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Location of Sub-Slab Insulation in Relation to Footings

Bluegoose68 | Posted in Energy Efficiency and Durability on

I have a question regarding the placement of the concrete slab in relation to the concrete footing in a basement.  I’m attaching 2 sketches that show different locations for the slab vs the footing.

One sketch shows the sub-slab insulation is BETWEEN the top of the footing and the bottom of the slab.  The other sketch shows the bottom of the slab in direct contact with the top of the footing (NO insulation between them).

1)    Is there a strong preference for one location vs the other?  By placing the insulation between the slab and the footing, the direct thermal conduction path is broken.  That would improve the overall R-value of the wall and footing but are there undesirable side effects?  Is one better from a structural point of view or does one method better fit the sequence of operations in constructing a basement?

2)    One sketch shows a vertical piece of rigid foam placed between the side edge of the slab and the concrete wall and the other shows no foam (only a caulked joint).  Again, the foam breaks the thermal conduction path but it seems like it would be begging for termites to enter anywhere under the slab, go through the foam between the slab and wall and continue up the interior basement wall foam insulation to anywhere in the house.  A metal termite shield could be placed on top of the basement wall but apparently the effectiveness of the termite shield is questionable.  Any tips to reduce the possibility of termites entering through the foam?

I understand that building is a compromise – you can’t have it all.  If the goal is to build a high quality, long lasting, energy efficient building, what would you go with?  Location is southern, middle TN (very southern edge of climate zone 4A but also just across the line from what’s considered “very heavy” termite probability area.)

Thank you for your help.

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  1. Expert Member

    In a basement, there are a few reasons we use a slab at the bottom. One reason is to assert that the walls shall not move inward from the soil pressure. Another reason is to provide a stiff walking surface, which is somewhat impermeable, flat, and durable.

    The first assertion can be dealt with using a keyway in the footing, as a secondary (but stronger) method.

    So really, as long as the slab is pretty close to the bottom, I'm not certain it matters. Ideally, you want as close to 100% insulation coverage as possible, and for that reason I would go with the option that puts insulation over the footing edge, otherwise you'll have a short to ground, thus bypassing any resistance offered by insulation.

    The insulation layer is only a few inches at most, and the slab will be effectively in the same place, from the wall and earth perspective.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Apart from better isolating the slab, having a layer of insulation between them also does two useful things.

      - It moves the slab up above the joint between the footings and stem-walls, meaning potential leaks there aren't as problematic.
      - It provides a cushion which makes the slab less prone to differential settlement and cracks at the footing.

      If you are going with foam under the slab and up the walls, I would make the two continuous.

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