Do I need a smart vapor barrier in my wall?
user-6323668
| Posted in Green Products and Materials on
I am building a net zero house, 2000 SF, 2 stories in Mystic CT. It has 2 full baths, heating and cooling by 2 mini splits, ventilation with an HRV, range hood in kitchen. The walls are double stud with Zip/OSB sheathing and densepack cellulose and sheetrock on the inside. Do I also need a smart vapor barrier such as Intello behind the sheetrock?
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Replies
Courtney,
To reduce moisture accumulation in the exterior sheathing during late winter, a smart vapor retarder on the interior side of a double-stud wall is a good idea.
It's also a good idea to include a ventilated rainscreen gap between the siding and the WRB.
Most experts would also advise you to choose a different type of sheathing -- something more vapor-permeable than the OSB sheathing you are planning to use.
For more information on these details, see How to Design a Wall.
thanks, Martin. The ZIp wall is already in progress so I guess that makes the other provisions more critical. I have read that the north wall is more prone to moisture accumulation due to lack of direct sunlight. Is there any logic in applying the smart vapor retarder only on the north wall to save money?
Using a much cheaper smart vapor retarder such as MemBrain is probably a smarter way to save the money.
If the south side gets good sun you can probably skip it there.
If the siding is rainscreened (or vinyl) forget about it entirely. ZIP sheathing tests at 12-16 perms (unlike ZIP-R), which is more permeable than most OSB products:
http://www.huberwood.com/assets/user/library/ZIP_System_Sheathing_-_Product_Data_Sheet-v2.pdf
(See the last page, lower right corner: "Vapor transmission | ASTM E96-B | 12-16 perms" )
Mystic CT is on the very warm edge of zone 5, and with back ventilated siding the risks are pretty low, and would meet IRC code for 2x6 walls without anything more vapor retardent than standard interior latex on wallboard as the interior vapor retarder. See TABLE 702.7.1 CLASS III VAPOR RETARDERS, (about 1/4 of the way down) and the exception for zones 4 & 5 that reads : "Vented cladding over wood structural panels":
http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html
Vinyl siding or rainscreened siding both meet the code definition of "vented cladding", and OSB definitely falls under "wood structural panels."
If you won't sleep at night unless you know with a higher degree of certainty you could try to find a competent energy nerd or engineer who understands the proper use of (and potential limitations of) WUFI.
Dana,
Be careful! You've gotten caught in Huber's obfuscation. The rating of 12 to 16 perms is for the "panel overlay" -- in other words, the facing they attach to the OSB. The entire sheathing (OSB + overlay) is 2 to 3 perms -- the same as other brands of OSB.
OK- makes sense- thanks!
They use the same terminology on their ZIP-R, which I had taken to mean the OSB as the "overlay" of the polyiso. My bad!
The overlay doesn't impede the drying of the OSB by much, so it's really not much different from brand-X OSB with #15 felt as the "overlay", which would also meet code in zone 5 using standard interior latex as the interior latex, as long as the siding is back ventilated.
At ~12-15 cents per square foot Certainteed MemBrain could be seen as cheap insurance. It's only been around for 12-15 years, so there isn't a 100 year track record, but it's nylon, a material with fairly well understood properties (and a 75 year history in other applications). It doesn't degrade quickly unless subjected to high temperatures, high acidity or ultraviolet light, none of which would be normally found in most wall stackups. Even if it degraded and crumbled by 2050, under recent global warming projections, Mystic CT could be (sadly) on the cool edge of climate zone 3 by then. In a stiff north wind you can already spit to zone 4 from there (Fishers Island NY, only ~3 crow miles away. :-) )