7 ' ~ .

BASEMENT

The basement heat lass remained unchanged from locality to locality f o r

t h e following two reasons.

[ a ) In practice t h e below-grade h e a t l o s s depends on f a c t o r s

which are d i f f i c u l t to account for v i z ground water movement

and t h e conductivity of b a c k f i l l e d soil. These appear to vary

as much within t h e localities chosen far climate modeling as

from one climatic zone to the next.

( b ) A t t h i s t i m e , t h e r e are v e r y few reliable measurements o f below-

grade heat loss.

A steady h e a t lass of approximately 1 kW based on measurements b y t h e

INFILTRATION

In the ENCORE simulation, h o u r l y infiltration rates are calculated from t h e s p e c i f i e d leakage openings in t h e envelope and t h e

driving forces

and temperature. The procedure is d e s c r i b e d by Konrad e t al.*

The mean h e a t i n g season infiltration r a t e was 0.18 a i r changeJhour. Variation between buildings at each l o c a l i t y caused by h i g h e r mean indoor

temperatures i n the b e t t c r i n s u l a t e d cases was less t h a n 0 - 0 1 a i r change/

hour. Variation f r o m locality to locality amounted to a standard d e v i a t i o n

of 0.02 a i r change/hour

.

The h e a t released by t h r e e occupants h a s been modeled as a series of hourly schedules for:

Sensible and l a t e n t heat

E l e c t r i c appliances

Contribution from h o t water Electric lights

There are holiday and workday schedules f o r each of t h e s e consisting of hourly average values. E l e c t r i c l i g h t s are t u r n e d on when called f o r by the schedule or when insufficient daylight i s available. The amount o f hot water h e a t i n g

energy f i n a l l y made available as space heat includes the standby losses and

50 per cent of energy to h e a t water.

*

Konrad, A . , Larsen, B.T. and Shaw, C.Y. Programmed Computer model of a i r

infiltration in small residential buildings with oil furnace. Procs, Third

I n t e r n a l S p p . Use o f Computers f o r Environmental Engineering Related t o

The following t a b l e gives mean d a i l y contributions to space h e a t .

The annual total electrical consumption breaks down to around 8000 k1V.h for

h o t water and 3800 kW.h f a r lights and appliances. The total of 11,800 kW,h l i e s close to t h e mean annual electrical consumption f i g u r e s c i t e d in a

survey o f single family homes by Ontario ~ ~ d r o '

'

$ c n s i b l e and l a t e n t heat E l e c t r i c appliances

Approximate

Mean Daily Contribution

to Space Heat

7.3 kW.h/d

6 . 3 kW.h/d Hot water system

Electric lights

I Total.

14.3 kw.hJd

2.9 klVlh/d

0

0 20 40 60 80 100 120 140 1.60

ABOVE-GRADE CONDUCTANCE l EXCLUDl NG I NFILTRATION1. WIK

e c s A B U I L D ~ N G

I I I