foam insulation between studs


Old 09-16-19, 11:18 AM
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foam insulation between studs

Hi, I have a new build ICF basement and 6" stud wall framed upper. I live near Ottawa and Zone 6. On the outside of the stud work is 1" of foil faced iso foam R6.5. As I read I am increasingly concerned that this vapour barrier on the outside will cause condensation problems if I fill the studs with glass fibre or mineral wool as it will cause the Polyiso insulation to be extremely cold and lose a lot of its R-value. The vapour will migrate to the cold OSB sheathing and condense. Spray foam would answer this problem but it would be too expensive for me. Solid XPS foam well sealed is a possible answer but I have bought Durospan GPS R10 which has 0.5 perm VB and would like to know if there is any problem with using this to R20 between the studs and sealed with foam. I think this would solve my problem as it would give excellent R-value with water and air barrier protection but I hope the experts on this page can clarify if I would still need a 6mil poly barrier under the sheetrock?
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Old 09-17-19, 07:57 AM
Join Date: Oct 2008
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Need to sort this out a little.
You start by mentioning you have an ICF foundation but then 6" stud work on top. Are you saying the house is framed with 6" walls or there is a short wall on top of the ICF?

Having a layer of foil faces rigid insulation on the exterior introduces two concerns, the barrier vapor and the ratio of the permeable insulation to the r-value of that rigid. GBA has a couple of articles on that ratio but you will need to sign up for their free intro. This link leads to the same GBA articles but has included the chart.

In Zone 6 with a 6" wall they are looking for r-25 for the rigid. Their objective is to ensure the interior surface of the rigid remains above the dew point to avoid condensation.

So, clarify where this 6" studded wall is and then we can discuss the other details.

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