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Community and Q&A

Insulating Slab on Grade and Basement

JamesBoy360 | Posted in General Questions on

Hi there, we are currently renovating a 1954 sidesplit in Toronto Canada (Believe this is Zone 6A). On the main floor, there is a 6″ slab which doesn’t have any insulation or poly below. Likewise, the basement is similar, but only has a 3″ slab. We have had to cut channels in the existing concrete slab and will need to pour the new floor for the addition.

We are now looking to insulate both floors. The plans has been to use the following:

Concrete > Poly Vapour Barrier > 2″ EPS (Amvic Silverboard) > 3/4″ plywood T&G tap-conned > 3/4″ Hardwood or tile. In the basement, we would sub the hardwood for laminate.

In the basement, our walls will be 10″ Block Foundation > 2″ EPS Silverboard > 2×4 with R14 Roxul > Drywall sitting on top of the subfloor.

Some questions:
1) Do I need to worry about bathrooms flooding etc. and have some type of airgap below the foam (esp in basement)? Maybe a delta product?
2) Will there be any issues with tile cracking over the plywood subfloor? I understand that I will need to use a decoupling membrane and lots of tapcons, but am concerned that it will flex or move causing cracks in the tiles.
3) Any concerns with putting the subfloor over recently poured concrete? I can’t leave it to dry for months, but a week or so may be possible.
4) Anything else I should be concerned about?

Thanks in advance for all your help – I have learned so much on this site.

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  1. user-2310254 | | #1

    James. On #2, the decoupling (uncoupling) membrane will be bonded to the tile so it can move independently of the subfloor. It shouldn't crack, but is is always a good idea to keep some extra tile on hand just in case.

    Here is a GBA article with more information:

  2. JamesBoy360 | | #2

    Thanks Steve! I was hoping you would say that...but just want to ensure that I am not missing anything. As we have a large portion of our house sitting on slab-on-grade/basement, I want to ensure we get this right the first time.

  3. Expert Member

    GBA is where you get the best advice on energy efficiency. For questions about all things tile related you might want to look at:
    They have extensive advice on substrates and techniques for avoiding trouble.

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