A question about insulation of a basement.

First, the house is built near Montreal, Québec, Canada. Climate, cold in winter and hot and humid in summer. To give an idea, -30 C to the coldest in winter, and + 30 C maximum in summer.

The wall of my basement is constructed as follows:

  • The concrete wall is underground for 2/3 (2/3 of the bottom ... of course!). A membrane was installed on the outside and a drain installed to correct water infiltration.

  • Inside, a 1.5 inch air space was left between the concrete wall and the wooden wall.

  • The interior wall of 3 1/2 inches is made of wood insulated with rock wool (Roxul).

  • A plastic air barrier was sealed on the wall, so inside.

I suspect that the air space is harmful: possible convection and condensation. Can you find me serious links, supported by science explaining why this is bad, if that is? What is the risk of this construction?

Now what can I do, do I have to do with this wall?

(sorry for my english... google translate help me a little bit!)

1 Answer 1


I'm in a similar climate, and what you describe is standard aside from the insulation. Rock wool is no longer used in favor of fiberglass or spray foam where I am. I have no reason to think that it's a problem, though.

Convection isn't a concern outside the heat envelope. Condensation isn't either because it's on the cold side.

That said, some recommend against vapor barriers in basements due to the dual-membrane situation they create in conjunction with the foundation waterproofing. This can trap moisture and promote mold and decay. It was probably installed as temporary covering for the insulation.

  • I would never use plastic for any vapor barriers. It traps moisture and promotes mold. This site has excellent articles. buildingscience.com/document-search
    – ArchonOSX
    Feb 25, 2017 at 1:34
  • A vapor barrier traps moisture? Crazy. :D In seriousness, polyethylene sheeting is a critical component of the building envelope by the millions in the northern part of the U.S. I'm not sure what you're saying, exactly. How would you prevent condensation against cold exterior surfaces?
    – isherwood
    Feb 25, 2017 at 15:56
  • If you read some of the articles on the site I linked they show failure of plastic sheeting. Especially in a basement where the wall has no choice but to dry to the inside. Once moisture is trapped behind the plastic. It has nowhere to go. They use EPS and XPS foam since it allows vapor to move through it. Upstairs I would use Kraft faced insulation and house wrap. If they have too much humidity in the winter, use ventilation to equalize humidity. A lot depends on location. Breathing is better for walls.
    – ArchonOSX
    Feb 26, 2017 at 11:47
  • What??? Condensation is on the warm side, not cold side. When you take a coke bottle out of the fridge, condensation occurs because it's in a warm room. ArchonOSX is right, plastic sheet on interior side is not appropriate. VAPOR will move through the wall, hit the dew point and become water. If there is a plastic sheet on the interior of the wall, the water is trapped and can't escape. Around here they cut the plastic sheeting out with knives. The problem is that the stud space is not completely filled with insulation. We vent attics, but can't vent stud spaces so we completely fill the void.
    – Lee Sam
    Feb 27, 2017 at 7:54
  • Condensation occurs when the cold side is encountered by air from the warm side. Your analogy is flawed because there's practically no insulation involved. If fiberglass is installed without a vapor barrier, the dew and frost points occur somewhere inside the insulation layer, not on the face of the warm side.
    – isherwood
    Feb 27, 2017 at 14:36

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