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Community and Q&A

Upgrading a dirt-floor basement

boadwinhouse | Posted in Energy Efficiency and Durability on


I live in an 1890 home north of Toronto, Ontario and am beginning a basement renovation.

The home has a full basement with rubble foundation walls and a dirt floor. Some massive benching (24”wide by 36” tall) has been placed around the perimeter to support the base of the walls and lower the original floor by 8” or so.

The floor is usually damp in spring/summer and sometimes wet. Twice heavy rain and poor gutters have caused severe water penetration through the rubble walls.

The space will be used for a laundry room, utility room, and storage. The slab will be the finished floor (with maybe an epoxy coating). I have forced air propane heat. I want the space to be dry, healthy, clean and energy efficient.

I’m planning to take Lstiburek’s approach to this type of retrofit outlined on the Building Science site: capillary break at the rim, spray foam on the walls down to the drainage mat running from under the new slab up the walls a bit, interior perimeter drain to a sump, etc.

I’m wondering about a few things.

Should I insulate under the slab? How much? Lstiburek recommends a “foam bathtub”.
Should I add gravel under the insulation (or slab if uninsulated)? The dirt floor is hard packed.
What type of drainage mat/vapour barrier should I use? Is one needed in addition to the foam if taped etc.?

I’m suffering from a bit of info overload/analysis paralysis.

Any advice would be appreciated.


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  1. BrianPontolilo | | #1


    You should definitely put some rigid insulation under the slab, I think R-10 is a recommended R-value for basement floors. And you should add a layer of poly sheeting above the foam, both of which will help keep moisture from migrating up through the floor. Yes on the gravel to and consider planning for radon mitigation. You can test radon levels first, but you'll be sealing things up tight it sounds like, so it may be worth planning for future needs (this is an affordable precaution).

    You may find these links helpful:

  2. Expert Member
    BILL WICHERS | | #2

    I’d make sure your drain system goes around the full perimeter at the bottom of the gravel you’ll put in under the slab. Since you’re already seeing water problems, you absolutely need the gravel (to keep the water away from the floor), the drain (to carry the water away), and the sump (where the pump will go to expel the water). It’s almost impossible to seal things enough to keep water out. It’s far easier to provide drains to give the water an easy path out.

    If your walls weep, make sure your perimeter drain has some type of “drain slit” on the perimeter to carry away water that comes through the walls without it pooling on your slab.


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