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Technical Note (National Research Council of Canada. Division of Building Research), 1961-11-01
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Condensation Control in Stressed Skin Panels
Platts, R. E.
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https://nrc-publications.canada.ca/eng/view/object/?id=8acbd58d-d084-450c-aa3a-58c96ad04418 https://publications-cnrc.canada.ca/fra/voir/objet/?id=8acbd58d-d084-450c-aa3a-58c96ad04418
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'fEClHIN II CAlL
NOTlE
No.
350
NOT FOR PUBLICATION FOR INTERNAL USE
PREPARED BY Ro Eo Platts CHECKED BY HBD APPROVED BY NEH
PREPARED FOR General Distribution
ｾ November 1961
SUBJECT CONDENSATION CONTROL IN
STRESSED SKIN PANELS
Stressed skin and sandwich design conflict with the usual recommended arrangement of "skins" for condensa-tion control, in which the outer skin is relatively permeable compared with the inner skino The stressed skin panel in its simplest form presents two roughly equal, tight skins with a water vapour permeance of less than one permo Vapour entering the panel during the winter should condense within ito Accordingly, many stressed skin building manufacturers expend considerable effort in back-painting the interior skin with an asphalt vapour barrier paint; others cut vent slots in the framing next to the outer skin and detail the assembly to vent these to the outside airo These steps add to the complexity and cost of the panel and evidence here summarized suggests they are unnecessaryo
The Division of Building Research first considered this problem in relation to the ArmyUs "General Purpose"
stressed skin hut designo Calculations show that one winterYs condensation can only amount to approximately 001 Ib/sq ft of panel, assuming 30 per cent RH and 70°F inside and a zero
vapour pressure differential across the outSide skino 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 7 per cento
In anyone winter then, the water vapour condensation in a closed stressed skin plywood panel should not be harmful0
The next question is more difficult to resolve: is there a long-term moisture build-up, because of the inability of summer drying to completely remove the wintervs condensate?
2
-Variation in summer drying conditions is great and the effect on closed panels is not amenable to calculation. Because of this, the Division could not recommend the elimination of vapour barrier back-painting in the "GP" huts, and it was retained at the time
(1950-1951).
Private companies continued to make the "GP" type hut for general marketing as all-purpose huts when the Army contract was completed, and still do so with some modifica-tions. One company soon dropped the vapour barrier painting while another retained it. Both report that they have never heard of condensation in any panels, floors, walls, or roofs. These bUildings are often used as kitchens, diners and shower halls. These companies completely prime paint in the shop
so that both skins are still more impermeable than the ply-wood alone. All hidden joints in the completed building are protected by this paint. A third major manufacturer incor-porates small vents in most panels on the past advice of the Division.
Recently, 0plnlon has formed that mass air flow, rather than or in addition to vapour diffusion, is nearly always the cause of severe condensation. The water and ice that so often build up in attics especially can only be explained by such mass air flow through actual openings. Electrical fixtures, holes for plumbing stacks, breaks in vapour barriers, trap doors and hollow partition tops provide openings into most attics. The gravity or "stack action" effect of the heated air provides a continuous force all winter to move the air and the moisture into the attic; its velocity through such holes can often be felt. This steady stack action force does not act as strongly across walls but here too it is the actual holes in the interior skins that cause major condensation. Real trouble in walls is much less common than in attics.
These findings have certain adverse implications for conventional structures (venting can aggravate stack action, for example), but they help explain the lack of problems with closed stressed skin panels in this way:
(a) Since fixtures and services cannot easily be placed in stressed skin panels, and since the panels are closed so that no joints lead into them, the inner skins usually form a true and unbroken barrier to air and vapour.
(b) Since stack action cannot create a force
differential across an interior skin unless the panel is vented to the atmosphere, closed
". ,
3
-stressed skin panels should allow little mass air flow even if small breaks are present on the interioro
This suggests that the major problem of air leakage is obviated in stressed skin panels (if free of services and fixtures) 9 and only the lesser problem of vapour diffusion existso Given the small amount of winter condensate by diffusion only9 the question of summer drying
still remainso
Long=term use in adverse conditions allows the best check on the adequacy of summer drying in removing each wintergs condensate from these panelso A performance check was made on buildings at Churchill and Resolute Bay during a field trip undertaken in May 19600 Resolute in particular was chosen because it has the oldest stressed skin plywood bUildings in the Far North, and some of these were used continuously0 The most valuable example checked on this
trip was a "Timber Structures Incorporatedtt (UoSoAo)
stressed skin structure erected in 1947 and used as kitchen-mess hall until 19579 and from then as combined school,
shower-ｲｯｯｾｭ｡ｲｲｩ･､ teachersu quarters with 28 Eskimo pupils showering each weeko Holes drilled in the wall and roof panels of the shower room showed no interior wetting; even the outer plywood was cleanv dry and light in colouro These panels were completely filled with uncovered mineral wool and had no vapour barrier and no vents of any kind0
Similar checks of floors9 walls and roofs of other stressed skin bUildings in constant use in Resolute and
Churchill showed similar dryness; most of these had back-painted vapour barrierso In contrast9 most conventional
site=finished prefabs showed some wetness in the wallsg but
no resulting damage9 indioating they evidently dry well in
the summers0 More important9 all conventional attics showed
moderate to very severe wetting, no matter how well ventedo Evidence obtained during this field ｳｾ､ｹＬ which was part of an ｯｶ･ｲ］｡ｬｾ investigation of prefabricated buildings for the North
*9
indicates that closed stressedskin panels can remain free from condensation damage without vents or vapour barrierso This obviously holds true for all closed, glued panels with complete edge tightness and fairly impermeable skins, and can therefore be applied in structural sandwich panel designo
*
Platts9 Ro Eo Prefabrication in northern housing. NationalResearch Counci19 Division of BUilding Research,
