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Wall assembly strategy using Dow structural insulated sheathing

I live in MD, climate zone 4 and want to use the DOW SIS 1" with blown in high density insulation. I can use a 2x4 wall to achieve R20 in the wall. Please help me settle a disagreement with my architect. He is concerned with using the DOW SIS product because the product has such a low perm rating that it will act as a vapor barrier and the wall won't dry to the exterior. He has traditionally used an interior poly so the wall needs to dry to the exterior. I say leave off the interior poly so the wall dries to interior.

He doesn't believe that the wall will dry to the interior, particularly in the bathroom where you have tile. He is concerned that it will produce mold and will be liable for damages. I say I will sign something to remove him from liability.

I have spoke with the product manufacture (no longer DOW) for guidance but not sure how helpful they will be.

Separately, can I use a 2x6 wall for added R value? Concerned that with 1" sheathing, the dew point will remain in the wall. From reading posts by Martin, in my climate zone this should not be a problem.


Asked by Ryan Sober
Posted Sat, 01/19/2013 - 09:39
Edited Sat, 01/19/2013 - 10:57


4 Answers

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In your climate zone, 1-inch-thick Dow SIS (with an R-value of R-5.5) has a high enough R-value to keep your stud bays above the dew point during the winter. Dow SIS will work fine, whether your walls are framed with 2x4s or 2x6s.

However, your architect is about 25 years behind the times in his use of interior polyethylene in Maryland. In your climate zone, that's a big no-no. Your architect should read the following two articles:

Do I Need a Vapor Retarder?

Vapor Retarders and Vapor Barriers

Answered by Martin Holladay, GBA Advisor
Posted Sat, 01/19/2013 - 11:06

Helpful? 0


Thanks for the answer. What about drying to the interior in a bathroom with tile? Doesn't the tile create a vapor barrier situation to prevent drying on the inside? I was thinking of using open cell phone in those cavities where the shower meets an exterior wall.

Answered by Ryan Sober
Posted Sat, 01/19/2013 - 11:19

Helpful? 0

open cell phone ???? :p
auto correct ? haha

i would only recommed it if you have a unlimited long distance call plan :)

Answered by Jin Kazama
Posted Sat, 01/19/2013 - 11:55

Helpful? 0

You're right that wall tile limits inward drying. Although the reduced rate of inward drying is not ideal, the wall will usually be OK -- as long as the materials aren't wet when the wall is assembled and closed in.

Just because some walls have tile on them, doesn't mean that all walls should get polyethylene. If the bathroom wall has reduced inward drying, that's no reason to make every other wall in the house just as bad.

Open-cell foam won't help or hurt. The permeance of the foam doesn't change the permeance of the wall tile.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 01/19/2013 - 16:45

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