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Community and Q&A

Insulate top of basement footing?

Andy Freeman | Posted in Energy Efficiency and Durability on

We have removed the slab from our basement and will be adding sub-slab insulation before pouring new concrete. I would like to preserve as much ceiling height as possible while still having a dry and comfortable basement. There will be a layer of 2 3/8″ EPS (R-10) under the entire slab that is even with the top of the footing. A dimpled membrane will be placed on the interior concrete block walls and lapped over the top of the footing to direct any water to the interior drain tile. Concrete will be poured directly up against the the interior drainage membrane. The interior walls will have 4″ of EPS against the drainage membrane with a 2X4 wall built in front of the foam. The original plan was to place 1″ thick EPS on top of the 2 3/8″ EPS so that insulation was present on top of the footings. With this approach the height of the finished ceiling is going to be 7′ 4″. I could gain another 1″ by removing the top layer of insulation.

How important is it to have insulation on top of the footings with this design? I’d prefer to skip the 1″ EPS and save ceiling height. We have had previous humidity and mold issues. My primary goal with the insulation is to prevent any condensation, and subsequent mold, from forming on the floor. The footings protrude about 6″ from the block wall; my vertical insulation from the walls will be 4″ thick with the 2X4 wall in front of that; could I just put a 2″ thick X 3.5″ strip of EPS under the wall plate to provide some insulating value from above?

Thank you for any responses, I am located in St. Paul, MN.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    As drawn the uninsulated thermal bridge of the slab-edge to the foundation wall is moving more heat than the thinness of the foam between the footing & slab. At the very least the wall foam should extend down to the footing to insulate the slab edge, continuous with the slab foam.

    The new slab doesn't have to extend any further than what it takes to support the bottom plate of the studwall. Since the studwall isn't structural (it's holding the drywall up, not the house), it can even be cantilevered off the slab edge a couple of inches, which would allow the entire top of the footing to be insulated to a higher R, not that a stripe of 1" foam is a show stopper. Making the wall foam continuous with the slab foam is the bigger deal.

  2. Andy Freeman | | #2

    Thank you Dana. So you'd go with this type of approach, drop the wall vertical insulation down to the top of the footing and pour up against that. I've also got a load bearing block wall down the center of the basement with a similar cross section. I was just going to leave the 2 3/8" EPS flush with the top of the footing there as it seems less critical than the exterior wall.

  3. GBA Editor
    Martin Holladay | | #3

    Andy,
    When it comes to addressing the thermal bridge at the bottom of the wall, your second attempt is better than your first attempt. The standard approach is shown below -- a detail that avoids the thermal bridge.

    Of course, you may not want to insulate as shown in this detail, because of your ceiling height concerns.

    .

  4. Andy Freeman | | #4

    With my footing width I would have a 3” wide strip where their tops would be uninsulated. How big of a problem would this be for heat loss? I can stick with the original plan, 1” insulation on the footings that would now butt up to wall insulation, if it is the superior approach.

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