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Community and Q&A

Retrofitting XPS foam to slab-on-grade house

zktDtTeMu7 | Posted in Energy Efficiency and Durability on

I am considering adding a couple of inches of foam to the inside of my 20 year-old slab-on-grade house. HOT2000 modeling says that I’m losing a significant amount of heat through the slab, which has minimal if any insulation, and it seemed like a reasonable idea to add the XPS inside as part of a flooring renovation. Certainly, there are comfort issues as well, as the slab is always cold (it just has carpet over a thin layer of regular underlayment). I am targeting other energy nosebleeds as well, such as air infiltration, etc; the floor is just one of 4 areas I’m looking at.

For the current layout, I have mixed tile (in the main entrance), carpet, and linoleum. I have no problem tearing out the linoleum and carpet, but I’m a bit at a loss for what to do with the entry way.

First off, will I get a decent improvement in reducing the amount of energy and improving comfort in the house if I just insulate part (approx. 80%) of the floor area, or do I need to push the insulation into the entryway as much as possible? If the latter, I think I’d end up either trying to reposition the front doors (lots and lots of finicky work) or else end up with an entry step-up well. Has anyone had any experience with these types of retrofits?

Also, I’ve read and modeled the effect of foam insulation around the slab perimeter. For retrofit, does it make sense to try to do so?

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  1. GBA Editor
    Martin Holladay | | #1

    1. What's your climate or location? It matters.

    2. Usually, the first step would be installing vertical insulation around the perimeter of your slab. It's possible to do that work as a retrofit, as long as you don't live in a climate with termite problems.

    3. I don't recommend creating a 2 1/2-inch or 3-inch bump between your entryway and the other rooms of your home. That would definitely be a tripping hazard.

    4. If you decide to install 2 inches of rigid foam on top of your slab, I would recommend doing it in every room. That will be a lot of work, however, since you'll have to adjust every door, including your entry door. The work will also effectively lower all of your windows, which will change the view and the architectural effect of the windows.

  2. Riversong | | #2


    How many times must we ask a questioner where they live and in what climate zone?

    It can't be more than a ten minute upgrade to the website to create required fields for this.

  3. Riversong | | #3


    Most heat loss from a slab in a cold climate is from the slab edge, though isolating the slab from the conditioned space by placing insulation above it will dramatically decrease slab edge losses.

    But that strategy will work ONLY if you insulate the entire slab. Leaving a 20% "hole" can effectively reduce the whole floor R-value by 80%.

    Insulate the entire floor and raise the entry door to the new level.

  4. zktDtTeMu7 | | #4

    Sorry, guys. I was trying to trim my question length.
    I am in Vancouver, Canada. Heating load is approx. 70 GJ, backed up by energy bills and modeling.

  5. Riversong | | #5

    For those who don't know giga joules from the family jewels, that's about 664 therms.

    And the climate of Vancouver has 2631 HDD(18°C) or 4736 HDD(65°F)

  6. zktDtTeMu7 | | #6

    Thanks all for your answers. I'm currently following Bruce Harley's advice and crawling through the house looking for holes to plug. The inside slab insulation thing seemed like something amenable to DIY (unlike the windows), as I had installed the flooring upstairs and on the stairs with a friend last year. I was definitely expecting to have to raise the doors inside, but am a bit more leery of redoing the entry door as well. Definitely have to give it more thought.
    Would you expect that insulating the slab edge ought to noticeably raise the temperature of the slab surface inside the house, or should I still be planning to insulate the slab surface for comfort even if I get the edge done?

  7. Riversong | | #7


    Insulating the slab edge will reduce heat loss to the outside air, but do little to raise the slab temperature if it's still thermally coupled to the ground beneath.

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