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Community and Q&A

Basement insulation

Ashr | Posted in General Questions on

We are planning to have our basement finished.  We are in climate zone 6 (Toronto, Canada). After reading various sites, for basement insulation, we are leaning towards 2″ rigid board (XPS or EPS). Framing against the board, fill cavities with mineral wool batts and then drywall. Couple of questions
1. Do we leave space (say 1/2″) between the board and framing?
2. Do you prefer EPS over XPS
3. Do we need vapour barrier between the drywall and batt insulation?


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  1. Expert Member
    Peter Yost | | #1

    HI Ashr -

    1. 1/2-inch space - there is no reason not to put your framed wall up against your rigid insulation.
    2. EPS vs XPS - assuming you can get Type IX EPS (see:, the lower global warming potential (GWP) of the EPS makes it a better choice, if that aspect of environmental performance is important to you.
    3. need for interior vapor barrier - no, no need for any vapor retarder other than Class III (latex painted gypsum board).


    1. Expert Member
      Dana Dorsett | | #2

      There is absolutely no rationale for using Type IX EPS for interior-side insulation on a basement wall. Even crummy Type-I goods are good enough, though a bit fragile.

      Going with Type VIII (commonly used as roofing insulation on commercial buildings) is plenty, at nearly the same R-value as Type IX, and use only 63% of the polymer. Used/reclaimed Type VIII is commonly found on the secondary market at about 1/3 the cost of virgin-stock goods. At 2"
      Type-VIII goods would be labeled R8.3, compared to R8.4 for Type IX, less than a 2% difference.

      Also note, Toronto is now already in zone 5 based on the most recent 25 year weather averages (thanks to global warming.) The boundary between zone 5 and zone 6 is a decades long average of 4000HDD/year base 18C. Since 1997 Toronto has only nudged over that mark once:

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