Thermal envelope houses

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Thermal envelope houses

Chown, G. A.

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BUILDING

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PRACTICE

NOTE

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THERMAL ENVEU3PE HOUSES

G.

THERMAL ENVELOPE HOUSES by

G.A, Chown

INTRODUCTION

The thermal envelope house is the p r o d u c t of an innovative approach to

enerw efficient d e s i g n . The concept was developed in t h e 70's in the U.S.A. Known also ;as envelope, double envelope, or double s h e l l houses,

these b u i l d i n g s are i n t e n d e d to provide arch%tecturally p l e a s i n g spaces a d a high degree of comfort w h i l e minimizing heating c o s t s . The

purpose of this Note is to describe the construction and operation of t h i s type

of

CONSTRUCTION

From the outside, a thermal envelope house may look l i k e any other. The

form of the b u i l d i n g is deterrmlned primarily by personal preference and

local architectural trends. The Figures in t h i s Note show a varfety of p o s s i b l e f o m .

In cross-section, a t h e r d envelope house looks much like a house within a house (Figure 1).

. -- A T T I C A I R S P A C E

N O R T H W A L L

The e a s t and west walls are single w a l l s similar to those

in

The north wall consists of two separate p a r a l l e l walls. They are positioned a b o u t a f o o t apart creating an a i r space in between. Each

wall is insulated and contains a continuous airlvapout barrier to

prevent t h e f l o w of air and moisrure between the l i v i n g space end t h e a i r space, and between the air space and the outside.

The w a l l on the sauth side is also made up of two walls. Here, however, they are further apart creating a usable l i v i n g area referred to a$ t h e sun space. The outer wall is mostly glass and may be vertical or s l o p e d depending on the owner's preEerence and how the space is to be used. Both the c e i l i n g and the roof are insulated and incorporate continuous

air/vapour barriers. The attic space between i s o p e d t o the north wall air space and to the sun space.

L i k e . the a t t i c , the basement or crawl space under the house is open t o

the a i r spaces in the north and south walls. The foundation w a l l s and the floor between the l i v i n g s p a c e and the basement are sealed and i n s u l a t e d . The basement f l o o r may be f i n i s h e d but is not insulated. THEORY OF OPERATION

The design of the thermal envelope house is based on the concept t h a t

the heating or cooling of t h e a i r in the sun space w i l l i n i t i a t e convective air flow between the i n n e r and outer envelopes (1).

Proponents of the design claim that the f l o w of warm a i r reduces heat loss from the house and provides the c a p a b i l i t y t o store heat on sunny days and recover it on cloudy d a y s o r at night. Air flow between t h e inner and outer e n v e l o p e s is also intended to provide cooling during the smmer.

Heating and Heat Storage

On sunny w i n t e r days, radiation from the sun warms the a i r in the aua space. Since warmer a i r r i s e s , t h e a i r in the sun space t e n d s to r i s e

i n t o the a t t t c . As the a i r in the north wall lases hear through the i n n e r and outer envelopes, it f a l l s toward the crawl space or basement- In t h i s manner the a i r circulates around the house through the air

spaces (Figure 2). Where temperatures in the a i r spaces exceed those in the l i v i n g spaces, heat I s transferred into the l i v i n g spaces. As the

e a r t h or floor i n the crawl s p a c e or basement w i l l he cooler than the cycling a i r , it w i l l c o l l e c t heat from the air and store it.

A I R I N N O R T H W A L L A I R S P A C E L O S E S H E A T A N D F A L L S I R I N S U N S P A C E G A I N 5 HEAT AND R I S E S H E A T STORED I N BASEMENT F L O O R O R GROUND I N C R A W L S P A C E

Figure 2. Winter Heating and Storage

Heat Recovery

A t night or on cloudy daya in winter, the large areas of glazing in the sun space allow rapid heat loss. The cooled a i r settles into the

basement or c r a w l space setting up -convective air flow fn the d i r e c t i o n o p p o s i t e t o that which occurs during heat gain.

The

A I R I N S U N S P A C E L O S E S H E A T A N D F A L L S T T H R O U G H [ N S U M S P A C E H E A T R E C O V E R E D F R O M BASEMENT FLOOR OR G R Q U N D IN C R A \ V L S P A C E

Summer Coo l i n g

In the summer, the a i r in the wall and attic spaces picks up heat from t h e house, o u t s i d e a i r , and sun. Ry allowing t h e w a r m a i r to escape through a vent at the t o p of ~ h e attic space and by r e p l a c i n g it with cool air drawn through a duct tn the ground, the house is kept cool (Figure 4 ) . W A R I I I A I R R E L E A S E D T H R O U G H V E N T I N A T T A I R G A I N S HEAT FROM H O U S E A N D S U N AND R I S E S A I R S P A C E S T H R O U G H VENT P l PE I N G R O U N D

Figure 4. Summer Cooling

CRITICISM OF THE THEORY

A number of questions have been raised regarding the theory of t h e operation of double e n v e l o p e houses. The major criticisms are r e l a t e d

to heat transfer from the a i r spaces t o the living spaces, t h e p a t h s of

the convection currents and the a b i l i t y of t h e soil o r basement floor to store and release heat.

The amount of heat transferred to t h e l i v i n g spaces or the outside air from the a i r between t h e envelopes depends on the temperature

differences across and the thermal resistances of the envelopes.

Because the temperature d i f f e r e n c e between t h e o u r s i d e a i r and t h e a i r spaces is much greater than t h a t between the air spaces and the l i v i n g spaces, substantially more heat will be lost t o the outside than w i l l be transferred to the i n t e r i o r (2). The efficiency of t h e double envelope d e s i g n as a means of heating the l i v i n g spaces is questionable.

In a number of buildings which have been examined, the temperature dtfference between the north and south air spaces is not sufficient to set up a complete convective air flow l o o p ( 3 ) ; any convection currents tend t o be l o c a l i z e d (Figure 5). Some advocates of the d o u b l e shell system have recognized t h i s problem and recommend the use of f a n s to move t h e air ( 4 ) .

HEAT G A I N - L I T T L E H E A T S T O R E D I N G R O U M D

H E A T L O S S - 1 I T I L E H f A l R E C O V E R Y FROhl G R O U N D

Figure 5. Localized Convection Currents.

Assudng that t h e air w a m d in the sun space does c y c l e around the inner s h e l l and enter the basement, the warmest a i r will pass through t h i s space very close t o the floor above. The cooler air already in the basement w i l l tend to s t a y close to the basement floor or ground.

Therefore the amount of heat which is stored in the ground is small (5).

PERFORMANCE CLAIMS

Proponents af thermal envelope houses claim a variety of b e n e f i t s from t h i s deslgn.

Heating - As t h e house is heated by the sun tn the winter and cooled by the earth in summer, l i t t l e or no auxiliary heating or cooling is required.

Comfort

-

construction was used. With warmer walls the radiative heat loss from the occupants is reduced and comfort is increased.

Air Leakage

-

Sun Space

-

Although thermal envelope homes have proved to be energy efficient, f e w of the benefits can be a t t r t b u t e d s o l e l y to the t h e m 1 envelope design; the concept also presents some problem of its o m .

Because b o t h the inner and outer envelopes are insulated, the combined thermal resistance of t h e two envelopes is higher than it would be in standard single s h e l l construction. Optimistic reports on energy use have tended t o be based on c o ~ a r i s o n s with houses of standard

construction which have half t o one t h i r d of the i n s u l a t i o n . Table 1 shows the d i f f e r e n c e s in i n s u l a t i o n l e v e l s i n four d i f f e r e n t house types: standard t o 1978; CMHC recommended for 5000 "C degree days (61, double envelope and s i n g l e envelope low energy,

Table 1. Comparative Iesulatton Level

cmc

Typ f cal Re commended Typf cal Average Standard Cons tructian Douhle She1 1 Low Energy C o n s t m c t l s n ( f o r 5000 Cdd) Construction Construction

RSI

(a)

( a )

Building Gomponent Roof

-

-

-

-

Total

Basement Walls -

Above grade Below grade Basement Floor

With t h e m 1 envelope houses, particular attention is p a i d t o the

orientation of the glazing. The number and s l z e of north-facing windows is reduced to a minimum and a large percentage of the south wall is

glass. The solar heat gain is therefore considerably higher than in standard construction so the net heat losses are cor~espondirrgly lower. This technique for reducing energy requirements, however, is n o t

restricted to double s h e l l houses. The same advantages can be achieved

in any type of house through proper planning and orientation.

A comparison of double envelope housing with s i m i l a r single e n v e l o p e housing rather than standard construction would p r o v i d e more v a l u a b l e information. A computer simulation developed for t h i s t y p e of analysis i n d i c a t e s that a super ZnsuLated house with a fan-forced rock heat storage would r e q u i r e significantly less auxiliary energy than t h e equivalent double envelope house ( 7 )

,

Comfort

Thermal gradient oalculations (2,9) indzcate that two w a l l systems with

the same therml resistance, one w i t h an air space and one without, will

provide the same degree of comfort. If the double system d i d provide a

significantly higher level of catnfort, it stands to reason t h a t t h e approach would be used in the east and w e s t walls as well as in t h e north and south.

Air Leakage

The comparison of double shell houses to standard construction w i t h respect to draughtiness and heat l o s s by a i r leakage is also misleading, Host double envelope houses have been custom projects. The quality c o n t r o l in these houses tends to be b e t t e r than in standard h o w s so a i r leakage will tend to be lower. Air sealing i n l o w energy housing has been significantLy improved over standard constructfon. The need for

two w a l l s and an i n t e r c e p t i n g blanket of a i r has not been demonstrated.

The usefulness of the sunspace as a 13Lving space or as a greenhouse i s l i m i t e d by the temperature swings to which it is subjected. Except in more moderate climates or where fans

are

Canada, additional heating, coolfng and i n s u l a t i n g systems would be r e q u i r e d t o keep a greenhouse productive year round.

The location of the sunspace with respect t o the rest of the house and t h e yard can a l s o present c o q l i c a t i o n s . Because the sun space extends across t h e f u l l width of the south wall, any d i r e c t t r a f f i c between t h e l i v i n g spaces and t h e yard to t h e s o u t h m u s t p a s s through the sunspace.

I n many d e s f g n s the sunspace becomes p r i m a r i l y a t r a f f i c corridor with Llttle room for other a c t i v i t i e s .

The sun space has been identified as an archttectural highlight of thermal e n v e l o p e design, This f e a t u r e , however, t s not peculiar to these houses and can e a s i l y be incorporated into s i n g l e s h e l l designs.

Other C o n s i d e r a t i o n s

The crawl space, because it is often l e f t open to the earth, may be a source of i n s e c t or animal i n f e s t a t i o n or may a l l m excessive amounts of moisture into the building. Because most of the a i r circulation space

is inaccessible to adults, it may be i m p o s s i b l e to remove any insects ar small animals which get between the north walls or into the a t t i c .

The lack of f i r e s t o p s fa the north wall may lead to r a p i d flame spread in the case of fire. It may be necessary to line both sjldes of t h e a i r space with gypsum board and i n s t a l l automatic dampers to reduce the risk.

Because of the extra f l o o r area required for the a f r spaces, t h e additional foundation wall h e i g h t for t h e crawl space, and the e x t r a windows and wall f i n i s h e s , the cost of a double s h e l l house will be greater than t h a t of the e q u i v a l e n t single shell house. Unless the sunspace can be used year rauad it becomes an expensive luxury. These

costs should be considered in conjunction with the auxiliary heating

requirements and the functional aspects of the d e s i g n .

As there are very few independent analyses of double shell houses, there are s t i l l many unanswered questions. Double shell houses would appear to use less energy for space heating than houses of standard

coastructian. The c o s t effectiveness of double s h e l l construction compared to other low energy approaches has not been demonstrated.

REFERENCES

{ I ) Butler, Lee Porter. ~kose'a H o m e s

-

~ k e s e ' a Inc.

,

( 2 ) Reno, V i c . Shakedown for t h e Envelope &use

-

Solar Age, p , 14-21, November 1980.

( 3 ) Scanada Consultants Ltd. Double S h e l l Passive S o l a r House: A Prabf sf Concept T e s t . 435 MacLaren St.

,

( 4 ) Hix, John. Thermal Homes, THERWL-Homes, 207 meens Quay West,

Toronto, M5J 1A7, 1980.

(5) Rodgers, Russel. Performance of a Double Skinned Envelope House in Nelson, B . C . Proceedings af the J o i n t S d f a r Conference, Solar Energy S o c i e t y of Canada Inc., and the P a c i f i c Northwest Solar Energy Association, August 1980, p. 87-91.

6 Canada Mortgage

a d

(7) Koher, J . and Lewis, D. Shakedown for the Envelope House - T r i a l by Computer. Solar Age, p. 22-27, November 1980.

( 8 ) Reno, V i c . Shakedown for the Envelope House

-

Solar Age, p , 14-21, November 1980.

( 9 ) Shurcliff, William A. Superinsulated Houses and Double-Envelope

Houses - A Preliminary Survey of Prlnciples and Practice. W i l l i a m A. Shurcliff, 19 Appletou St., Cambridge, MA 02138,