Help with wall details
ovanlier79
| Posted in Energy Efficiency and Durability on
We’re in a maritime climate on the west coast of Newfoundland, Canada. Climate is zoned as 6 but we have mild winters with averages around -10 C. We get warm spells followed by cooler days, not your average climate 6 winter. We heat with a wood and electric furnace.
Contractors have left us with 1/2″ spruce clapboard, 3/4″ rain vent, 1″ rigid foam (classmate, perm of 1.0), osb sheeting, 2×6 stud with R20 batts. The plans call for poly before T&G pine… But after reading a few articles and the potential for osb turning to oatmeal: we’re concerned!
What would be the best way to proceed?
Thanks for any insight.
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Replies
Oliver,
I have no idea what you mean by Classmate rigid foam. There are three types of rigid foam: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate.
In a cold climate, 1 inch of rigid foam has (at most) an R-value of about R-5. If it's EPS, it might be R-4.
In Climate Zone 6, the rigid foam on your wall assembly needs a minimum R-value of R-11.25, so your rigid foam layer isn't thick enough. To read more about this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
There are two other issues you need to address:
1. Ideally, the interior side of your wall assembly will have an air barrier. Polyethylene plus tongue-and-groove pine is not a very robust choice; taped drywall plus T&G boards would work better.
2. There is another problem with the polyethylene layer: a wall assembly with exterior rigid foam needs to be able to dry to the interior. You need to omit the polyethylene -- or, if required by your building inspector, you need to substitute MemBrain (or a similar smart retarder) for the polyethylene.