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Community and Q&A

Under slab insulation

ShanePhilipMac | Posted in General Questions on

Im currently building a new house on the east coast of Canada(PEI). We are considering using Durospan GPS r- 10 under the basement slab. I’m curious to hear peoples thoughts on this type of insulation used in this application.
The home is constructed using Nudura ICF block to the rafters, Marvin triple pain Windows and the heating source for the home will a central air to air heat pump. I’m not sure how much information you need on the home to give a a response but I thought I would share a couple points to show the overall  scope of the build. We are trying to achieve a high level of comfort and energy efficiency without breaking the bank.

Shane MacDonald

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  1. kyle_r | | #1

    For a basement slab, i would say R10 is sufficient. This is a great application for reclaimed EPS or XPS. I would shoot for 3 inches to account for the aged r value. And don’t forget the vapor barrier on top of the insulation.

  2. onslow | | #2


    I agree with used XPS as a good choice. It is much tougher than EPS and if used, the eco-hit is already done. Dow does now make a lower GWP version of XPS and I think it is available in Canada. Definitely will be more than used, but PEI is a bit removed from major metro areas that generate the used insulation.

    I have 3" reclaimed XPS under my slab in CZ6B and wish I had 4". Your PEI environment maybe overall less freezy, but yearly the chill factor of maritime conditions might make it worth it. The ground losses from inside house temperatures through a slab are constant with virtually no swings. If you can, run a calculation of heat loss as you would a wall area. Pick 50F as the "outside temperature" and find the BTU losses for both thicknesses. Multiply by 365 days. I would give 3" aged XPS a 12.5R rating. I won't get into the raging debate about water retention in EPS vs XPS since you should be placing your sub-slab insulation on a 3" minimum washed rock bed.

    Mind your ground water and radon potentials.

  3. Expert Member
    BILL WICHERS | | #3

    XPS is really a better choice for under slab use even if bought new. The reason is that XPS is much less prone to moisture absorption over time compared to EPS, so it holds its R value in this application much better.

    I do agree the reclaimed XPS is the best choice it to have access to it. The dings and scuffs in reclaimed material really don’t matter when you’re going to pour concrete all over the tool of it anyway :-)


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