Condensation control in stressed skin and sandwich panels

Publisher’s version / Version de l'éditeur:

Forest Products Journal, 12, 9, pp. 429-430, 1963-01-01

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

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 [email protected].

Questions? Contact the NRC Publications Archive team at

[email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information.

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.

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

Condensation control in stressed skin and sandwich panels

Platts, R. E.

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.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=bf305fbe-37ac-4d30-80cd-639b5d8b5d08

https://publications-cnrc.canada.ca/fra/voir/objet/?id=bf305fbe-37ac-4d30-80cd-639b5d8b5d08

Ser

I

TH]"

N21t2 ,

no. l-\9

e . 2

I

i n r c c 1

NarroruAL REsEARcH Cour.rcll-

CANADA

DIVISION OF BUILDING RESEARCH

CONDENSATION

CONTROL

IN STRESSED

SKIN AND

SANDWICH

PANELS

B Y

_{A N A L Y Z E D}

R. E. PLATTS

R E P R I N T E O F R O M

FOREST PRODUCTS JOURNAL, VOL. XII, NO. 9, SEPTEMBER 1962,

P. 429 - 430.

D I V I S I O N O F B U I L D I N G R E S E A R C H

OTTAWA

J A N U A R Y I963

i-S'$-5t

q A/) L)34

Thle publication ie being dlstributed by the Dlvlsion of Building Reaearch of the National Reaearch Gouncil. It ehould not bereproducedin wholeor ln part, without permis-aion of the original publisher. The Divieion would be glad to b e o f a e a l e t a n c e i n obtaining euch permiseion.

Publicatione of the Divieion of Building Reeearch may be obtained by mailing the appropriate remittance, (a Bank, Expreaa, or Post Office Mcney Order or a cheque made pay-able at par in Ottawa, to the Receiver General of Ganada, credit National Reeearch Council) to the National Reeearch Council, Ottawa. Starnpe are not acceptable.

A coupon ayetern hae been introduced to rnake pay-menta for publicatlong relativelye irnple. Ccupone are avail-able in denorninatione of 5, 25 and 50 centa, and may be ob-tained by making a rernittanrce aa indicated above. Theoe coupons rnaybe ueed for the purchase of all National Reaearch Council publlcaHona including specifications of the Canadian Governtnent Specificatione Board.

Condensation Control in

Stressed Skin and Sandwich Paneld

R. E. PLATTS

National Research Cotrncil Ottawa, Canada

Qrnesseo sKrN AND sRNlwcn de-tJ sign conflict with the usual recom-mended arrangement of "skins" for condensation control, in which t h e

Accordingly, m a ny manufacturers of stressed skin bgilding _{.panels expend} conslcefaDlc ettoft tlttrng vapor bar-riers or back-painting the inteiior skin with an asphalt vapor barrier paint;

rA contribution from the Division of Building Research of the National Research Council of Canada; pub-lished with the approval of the Direc-tor of the Division.

Are vopor borriers qnd vents needed in tighnreighr

building ponels? Here is evidence thqt shows they mighr not be

-even under severe conditions in northern cqnqdq, where

con-densqtion is o problem in conventionol wood-frqme sysfems.

4N

NRC 7057

others cut vent slots in the framing next to the outer skin and detail the assembly to vent these to the outside air. Sandwich manufacturers

some-Vapor Diffusion Plays Small Part The ,Division of Building Re-search first considered the problem of condensation control in fl o s e d panels in relation to the Canadian Army's "General Purpose" stressed skin hut. Calculations show that one winter's condensation can onlv amount io ap-proximate]y O.O4 psf of panel, considering diffusion alone and assum-ing 30 percent relative humidity and 70'F. inside, and 0'F. and 80 percent relative humidity outside. The ialcula-tions use the accepted permeance of

0.7- perms for exterior grade plywood, and assume that for a 3-month period all vapor entering the panel condenses within it and none escapes across the outside skin. Even if all this water is held by the framing and the inner veneer of the outer plywood skin and none by the insulation. the moisture content of this veneer would be raised by only 3 percent. In any one winter then, the water vapor condensation in a closed stressed skin plvwood panel should not be harmflul. the next iues-tion is more difficult to resolve: is there a long-term moisture build-up be-cause of the inabilitv of summer- drv-ing to completely rehove the winteis condensate? Variation in summer

dry-The Author: R. E. Platts is re-search officer, Housing Section, Di-vision of Building Research, National Research Council, Ottawa, Canada. F O R E S T P R O D U C T S _{J O U R , N A L}

tng condiiions is great and the effect on" closed panels i-s difficult to calcu-iate. Because of t h i s, the Division could not recommend the elimination <-'f vapor barrier backpainting in the_''GP"-huts,

and it wairetained at the t i m e ( 1 9 1 0 to 1 9 t 1 ) .

Recent Views on Condensation Damaging condensation is a com-mon occurrence in conventional con' structions in Canada, particularly in attics and roof spaces.- Emphasis- on preventive techniques has centered on ionventional structures that allow easy placement of simple vapor barriers to iestrict diffusion and allow the use ot

than in attics, but here too it is mass air flow through holes in the interior skins that cause"s the major problems'

a) Since fixtures and services are not usuallv placed in a stressed skin panel. the'inher skin can form an un-broken barrier to air and vaPor over the entire panel surface'

rather than assumed in site finishing.

430

The above does not imply that the panels are hermetically sealed - exper-i e n c e u ' i t h d o u b l e - g l a z e d w i n d o w s shows that hermetically sealed build-inch thick panel "pumps" all its air from the house intelior-(at 70"F. and 30 percent relative humidity) for 100 days, with nights of 0'F. and days at 20"F. and assuming that all air pumped in is moist. and all air pumped out is dry (dehumidified by the cold outer skin), then only two complete air changes are effected and the moisture accumulated by "pumping" is much less than 1/100 of the condensate from diffusion.

Even with the small amount of win-ter condensate by diffusion and pump-ing, the question of summer drying of closed panels still remains.

Field Experience with Closed Panels Long-term use in adverse conditions provides the best check on the ade-guacy of summer drying in removing each winter's condensate from these panels. Private companies have pro-i , pro-i d . d r n u . r y s t r e s s e d ' s k i n a n d s o m e sandwich panel units for severe uses across the North, since 1947. Some companies omitted the vapor barrier painiinq while others retained it. Most io not-use vents. All report that they have never noted evidence of condensa-tion in any closed panels, floors, walls, or roofs. These buildings are often used as kitchens, diners, and shower

A-performanee check was made by the author on buildings at Churchill and Resolute Bay during a field trip in May. 1960. In the Far North the summers are cool and short with very

since 7947). The most valuable ex-ample seen on this trip was a "Timber Structures Incorporated" (U. S. A.) stressed skin structure erected in 1947 and used as kitchen-mess hall until 1917. From that date it had been used

as combined school, married teachers' quarters and shower-room, with more than a score of Eskimo pupils showe:' ing each week. Holes drilied in the wall and roof panels of the shower

room showed no interior wetting; even the outer plywood was ciean, dry and light in color. These panels were com-pletelv filled with uncovered minerai il'ool'and had no vapor barrier and no vents of any kind.

Similar checks of floors, walls and roofs of other stressed skin buildiogs in constant use in Resolute and Chur-chill showed similar dryness; most of these had back-painted vapor barriers. In contrast. most of the conventional site-finishd buildings showed some wetness in the walls, but no resulting damage, indicating they evidently dr! well in the summers. These walls have plywood cladding on the outside, va-por barrier and gypsum board on the inside, 3- to 4-inch insulation, and no venting. More irnportant, all conven-tional attics showed moderate to very severe wetting, no matter how well vented.

Conclusions

Evidence obtained during this field study, which was part of an overall investigation of prefabricated buildings for the North' indicates that closed stressed skin oanels can remain free from condensation damage without vents or vapor barriers. This obviously holds true for all closed, glued panels with complete edge tightness and with inner and outer skins neady equal in permeance, at less than one perm. Even i,here the outer skin is tight (metal. for example) the amount of winter condensatE will be little more than be-fore, since little moisture would escape across a one-Perm ooter skin (since the vapor pressure differential across it would be close to zero). Sfith a metal outer skin, however, one cannot as yet say that summer drying will be com-plete. At this time it must be rc:om-inended that the skins be nearly egual in permeance or that the inner skin be the tighter one, where venting is not provided.

This recommendation applies equally well to sandwich panels with highly permeable cores if bdqes are serled. If ilosed cell cores are- used, however,

such panels should be satisfactory ei'er if services and outlets break through the inner skin, and with edges not sealed, assuming of course that the core completely fills the Panel.

2Platts, R. E. 1960. Prefabrication in northern housing. National Re-search Council, Division of Building Research. Nov. 23p. NRC 6059.