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Community and Q&A

Insulate on top of concrete in 2020

PhilMancini | Posted in Energy Efficiency and Durability on

We have a semi detached victorian town house located in the Montreal area in Quebec Canada. The house is about 100 years.
The basement is semi finished with spray foam on the foundation walls and  gypse walls are covering 
The flore is a concrete slab with no vapor  barrier 
There is a ventilation system that is running on smart mode
We do not have humidity issue coming from the slab
Redo the slab is no option because the City will impose us to excavate a lot ro reach  code and it will cost a fortune.

After too many years trying to design the ideal solution i need to make it happen now

the « solution » is  as follow:
apply xypex concentrate to seal the concrete inside the concrete
put a layer of eps or xps ridgid foam planks
put 1/2 fibrocement planks and glue them to the rigid foam planks
the sandwich will be floating

Not sure about the finishing , and open to suggestion

what are your comments?

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  1. Expert Member
    Peter Engle | | #1

    The Xypex and EPS is a good start. Even an inch of EPS will make a huge difference. I'm not thrilled with the fiber cement boards. Unless you use very high density EPS, it will tend to crush under the seams as the boards flex. Two layers of plywood with staggered seams, glued and screwed together is a much better approach. They don't have to be screwed to floor or the foam. You can use pretty much anything on top of that for flooring, though I have become a fan of laminate flooring for basements.

    1. PhilMancini | | #8

      Many thanks Peter i was trying to avoid plywood on the floor
      But it seems to be hard
      eps versus xps what would be a better choice?

      1. charlie_sullivan | | #9

        The climate impact of most XPS is terrible: the blowing agent used to foam is is more than 1000X worse than CO2 for global warming. So it's simply not an ethical option unless you can use reclaimed foam, or source XPS with a replacement blowing agent, which I don't think is available in North America yet.

        So EPS is the way to go. You can get graphite-infused EPS (GPS, or Neopor) to get a little higher R-value per inch (~5 vs. ~4), which is probably worth it if it's easily availble in your area.

        What's your concern about avoiding plywood? I suppose you could do two layers of fiber cement boards, with staggered seams, and that would probably workd, but its not ideal given how brittle it is.

  2. creativedestruction | | #2

    I'm curious on the preference for Xypex waterproofing. No doubt it will make the slab more vapor tight, but I would think an epoxy coating or sealed dimple mat does that better and leaves you a more level surface for the foam. I guess epoxy wouldn't work too well if you're dealing with big cracks.

    I'm a fan of the dimple mat as it controls any vapor pressure, allows drainage, and keeps foam dry for full insulating value. A 3/4" T&G ply subfloor over the foam allows whatever finish flooring you like, but preferably something vapor open that can dry upward.

    1. PhilMancini | | #4

      There is no crack in the slab
      I used this product for a small but important job last year
      I have this huge bucket in my basement waiting To be used for something.
      And eureka why not using it on the floor!!

      Delt fl is an option that i pursued but did not complete with finishing. I am concern with the insulation aspect of it
      May be it can be my first layer of the sandwich with the foam and the fiber cement board on top
      I want to avoid plywood if possible

  3. Expert Member
    Akos | | #3

    I've had good luck with LVT right over the concrete in basements. You can get thicker ones with a bit of foam backing which really helps with taking the edge off cold floors.

    If you want more R value, you can also go with one of the insulated subfloor panel systems. The OSB on top can be sanded and cleared for a budget finish if you don't need to use the space a lot.

    1. PhilMancini | | #5

      Many thanks for your input
      What does lvt stand for?

      1. Expert Member
        Akos | | #6

        Vinyl click tiles. There is also loose lay which works better if your concrete is very uneven.

    2. PhilMancini | | #7

      Thanks i need R to even better manage condensation fro the slab
      Not sure the vinyl will be my choice because it forms a vapor barrier for what i learned from other post and it appears to be not good to manage mold

      1. Expert Member
        Akos | | #10

        If you have condensation on your basement slab (not talking about moisture from the soil bellow) your basement is way too humid. You need to deal with that first.

        Generally a slab with a vapor barrier bellow will never get cold enough for this, so it is fine to use a non permeable flooring on top.

  4. Expert Member
    KOHTA UENO | | #11

    Don't know if you're already seen this column, and it is behind the paywall, but it seems applicable:

    Installing Rigid Foam Above a Concrete Slab
    You can sandwich rigid foam between an existing concrete slab and new plywood or OSB subflooring

    See Comment #10 for my input on this topic.

    What's the intended use of this space? Storage and mechanicals? Or finished space? Combination of both? In general, doing a 'flop down' floor of cement backer board is a great, quick cheap way to make a walking surface in secondary spaces, and it has the benefit of being completely flood-proof. But if you wanted more of a finish floor, structural sheathing as a nail base/flat surface might be required.

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