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3/4″ of XPS on a basement floor vs. 1½” in a Climate Zone 7 basement

kenorakq | Posted in General Questions on

A friend is planning a basement reno..

it has no insulation at all on the interior or exterior or under the slab. It was built in about 1995. The basement has about 18″ above grade, the balance is underground. It has never had bulk water seep in and its unlikely that there is poly under the concrete as this wasn’t commonplace here (Winnipeg Manitoba Canada).

He wants to add 3/4″ XPS to the floor (1150 sq/ft) and cover that with either 5/8″ or 3/4″ t&G OSB or plywood using only the T&G joints to hold the sub floor together as a unified surface.

I have questioned this and suggested the 1 1/2″ foam under 3/4″ ply with sleeps as suggested in the Strategies and Details section.

Additionally he plans (like most of the unwashed here in the north) a 2×4 frame wall with R12 fiberglass bat insulation for the interior walls: with 6 mil poly to 4 ft below grade (nearly to the floor).

Can someone please help me with some facts why the thinner foam and lack of sleepers is not a good idea and whether his plan for the basement walls makes sense (I would omit the poly all together).

I will get him to tape a square of poly to the floor to see if there is vapour moving through from the subsoil..

Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    You don't need sleepers over the rigid foam installed on the slab. The T&G plywood or OSB is just fine -- in fact, it's a better approach, because it allows for thicker foam. Here's what I wrote on the topic in an article titled Fixing a Wet Basement:

    "The usual technique is to install 1 or 2 inches of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCon fasteners. (If you are still worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it's important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.

    "For more information on insulating existing basement slabs, see:
    Finishing a Basement Floor;
    Green Basement Renovation; or
    The Stay-Dry, No-Mold Finished Basement."


    When it comes to walls, you are correct. You don't want to insulate the basement walls on the interior with fiberglass batts. For information on the correct approach, see this article: How to Insulate a Basement Wall.

  2. Expert Member
    Dana Dorsett | | #2

    Since XPS degrades to the same performance of EPS over time, you might as well go with 1.5" EPS, which uses a 100x more benign blowing agent, and is cheaper per labeled-R. At 1.5" you'd be at R6+ which is plenty for summertime dew point control at the subfloor/foam boundary.

    A 2x4/R12 studwall is only worth about R8.5-R9 after factoring in the thermal bridging of the studs. That's fairly pathetic performance for a zone 7 climate- int wouldn't even meet code in zone 4 under IRC 2015. Continuous 3" of EPS held to the wall with 1x4 furring through-screwed to the foundation delivers about R13 in a zone 7 climate, and takes up only 1/4" more depth than a 2x4 wall. With all of the moisture-susceptible wood on the warmer interior, no interior poly is needed.

    Putting sheet poly on the interior traps ground moisture in the studwalls. You might be OK with 18" of above grade exterior exposure for a drying path to the exterior, but it's risky.

    If you expect the inspectors to complain with a furring + foam solution, put the poly between the furring and the foam, or use foam with plastic or foil facers. At 3" 1.5lb density "Type-II" EPS is already under 1 perm, and would meet Canadian code definition as a "vapour barrier", but that requires producing the spec and educating the inspector.

    Trapping foam to the foundation wall with a 2x4/R12 studwall would be even better. With 2.5" of EPS (R10.5) you have sufficient dew point control at the foam/fiber boundary to skip the interior polyethylene, per the prescriptives in TABLE R702.1.2 :

    That would deliver a whole-wall R of about R20, which is still appropriate for your climate albeit higher than IRC code minimum (R15 continuous insulation.)

  3. kenorakq | | #3

    Thanks...I'll forward that info...

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