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Underground insulation around top of exterior basement perimeter

sharonsec | Posted in General Questions on

Our house has a full basement foundation in a cold climate.

The basement wall is concrete blocks and has been insulated on the inside only, with 8 inches of rigid foam board attached to the wall.

A concrete driveway adjacent to the basement is being repaired.

Before placing the new concrete, I am curious if putting foam board insulation under the concrete for a few feet adjacent to the top of the exterior basement wall would provide any insulation value.

Attached is my attempt to visualize placing the foam adjacent to the top of the exterior basement wall.

It would be somewhat like this, but adjacent to the house.

https://www.greenbuildingadvisor.com/content/underground-roof-rigid-foam-and-membrane

I ask if this is provides insulation value for two reasons:

1) this description how basement heat flows at a sort of 45 degree angle from the basement wall, it seems foam near the top of the ground would reduce heat loss

https://www.greenbuildingadvisor.com/sites/default/files/ASHRAE%20Fundamentals%20Chapter%2028%20Figure%205.jpg

2) how frost protected shallow foundations work

It may not provide worthwhile insulation value, but it is a one time opportunity to add insulation in this way, so I thought I would ask.

I understand adding insulation adjacent to the exterior basement wall would be the best option, and I plan to inquire about that with the contractor.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Sharon,
    Q. "I am curious if putting foam board insulation under the concrete for a few feet adjacent to the top of the exterior basement wall would provide any insulation value."

    A. Yes, the rigid foam would provide insulation value. Your idea would work. The rigid foam would also contribute to keeping the soil near your foundation dry.

    That said, your basement wall (at about R-32 to R-40) already has much more R-value than most basement walls -- more than any code requirement -- so it's unlikely that additional exterior foam would result in significant energy savings.

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