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Should I insulate one of my exterior foundation walls?

I am getting some water penetration through a block wall in my basement and through the floor near the bottom of the same wall. The ground on that side of my house is currently sloping towards the house and inviting water inside. I was thinking of excavating that side of the foundation and applying a waterproof membrane to the exterior and then regrading the ground to make it slope away from the house. I was wondering if it would be worthwhile to insulate the exterior of the foundation wall, or if it would be a waste to only do one side, since there is no way that I will be able to do the rest. I was thinking that the exterior insulation might help to better keep moisture out of the basement in addition to providing insulation. My concerns are that this side of the house has had problems with termites. I have read that rigid foam on the exterior of foundations can be a problem if there are known to be termites in the ground. I have had a termite bait stations put in and we had the sill plates, the tops of the basement walls and the bottoms of the first floor walls sprayed with a boron-based treatment when the house was gutted. However, when I was digging around this weekend, I found some termites in some wood chips right next to this side of the house. Having said all of that, would it be unwise to insulate the exterior? If I could find a way to get something like Roxul Drainboard, would that be any better? Thanks for any advice.

Asked by Kevin Schramm
Posted May 12, 2014 10:39 AM ET


5 Answers

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Heat loss is a delta-T x U-factor x area type of thing- all heat loss counts. Given the amount of effort it takes to excavate and waterproof that side, it would be a shame to pass up the opportunity to insulate it a the same time.

Termites don't usually dig through compacted sand, which may be useful as a select backfill. Adding a strip of copper Z-flashing extending to the exterior face of the insulation is also effective as a mechanical barrier (they won't chew through metal), but the leaching copper is also a toxin to the gut-flora that termites & carpenter ants/bees/wasps need to be able to digest wood.

Answered by Dana Dorsett
Posted May 12, 2014 10:50 AM ET


The experts here always recommend re-grading and redirecting water as a first step to any moisture problems, so you are heading in the right direction. I had two additional thoughts. First is that if you are excavating, you may consider putting in a drain at the same time. It's additional insurance and you will have already paid for the expensive part (excavating).
Second thought - the way the original post is written, it seems that there are wood chips in the landscaping next to the house. Having wood or bark anything next to the house is inviting pests, rocks would be my preferred material for any landscaping that abuts a house.

Answered by andrew c
Posted May 12, 2014 11:44 AM ET


Thanks for the replies.

Dana - are you recommending going all the way up to the siding with the insulation and then putting the z flashing in between. I've seen it where people say to leave a break between the siding and the foam so you can inspect for termites? Could that be achieved with the z-flashing?

Andrew - Yes, I was hoping regrading might help, but it's a little difficult because of my proximity to the neighbors. On this side of the house I have only 5 ft of property on my side and it is downward sloping from his house to mine. Because of this I was planning on putting in railroad ties on the edge of my property and removing soil from my side of the railroad ties. so that the ground slopes down to the property line. As it is right now, there is really no room to add soil because the soil is only a few inches below the siding.

About the wood chips - They were left when I had a tree trunk ground up next to the house a few months ago. I was in the process of removing the wood chips when I found the termites. Thanks again.

Answered by Kevin Schramm
Posted May 12, 2014 12:13 PM ET


As long as the Z-flashing is visible from the exterior, and is installed tight to the concrete just below the foundation sill it leaves no path for the termites to reach the wood that doesn't hit daylight- the exposed portion of the flashing becomes the inspection strip.

While the flashing is a thermal bridge between the concrete and the exterior at that point, it's certainly no worse (and probably better than) leaving a 3" stripe of bare concrete for termite inspection (often specified in local codes in lieu of a termite barrier.)

Answered by Dana Dorsett
Posted May 12, 2014 1:28 PM ET


One more question Dana. I am a little unclear about the z flashing. If the bottom goes over the insulation and the top tucks under the siding, wouldn't that create a space for the termites to crawl under? Wouldn't they be able to travel from the soil to behind the insulation, up under the z flashing and then into the house under the siding? Thanks.

Answered by Kevin Schramm
Posted May 14, 2014 12:40 PM ET

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