GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Struggling with how to use foam on walkout over Canadian shield zone 7

Tim Brown | Posted in Green Building Techniques on

I’m building a walkout basement (1000 sq ft) in zone 7 (NW Ontario Canada) and have purchased 1000 sq ft EPS at 5 inches and 1000 sq ft at 1 1/2″ XPS (I was told its much better to use above the XPS since its much more rigid when walking on during the pour)..
I am struggling on how to best use the foam on the walkout side (I got a great deal so I have lots of EPS and XPS)..
my site has about 2 ft of sand above the Canadian shield (although that varies by a ft or more) that I must contend with when attempting to add vertical insulation below the slab (parallel the walls and exterior to the footings).. since I hit rock …
in that case would you recommend rigid insulation extending further than the usual 4 ft horizontally and greater than R7.5 (1 1/2″ XPS or EPS).
Exterior walls (2×6) will have 2 1/2″ EPS under 1 1/2″ XPS on the exterior footing to roof..(that’s what came in the “deal”).

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    So, you dug down to ledge rock on your walkout side -- and that's about 2 feet, more or less, below grade. Is that right?

    Then you are forming up a concrete stemwall on the ledge rock -- right?

    And the basement slab is poured at grade level? If so, I can't understand why this is a "basement."

    If you site consists of solid rock covered by 2 feet of sand, you will need some explosives to get a basement -- unless I'm missing something.

  2. Tim Brown | | #2

    I just stumbled on this thread that I forgot to reply to...thanks for the reply..

    The basement is a walkout with the east side down the slope. The footing on that side is only about 2 ft above rock); as the basement extends west it goes up slope till the west side of the building has about 8 ft of soil above the footing (which is also about two ft above rock)... the south and north sides are a triangle with the most ground cover at the west end and the least at the east. The walkout portion is of course at the east side where the basement floor is just above grade.The main floor of the house is above this walkout.
    My concern is that since I am limiting heat loss to the subsoil under and around the slab that the entire east side and portions of the north and south sides will not get enough heat (through the floor and basement walls) to prevent freezing and may be at risk of frost heave.
    I have enough EPS and or XPS to insulate horizontally from just above the footer to 4 or 6 or even 8 ft away... is this recommended and if so how thick should the foam be...(I mentioned this to a local builder who said the foam should be as thick in inches as the frost penetrates in ft...(about 8" then).????

  3. Expert Member
    Dana Dorsett | | #3

    If the stemwalls are on solid Canadian Shield there's no risk of frost heave, since there isn't enough moisture or enough space to grow the ice crystals. If all the soil is sand, that's also protective, since it drains very well, limiting the available quantity of water for forming ice crystals.

    If the footing isn't on rock or below the frost line it's effectively a grade beam, even if it's a kinda-deep one.

    There are several approaches to frost protecting shallow foundations:

  4. Tim Brown | | #4

    Thanks for the links...

    thank goodness I have a laser printer :)

    I have some reading to do..

    Tim Brown

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |