Basement – Rigid Foam + Rock Wool + Smart Vapor Retarder?
anom12
| Posted in General Questions on
I am having my basement finished in Toronto (Zone 5 I believe). We have an interior french drain installed and dimple membrane on the block foundation walls going to the french drain. +
From reading on here, we have installed 2″ EPS (Durafoam) against walls (over the dimple membrane) and framed with 2×4 studs in front. We have filled the stud bays with rock wool (R14). I have read the articles and posts here recommending against the use of a traditional poly vapor barrier. However, is it advisable to use a smart vapor retarder (like Certainteed Membrain) or to use nothing and just drywall?
Thanks in advance for your advise.
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Replies
Anom12,
You don't need an interior vapor retarder. Just drywall is fine.
Martin, wouldn't this cause the inside warmer air in the winter to cause condensation in the rock wool?
I was reading this article: https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/smart-vapor-retarders
In which cases would an interior vapor retarder be necessary then?
Anom,
You won't get condensation unless the humid air contacts a cold surface.
The reason you installed 2 inches of EPS against your block walls is to make sure that there is an air barrier with R-value that prevents the warm humid interior air from contacting the cold concrete blocks. Once the EPS is up, the question becomes: "Will the interior surface of the 2-inch thick EPS ever be cold enough to permit condensation?" If your wall assembly is properly designed -- that is, if the foam is thick enough -- the answer should be no.
For more information on the theory behind these concepts, see Calculating the Minimum Thickness of Rigid Foam Sheathing. Since you have 2x4 walls with fluffy insulation, you want at least R-7.5 rigid foam in Zone 6 -- so you should be fine.
In Toronto you may to include a layer of MemBrain for INSPECTOR control, even though it's not actually necessary in that stackup & location for condensation control.
The 25 year average annual heating degree days in Toronto is still under 4000 HDD (base 18C), so it's technically still zone 5, but it's the cold edge of zone 5- some years actually bump over the 4000HDD mark:
https://toronto.weatherstats.ca/charts/hdd-wyearly.html
Either way, with 2" of even Type-I EPS you have plenty of dew point margin for a 2x4/R14 wall.
Thanks, appreciate the answers!