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Community and Q&A

Footing insulation

Pierre Gingras | Posted in GBA Pro Help on

I am building an energy efficient house on a slab with 6 inch of EPS insulating panel under the a 5 inch slab. My architect is asking to add a 3 X 3 footing (on compacted soil) under the slab where the posts that hold the stairway to the second floor is meeting the ground floor and a second one at the bottom of the stair.

How do we go about insulating these footing to at least the same level as the rest of the slab do we just put the same 6 inch insulation under these footing as well as their rim?

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  1. User avater GBA Editor
    Martin Holladay | | #1

    Talk to an engineer. It may well be possible to detail the rebar in the existing 5-inch-thick slab to handle the load from the post (without an additional footing under the slab). I can't imagine that a 5-inch-thick concrete slab can't handle a stair landing. Something smells wrong here.

  2. Malcolm Taylor | | #2

    In the absence of a favourable alterative such as that suggested by Martin, you have two choices:

    Run the insulation under the footing insuring you use a type with a high enough compressive strength to resist the loading.

    Locate the footing far enough below the slab to run the insulation over top and carry the point load to the footing with a short pier. The energy penalty for this would be quite small.

  3. Terry Lee | | #3

    Right, an Architect is not a licensed PE or structures engineer that needs to look at the "loads" vertical and lateral resulting from live, dead, cyclic, stress analysis on the stairs, and in the case of a pier on a footing....there is no point load (like a small rock hitting it), it will be as I described with the biggest challenge showing a rotational bending moment good based on out of plane seismic, wind, lateral static and dynamic loads. What makes sense from energy point of view can make structure worse, the strongest structural design especially concrete does not put it in tension or bending, monolithic carry loads the best and resist bending, couples. The stairs will be treated as a evenly distributed load (pressure over area for static, or should be designed that way) on slab, not point from post. Creep and MOE need to be looked at slab or footing on foam.

    That was 'proprietary" info shared with Malcolm ;)

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