Will I do more harm than good adding unfaced polyiso in my exterior walls?
Hello:
I am building a two story home trying to balance efficiency and cost. Outside is Hardie board strips that look like natural wood boards. Under the hardy board is TyveK, no additional insulation outside. Could not afford zip guard so we used standard particle board looking sheathing. 2×6 walls 16″ on center.
To get an R-value of 25 in the walls I found .5″ polyiso unfaced ( looks like thin cardboard on both sides). This was cut and placed against the searching between the studs on the inside to add a little extra insulation. Roxul r23 was the final step to get as close to r25 as possible. Heat will be radiant on the first floor and baseboard on the 2nd. During construction I have a large woodstove running 24-7 as winter in VT has begun. Only the first floor walls are insulated now. It’s been in for about a week.
After a cold night of about 2 degrees outside and 50 inside when pulling back the Roxul we found the inside of the polyiso was a little damp. My question is should I leave things as is but run a vapor barrier then 5/8 Sheetrock? Or remove the polyiso and only use the Roxul? I’m hearing many views on letting a house breathe or not. Thanks again.
Mike
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Replies
Mike,
To answer you question, you have to track down the name of the manufacturer of the product you describe as "1/2 inch polyiso unfaced (looks like thin cardboard on both sides)." You also need to learn the product name.
Once you learn the name of the manufacturer and the product, you can contact the manufacturer and ask about the product's vapor permeance. This product may be quite vapor-permeable, or it may be a vapor barrier. It matters.
In general, your approach to insulation is not standard. Inserting rigid foam between the studs is called the "cut-and-cobble" approach. If you really want to add rigid foam on the exterior side of your mineral wool, the usual approach is to make sure (in your climate zone, which is Zone 6) that the rigid foam has a minimum R-value of R-11.25. To learn why, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
An alternative approach -- using a thin layer of rigid foam on the exterior side of your assembly -- won't work unless the rigid foam is vapor-permeable, and may introduce more risks than benefits. I don't like that approach.
After you learn the vapor permeance of the product you have installed, you can decide what to do. If its vapor permeance is low, you need to remove it.
One option to consider (after you have removed the thin foam) is installing a continuous layer of rigid foam on the interior side of your studs. For more information on this option, see Walls With Interior Rigid Foam.
If you decide to pull it out, you aren't losing much. When it gets very cold, polyiso loses some of its R-value, so it ends up not much better then rock wool. Martin's final suggestion--foam on the inside--would give you more benefits particularly with polyiso, since the polyiso would be warmer and it works better when it's warmer.