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I’m thinking of building attached townhouses in NYC (3 or 4 floors).

GBA Editor | Posted in PassivHaus on

My foundation will probably be a matt foundation. I am trying to meet passive house standards. What is the best way to create a thermal break between the ground and the matt. I’m concerned. I would love to see some form of database of components, assemblies and appropriate products for Passivhaus construction.

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  1. GBA Editor
    Martin Holladay | | #1

    I assume you are talking about a mat foundation, also known as a raft foundation.

    I think you will need to discuss the issue with your engineer. I'm not sure whether a mat foundation can be installed over a continuous layer of rigid foam insulation.

  2. Marshall | | #2

    I think we are talking the same type of foundation. I was told there exists a product i assume high insulation value and yet can handle the downward forces without compressing the material in excess of any required limits. If not there should be such a product. I assume they have solved this problem somewhere and I hope the answer appears. Thank you Marshall

  3. Riversong | | #3

    Mat foundations are typically used over expansive soils, in which case they have to resist both upward (expansive) forces and downward (gravity) forces combined. So any subslab insulation must be able to resist the composite forces without deterioration.

    A waffle-type mat foundation will be very difficult to insulate because all the loads are concentrated at the ribs, unless you can find a spray foam with sufficient compressive strength to apply over the plastic waffle forms. A uniform thickness hollow-core mat foundation would be easier to insulate, and there are XPS boards available with compressive strengths up to 100 psi.

    Obviously, any such foundation system will have to be carefully engineered.

  4. Marshall | | #4

    Robert we do have poor soil conditions i think our
    soil is basically fill we are close to the water front i was told
    by our architect it was rated 1165 I looked at some of the owing corning
    products that seem to have a 100 psi strength and sent the specs to the
    structural engineer for review He was open minded but asked how much we expected to save
    by insulating under the slab ? He basically told me on structural projects he has been involved with they havent placed insulation under footings or slabs. I told him it was more about the future than the present. Even with footings I assume there should be materials that have the compressive strentgh as well as high insulating values . I guess then it comes down to availibiity and price. Thank you for pointing out the expansive force of the soil. I agree that any such foundation has to be carefully engineered. Im just thinking there should be more info out ( collective experience) there re : underfooting insulation materials and techniques as well as sub slab (bearing) insualtion materials and techniques thank you marshall

  5. Riversong | | #5


    If you're aiming for PassivHaus standards, or even just superinsulated standards, and if your mat foundation slab will be a finished floor within the thermal envelope, then it has to be thermally insulated from the ground.

    In most 1 or 2 storey residential applications, even standard 25 psi XPS can easily support the live and dead loads from a conventional spread footing. You're going up with twice as many storeys, so a more dense foam board would be in order.

    I assume your soil rating (1165) is the psf bearing capacity? That's pretty weak soil, as it translates to a load-bearing capacity of only 8 psi, which is 1/3 the bearing capacity of standard XPS. If your architect doesn't understand the value of subslab insulation, perhaps you need another architect or an enginneer who is better qualified to evaluate your foundation requirements.

    Dow makes 40 psi, 60 psi, and 100 psi HighLoad XPS specifically for such applications. The NAHB design manual for shallow, frost-protected foundations requires that unheated buildings, like garages, have foam board under the entire structure and extending out beyond the footprint several feet depending on climate zone. This is accepted by the IRC and ICC.

  6. Anonymous | | #6

    Another approach would be to install insulation above the rough structural slab; which could then be covered with a topping slab/screed or other floor system.

  7. kevin_in_denver | | #7

    Legalett is in Ontario, and they have quoted the engineering and subslab insulation for me on a project. It sounds like what you have in mind. I'm pretty sure they have an engineer with a NY license.

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