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Effect of exfiltration on the hygrothermal behaviour of a residential wall
assembly : Results from calculations and computer simulations
Ojanen, T.; Kumaran, M. K.
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Effe c t of e x filt ra t ion on t he hygrot he rm a l be ha viour of a re side nt ia l
w a ll a sse m bly : Re sult s from c a lc ula t ions a nd c om put e r sim ula t ions
N R C C - 3 8 7 8 3
O j a n e n , T . ; K u m a r a n , M . K .
S e p t e m b e r 1 9 9 5
A version of this document is published in / Une version de ce document se trouve dans:
International Symposium on Moisture Problems in Building Walls, Porto,
Portugal, September 11-13, 1995, pp. 157-167
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Tuomo Ojanen and Kumar Kumdran
BPFECT OF EXFILTRATION ON THE HYGROTHERMAL BEHAVIOUR OF A
RESIDENTIAL WALL ASSEMBLY: RESULTS PROM CALCULATIONS AND COMPUTER SIMULATIONS
VTT Building Technology Laboratory, Indoor Environment and Systems, ?L 1804, F -02044 VTT. Finland
Building Performance Labora ory, Institute for Research in Construction, National Research Council Canada
157
ABSTRACT
The ｨｹｧｾｯｴｨ･ｲｭ｡ｬ behavio 0 timber frame all is
analysed using a steady-state calculat' on method dnd a
ｾＭ､Ｇｭ･ｮｳｩｯｮ｡ｬ heat. a'r and moisture transport compu er model, The conditions leading to exfiltration of warm nd urn'd indoor air are examined. The physical quantities calculated inc de the amount of mo'sture accumulated in t e wall cavity during the heating season nd he heat loss across the wa 1. Several interesting correlations emerge, These corre ations show the advantage of using he analytica methods in deriving design gu'delines or building corn 0 ents. The results from he analysis re
used to identi y and quantify various parameters ha govern t pe -ormance 0 i- barrier systems.
RESUME
Les auteurs analysent Ie comportement hygrotherm'que 'un rour
a
charpente e bois d'oeuvre au moyen d'une methode de ca u en reg' e permanent e d' un modele in ormatique bidimensionnel de transport ae chaleur, d' ir e d'humidite. s examinent egalement les conditions prop'cesa
ＱＧ･ｸｦＧｬｴｲ｡ｴｾｯｮ de I'air interieur chaud eL humide, Parmi les grandeu s physiques mesurees se trou en la ancite ' humidite accumulee dans 1a cavice murale au cours de 1a saison froide e les pertes de chaleur u travers u mu. Plusieurs ｣ｯｲｲｾＱ｡ｴｩｯｮｳinteressantes emergent de ce te etude et man rent comment i1 es t a vantageux e recourir aux methodes ana 1y iques pour me tre u point des Ii nes d'rectrices isanc la conception des composants u ba 'ment, Les esultats de I'analyse ser ent
a
dete miner eta
quant' ie les Ivers parametres qu regissent la performance des systemes d'etancheitea
l'air.1. INTRODUCTION
The most common construction ｴ･｣ｨｮｾｬｯｧｹ used for residential buildings in Canada consists of the timber frame that creates a wall cavity which is filled with an insulating material. The insulation is usually sandwiched between an interior gypsum board and an exterior sheathing. During the heating season, moisture from the warmer indoor air can end up inside the cavity as condensed moisture. This happens through two processes: diffusion and exfiltrationloutward air movement). The process of di ffusion can occur throughout the wall surface. This can be effectively controlled by using a continuous vapour retarder at the warm surface of the cavity. This is usually achieved by placing a polyethylene membrane between the interior gypsum board and the insulation. By following this procedure. the condensation due to vapour diffusion may be effect.ively conr:::rolled{l]. When it occurs. t.he amount condensed is usually negligible. But the same can nor::: be said about condensation due to exfiltration. Exfiltrar:::ion mostly occurs at joints, holes and cracks. It is generally acknowledged that the amount of moisture condensed due to exfiltration is many times larger than that due to diffusionl21 (for, example see the calculations given below). Hence, in building technology, methods have been developed to effectively control the rar:::e of exfiltration. This is achieved by the installation of an air barrier in the wall assembly. The technology associated with the air barrier is rather complex in comparison wit.h r:::hat of r.he vapour retarder. It may not be appropriate to r.alk about a single material in the wall assembly acr.ing as an air barrier. The whole assembly participates in controlling the air movement. However, many innovative products are being introduced in the market place and sold as air barrier systems. Recent research activities at the Institute for Research in Construction were directed at the evaluation of the performance of these systems. This paper documents some results from a series of preliminary calculations and qualitatively shows the relation between exfiltration, heat transfer and moisture accumulation.
'I'he calculations were done eit.her using simple sr:::eady-state methods or a complex. two-dimensional heat. air and moisture transport model called TCCC2D(Transient-Coupled-Conduction and Convection in 2- Dimensions). developed at VTT Finland. Several applications of this model have been reported elsewhere(!, 3-6J.
2. RBLAT:IVB MAGN:I'l'ODES OP HOISTORE ACCOHULATION COB TO DIFFUSION AND EXPILTRATION
Let us consider a 2X4 residential wall with a type II (water vapour permeance _ 60 ng.m-1•Pa-I • 5"11 vapour
retarder. Let the indoor air be at. 21 °c wiLh d WdLtH
vapour pressure of 1200 Pa (i.e. 48' RHI. If the exterior te=perat.ure is -15 °C and the water vapour pressure stands at 100 Pa(i .e. 60 , RHl. moisture condenses within the cavity at a rate - 4 ｧＯＮｾＯ､｡ｹＮ Now suppose that the indoor air moves into the cavity at a rate - 1.4 ｌＯｭｾＯｳＮｉｔｨｩｳ is roughly equivalent to an orifice flow induced by 10 Pa pressure difference through a hole. 2 cm in diameterl. The rate of condensation increases to = 480 ｧＯｭｾＯ､｡ｹＮ
3. INTER-RELATION BIMWEEH EXPILTRATI:ON RATE, HEAT TRANSPeR AND HOI:STIJRE ACC'DH1JLATI:ON
3.1 Steady-.tate calculations
Consider the same wall assembly used in the previous calculation. All boundary conditions are the same. except. the indoor relative humidity - ]6 1. Let the air leakage characteristics of the assembly be varied between 0.001
ｌＯｭｾＯｳ and 10 L/m1/s at 15 Pa pressure difference. Let us
now subject the wall to a pressure difference of 50 Pa to induce exfiltration. The heat flux and moisture accumulation can be qualitatively calculated by modifying the steady-state Glaser methodl?) - mudi[ied in the sense that the contribution of the heat capac:ity of the exfiltrating air towards the heat transfer can be
expressed as an inc:rease in the t.hermal conductivity of the wall assembly. The result.s, though not. quantitative. clearly establish the inter-relation between the
exfiltration race, heat transfer and moisture
accumulation. as shown in Figure 1.
As the air flow rate increases, the heat flux also proportionally increases. On the other hand. as the air flow rate increases. the moisture accumulat.ion rate increases up to a point. beyond which it decreases and then abruptly goes to zero. The rea501l fOt" this will
become clear as we look at the more quantitative calculations given below.
3.2 TCCC2D calculaeioD.
Figure I also contains results from more ｡｣｣ｵｾﾷ｡ｴ･
calculations done. using TCCC20. The trend indicated by
:.us
Pall
0.06 0.08 0.1 0.12 OUTER SURFACE. mP
20 ui § 10i
w 0｜ｲＬｬｹＺＮＮＮＬＮＮｾＢＧＢＢＢＢｾｾＭＭＭＭＭＭＭＭｬ Q.. :! ｾ ·10 -20'--_--L.._---'-_----JL--_-'-_-.L._---J o 0.02 OISTIv'DISTIJRE PROBLeAS I BUILD' G W
Fig.2- Changes in the temperature dist ibution within the cavity with changes in the rate of air
flow.
160
Fig.l- Inter-relation between air leakage rate, heat transfer and moisture accumulation.
30,....---.
>:" 1 ...-- ---.400 c( q 0.81
1!>Q..6 .:>C ｾ 0.4ｾ
02 ｾ 0ｬＭＭＭ｡Ｚ］］ｉ［［［［［ＡｾｾｾＮＮＮＮｩＮＮｊ｟ＭＭＭｉ 0 0.000 0.001 001 O. 10 100lEAKAGE RATE. U(rnZ.s.75Pal
Figure 2 clearly shows the reason or the pa ern 0'::
dependence 0 the mo' sture accumu ation on ir flo ra e.
As the air flow rate incre ses, he indoor boundary conditions encroach more and more in 0 the cavity. Consequently, the cavity becomes increasingly w rmer. At some stage i t becomes so warm ha condit'ons for condensatio cease to exis .
seen that the rate of moisture accumulation is grossly over-predicted by the steady-state method. A he same time, in the present example, the heat flux calcu a ed by
4. EFFECT OP EXPILTRATION PATH ON THE RATE OF CONDENSATION In the examples given above the exfiltration was diffused or distributed across the whole surface. As mene' oned in the introduction. the exfiltration usually occurs through cracks and holes. In such a case the amount of moisture condensed within the cavity depends on the deta'ls of the air flow path. In a series of simulat' ons using TCCC2D
five different air f ow paths, as shown in Figure 3, we e considered.
Fig.3- Five different patterns of ex i tra two parallel lines represent a cavity arrows show the points 0 ent.ry and exit
all processes consis 0 ex iltra ion.
on; n of the he ir;
For a 1.85 high sec ion of the ca ity 45 mm ｾ e, illed with mineral tiber ins lat' on, he pe cen age 0
moisture condensed is listed in Table 1. The ir ow entry corresponded to :; 6 g/h of moisture enterlng the cavity at a temperature of 20 °C. The exterior temperature was assumed to be -10 °C. The exUlt ation in 1 and 5 deposits more than double he amount of moisture deposited
in the other three cases; the ex r tion Pi'! hs being onger in the two cases, the cavity is in contac with he exfil ra log humid air for a longer per"ad and more oisture gets depos' ed . n the ca i y. ccord' ng to he present calculation, he ex-iltra ion path in -1 represents a worst case scenario of mois ure aceum la ion. This case was further examined with an emphasis on the ettee of geographical location on the amount of moisture accumulated in' the cavities due t.o exfiltration. The results were reported in reference[3]. Subsequent analysis of the results showed tha moisture accumulation is strongly correlated to the weather data, such as rhe beating de ree daysrSI.
Table 1. The effect of the ex iltra ion path on the amoun of moistu e ac ulac"on.
Exfiltration path Accumulation, %
5. INFLUENCE OF INDOOR HUMIDITY ON MOISTURE ACCUMULATION 140 120 100
E
ｾ
80x
::J 60 [[ 408
J: 20 0 10MJISTURE PROBLEMS IN BUILDING WALLS
0.001 0.01 0.1 1
LEAKAGE RATE, U(mZ.s.75 Pa)
o
ｌＭＭＭＭＮＮＺ［ｾｾＺＺＺＺＺｲＺＮ｟｟｟｟ｌ｟｟｟ＮＡ｟｟Ｎｊ
0.0001 0.8r - - - -__
s:-<
ｾ 0.6 E:a;
ｾ 0.4a:
ｾ
0.25
:EFig. - Thp. p ct of indoor humidlCy level on the
amount of moisture accumulated within a cavity due to exfiltration; the effect of the RH on the heat flux is negligible.
6. EFFECT OF ADDITIONAL THERMAL RESISTANCE PROVIDED BY THE
EXTERIOR SHEATHING
-eoMOISTURE (36 % RH) ｾ HEAT FLUX (36 % RH)
-9-MOISTURE (48 % RH)
"*
HEAT FLUX (48 % RH)As the simulations whose results are shown in F' gure were repeated with changing the indoor 'h idity leve roo 36 % to 48 • the mo' sture acc ula ion pattern changes as shown in Figure 4. his illustrates that the indoor humidity level of the built environment deserves proper consideration at the design stage of an air barrier system. The allowable leakage rate will strongly depend on the humidity level of the indoor air.
The above calculations were further altered by ssuming that the exterior sheathing was a 25 mm dense mineral fiber board. The results are shown in Figure 5. The effect is dramatic. The amount of moisture retained by the cavi y is signi ficantly reduced. The s e is true 'Lh the hea
flux, n added thermal resistance may thus be more forgiving to increased level of indoor humidity. These aspects can be advantageously used in the design of a proper air barrier system.
o
ｌ｟ＮｅｾｾｾＺＺＡＺｾｾｾｕ
0
0.0001
0.001
0.01
0.1
1
10
LEAKAGE RATE. U(rn2.s.75
Pal
>"
0.8 ...- - - , 150
C§
ｾ
0.6
=a;
.:.l: uj0.4
a:
ｾ
02
o
:E • UOlS1\RO (36*'
I£ATR.UX("8-eotEATR.UX(36ＧｉＮｾ . . UOISlU'Eｉｾ ｾ
. . UOISll.llE(ADDEDFWALUE) -t:rHEATR.\lX (ACOED FWALUE)
Fig.S The effecc of t.he add· tiona he m 1 resistance prov'ded by the exterior sheathing.
7. VAlUOUS CORRELATIONS PROM TCCC2D SIMULATIONS
If one carries out a systematic analysis of the hygrothermal performance of any given ype of construction. many useful correlations result. These correlat.ions are invaluable in arriving at. good design guidelines. An example of this approach is given be ow. The exfiltration simulated is shown in Figure 6.
<=
Fig.6- A timber-frame cavity wall with a crack on the inside finish allows air to enter the cavity. The exterior sheathing that acts as an air barrier allows only diffuse air flow out of the cavity.
Table 2. Various parameters used in simu ations; eaco simulation
is
given a code.The parameters used in the simulations are summarized in Table 2. The exterior boundary corresponded to Otta1fla weather data (temperature, RH, wind velocity a d direction). The interior temperature was always maintained at 21°C. The width of the cavity was 140 Mm.
7.1 Observatious 1
The moisture index for all the simulated cases are listed in Table 3. As the rate of exfiltration increases through B • 82. 83. 84 and 85. the amoun 0 moisture accumulated shows a corresponding incr ｡ｾｰＮＮ This correlat.ion can be used to specify the allowable air leakage through the wall. provided that the information on the threshold for the accumulation 0 moisture in terms of the durability of
various component:s exists. As the rate of exfiltration
t>AOlSTURE PROBlEMS IN BUILDING W US
The simulations were carried out to analyze the hygrothermal behaviour of the cavi y for one full year. on an hourly basis. and to derive information on t e moisture accumulated in the cavity. the distribut.ion 0
moisture and temperature within the cavity and the heat flux across the cavity. The results for the presen discussions are expressed in relatio to that for the case
BO, referred to as the base case. The heat flux and the moisture accumulated are averaged on a daily basis an integrated over the whole year _ Le us call hem a hea loss index and a moisture index. respect.ively. Two interesting sets of observations are summarized below.
1M
Code Air Permeance Exterior Interior Additional
of the air Vapour RH Thermal
barrier Permeance 'i; Resistance
L/(rn1.s.75 Pa) ng/ (m' .s. Pa) K.ml /o1
BO 0 170 35 0 Bl 0.05 170 35 0 B2 0.1 170 35 0 B3 0.2 170 35 0 B4 0.35 170 35 0 B5 0.5 170 35 0 A2 0.1 60 35 0 D2 0.1 400 35 0 E2 0.1 800 35 0 C2 0.1 1500 35 0 B2RH 0.1 170 50 0 B2R 0.1 110 35 0.75
.'
.
remains constant and the water vapour permeance of the exterior surface increases through Al, B2, 02, E2 and C2, the amount of moisture accumulated decreases. This suggest.s that higher vapour permeance of t.he exterior surface allows higher air-flo rate for a given threshold for o':'st-ure accumulation. As B2 and B2RH are compared, for t.he same exfilt.rat·on rat.e, an increase in t.he relative humidity of the exfiltrating air from 35 \ to 50 % results in an increase in the moisture ｡｣｣ｵｭｵｬｾｴ･､［ the effect s roughly equivalent to increasing the exfiltration rate five times, as seen by comparing B2, B5 and B2RH. The influence of added exterior insulation is clearly demonstrated by co paring B2 and B2R.
Table 3. The hygrothermal performance of the cavity simula ed using t.he various parameters in Table 2.
Code Moisture Heat loss
index index BO I 1 BJ. 1.24 1.04 B2 1.48 1. 07 B3 1.95 1.14 B4 2.55 1. 23 B5 3.07 1. 32 A2 1.64 1. 07 02 1.28 1. 07 E2 1.05 1.07 C2 0.82 1. 07 B2RH 3.02 1.09 B2R 0.78 0.89 7.2 Observations 2
The heat loss index for all the simulated cases are also listed in Table 3. The ma·n observation is the correlation between the ra e of ex!iltration and the heat loss, in the cases B to B5. If the designer puts a l'mi on the heat loss, . t automatically defines the allowable exfiltration rate. The latent heat part of the heat loss appears to be insignificant in the case analysed here, as shown by A2, B2, 02, E2 and C2; increased water vapour permeance of the air barrier system has negligible effect on the heat loss index. As shown by B2RH, increased relative humidity of the exfiltrating air has ittle effect on the heat loss. he case B2R indeed shows the effect 0 - the increased
thermal res' stance.
9. ACJrn'OWLEDGBKENTS 8. CONCLUDING ｾ ｓ
MOISTURE PROBLEMS I BUILDING W US
The authors gratefully acknowledge the ｣ｯｮｾｲｩ｢ｵｴｩｯｮ of Mr. B. Di Lenardo, Mr. W. C. Brown, Mr. . A. Dalgliesb and
r. G. F. Poirier of the Institute for Research i Construction "n defining the parameters ｬｩｳｌｾ､ in Table 2. in connection with the development. of the technical guide for air barrier systems [9] . Discussions Mr. J" C. Haysom are also acknowledged.
The hygrothermal behaviour of any building component is determined by the complex interaction of heat. air and moisture transport processes. The building componen considered here is a timber frame wall provided with an air barrier system. The results presented above clearly illustrate that technical requirements for he air barrier system for a given type of construction are de ined by
factors such as the effective air leakage rate, exterio and interior boundary conditions and heat and moist.ure transport properties of the bounding surfaces of the building component.. A 1 these factors can considerably
vary from locat.ion to location and from biding to building. Hence it may not be possible to design the air barrier system based on just experience tha is related only to some of the factors. The results presented in this paper show the potential advantage of using prope me hods of calculations and computer simulations for developing design guidelines. Some of the information presented here has already been sed to develop a technical 9ui ｾ Eo air barrier systems[91 and to re-define an art·c e i relation to the use of exterior insula ing shea hing in the National Building Code[lO) with reference to low-rise residential construction.
ith increased understanding of the phys'cs of comb·ned heat, air and moisture transport. through b ilding materials and components and rapid advances in computer technology and numerical methods, more and more sophisticated computer models are emerging[ll]. These models make it possible to analyse the hygrothermal behaviour of any type of construction det.ails subjected to any given sets of bounnary condit.ions. Full adval Lclge of these models should be taken to generate guidelines for the design stage. The models usually being com_ ex, this work should be ｵｮ､･ｲｾ｡ｫ･ｮ by the building researchers, wit.h appropriate input from building practitioners.
1.0. RBPBRENCBS
[lJ Karagiozis, A. N. and M. Kumaran. Computer model calculation on the performance of vapor barriers in Canadian residential buildings, ASHRAE Transactions. Vol 99(2), pp. 991-1003, 1993.
(2) Latt-a, K. Vapour barriers: what are they? Are they effective? Canadian Building Digest. Division of Building Research, NRC Canada, CBO 175, 976.
(3J Ojanen. T and M. K. Kumaran. Air exfiltration and moisture accumulation in res'dential wall cavi ies, Thermal Performance of Exterior Envelopes of Buildings V. Proceedings of ASHRAE/OOE/BTECC Conference. pp. 491-500.
1992.
(4) Ojanen, T., and R. Kohonen. Hygrothermal influence of air convection in wall structures. Thermal Performance of Exterior Envelopes of Buildings IV, Proceedings of ａｾｈｒ
Conference, pp. 234-249. 1989.
(5) Morrison, I . D., . N. Karagiozis and M. K. Kumaran. Thermal performance of a residential dynamic wall. hermal Performance of Exterior Envelopes of Buildings V. Proceedings of ASHRAE/OOE/BTECC Conference. pp. 229-234, 1992.
[6] OJ an en , T. and R. Kohonen. Hygrothermal performance analysis of wind barr' er struccu es. ASHRAE Transac ions
(in print) 1995.
[7] Glaser. H. Graphisches vefrhren zur untersuchung von diffusionsvorgangen. Ka tetechnik 10. pp. 345-) 9. 1959. Also. ｾＮ Seiffert. wasserdampf Diffusion 1m bauwesen.Bauverlag GmbH. Wiesbaden und Berlin, Chapters 6 - 18. 1982.
[8) Climatic Information for Building Design in canada
1977. Supplement o. 1 to the ationa) Building Code 0
Canada. CC o. 15556.
[9] Oi Lenardo. B., C. Brown. Dalgliesh. G. L
Poirier and M. K. Kumaran. Air barrier systems for exterior walls of low-rise build'ngs, CCMC Technical Guide Master Format 07195. Institute for Research in Construction. NRC Canada, 1995.
[10] ational Building Code of Canada. Part 9. Article 9.25.5.2. 1990.
[11] Hens. H. and . Janssens. gnquiry on HAMCaT Codes. Report Annex 24. Task 1. Modelling. International Energy Agency Annex XXIV on Heat, Air and Moisture Transfer in Insulated Envelope Parts.
