Horizontal insulation outside slab?

dougstove Posted By dougstove, Feb 26, 2012 at 5:09 PM

  1. dougstove

    Feeling the Heat 2.

    Aug 7, 2009
    New Brunswick, Canada
    I am hearing about laying near-horizontal insulation just underground, running out from the perimeter of a basement or slab, to keep frost from penetrating down along the basement wall.

    Any comments from someone who has done it? Or even the correct terminology so I may Google the approach?
    I have some landscaping and patio building to do around the perimeter of my house and was thinking the underground insulation might be worth doing.
    I live in a cold temperate climate with many freeze-thaw cycles, and frost alongside the basement causes various problems.
    cheers, Doug
  2. benjamin

    Minister of Fire 2.

    Nov 7, 2009
    SW WI
    Frost protected shallow foundations it the term. They've been around for long enough to be proven.
  3. flyingcow

    Minister of Fire 2.

    Jun 4, 2008
    northern-half of maine
    i have 2 inches of foam up against my foundation walls. Laying it out flat couldn't hurt if you have none now. generally speaking, frost doesn't travel sideways.
  4. DickRussell

    Burning Hunk 2.

    Mar 1, 2011
    central NH
    Doug, your use of the word "basement" suggests you have a full basement rather than a slab. In that case, you might consider excavating around the entire perimeter to the footing and laying a 2" layer of rigid XPS foam up against the foundation walls, from footing to sill. Use surface bonding cement with modifier (such as Silpro's C-21) over self-stick fiberglass mesh to protect the above-grade portion of the foam board. This will provide thermal protection of the basement walls and make the basement more comfortable (and save a pile of heat loss). If the excavation is out of the question, then yes the foam board can be laid out more horizontally. Here are two links:

  5. Noggah

    New Member 2.

    Jan 21, 2012
    Central Maine

    When I built my house last year I put two inch styro under my basement and garage floors and against the exterior of basement walls and edge of garage slab. I have radiant heat in all areas and did not want to push that heat into dear old mother earth. I have checked the walls with my infrared thermometer and found the walls to be well insulated even in very cold temps.

    Putting styro horizontal may help with frost penetration, but the earth's temp will remain constant underneath and will certainly be colder than your basement. I have been told that concrete is R-1, so putting stryo against the exterior of the concrete wall would make most sense to me. This would be a pretty expensive retro-fit, but might be worth while. I noticed the other day that a 4x8x2inch thick sheet of styro is now over $30.00 here. Plus the cost of excavation. OUCH!!!

    Just my thoughts. Hope it helps.
  6. dvellone

    Feeling the Heat 2.

    Sep 21, 2006
    I poured my monolithic slab-on-grade this past November in preparation for our passive solar house project which we're recommencing this Spring, and I did have to install horizontal insulation around the perimeter to "break" the potential frost penetration. The whole point being that the insulation along with potential heat loss from the slab will mimic adequate ground depth that would otherwise protect the footings from frost getting underneath.

    I'm using an engineer who specializes in passive solar design and the minimums for my area (West Central Adirondacks) are 1' wide 3/4" xps foam around perimeter and 16" minimum below grade. At the corners, and extending 3' 4" back from each corner the insulation is 1 1/2" xps, 2' wide.

    I have an unheated section and the xps required was 2 3/4" x 5' 4" wide with a 10" minimum depth.

    Without the engineer I surely would have overkilled the whole deal. In fact when I was buying the required 2" xps for the underslab and side of slab insulation I figured I'd also get enough to do the sub-grade perimeter insulating. More is better! Once I priced it all out I decided to go with the engineer's recommendations and that still cost plenty. I did jump up to 1" from the 3/4" recommended since the price difference was pretty small.

    I agree with the above post regarding ground warmth - that along with the slab's and house's mass and what small heat loss there is would prevent any frost getting underneath. The only potential problem might be if you shut the place down for the winter and left it unheated, but even then the mass of the slab, home, snow cover, and good drainage would probably keep you well protected. My project is new construction with a nosy inspector.

    One other thing to consider is what your drainage is like. I installed much more than the recommended gravel base to eliminate as much frost potential as possible.

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