I’m trying to figure out best way to connect Vapor barrier to the joists in a basement renovation. House is in Alberta Canada. 1970 build.

The basement wall consists of a concrete wall with a pony wall on top. As a result the framing leaves a gap between the framing wall and the exterior pony wall.

As it’s only half concrete wall I’m going to reinstall the Vapor barrier on the assumption that any moisture would dry up and out through the pony wall plus there was zero moisture issues when I took down the previous Vapor barrier.

However I can’t decide on the best way to seal the top. I have insulted the joist header space with 2in XPs with foam seal. Should I run the Vapor barrier straight up vertically between the joists in line with the framing wall or install a strip of poly horizontally under the joists between frame wall and pony wall given that the 2in XPs forms a vapour barrier?


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1 Answer 1


Just go straight up to the underside of the subfloor. The existing XPS was probably misguided. Its surface will still be freezing cold in the middle of winter, so water vapor would condense on it. The point of vapor barrier is to prevent water vapor from reaching a cold surface. It's a good air seal, though.

The area sandwiched between the two vapor barriers looks like it still has lots of area for moisture to escape, so I wouldn't rip anything out.

  • Thank you. So would the concern with the XPS only be if vapour could reach it, hence the reason putting the Vapor barrier in line with the interior frame wall. I presume there would be risk of Vapor coming from upstairs also through the sub floor? I could replace it with batts but surely the same problem would exist. Note the gap between the inside frame wall and exterior pony wall would be completely filled with batt insulation.
    – Paul
    Feb 25 at 6:10
  • @Paul, that XPS is wasted material. It could prove useful if air sealing was necessary in that neighborhood. If there was a full panel covering every square inch of the cripple wall, then that condition combined with your interior vapor barrier would be a problem. Then, any moisture that got in there would not have an easy path out.
    – popham
    Feb 25 at 8:40
  • @Paul, people often put XPS against the concrete down low to keep out moisture from the exterior soil (a vapor barrier would work just as well). In that case, you would use something like kraft paper for the interior vapor barrier side because the kraft paper still allows water to escape. Whether that's a good idea depends on your soil surface slope outside the foundation, soil drainage, and precipitation details, though.
    – popham
    Feb 25 at 8:42

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