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Community and Q&A

Effective Foundation Insulation Approach (Lakes Region, NH)

scottwoodward | Posted in Energy Efficiency and Durability on

I’m learning my way through the construction of a two-car garage with 740 sq. ft. of living space above. The foundation walls are done, but the slab has not yet been poured — that’s coming soon.

I’m looking for advice on the best approach to insulate the foundation. I haven’t settled on any one approach, but so far I’m leaning toward insulting the outside walls only, based on the information in this earlier post ( and based on this post as well (

Scott in New Durham, NH

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  1. kbentley57 | | #1


    Would you mind sharing a few pics of the foundation walls? That'll put everyone on the same page.

  2. scottwoodward | | #2

    Kyle, yes. Thanks for the suggestion. Attached are a few photos. The inside of the foundation has been filled back in, but it's not too late to insulate under the slab, just not the inside walls at this point. These are 8" thick walls and roughly five feet high.

  3. kbentley57 | | #3

    Thanks for those pics!

    I think external insulation is always the way to go, as it protects your concrete from freeze thaw damage over its life.

    Those wall ties are protruding quite a bit, do they snap off or are they fiberglass ties? I'm afraid they might interfere with the insulation a little bit, Ideally the walls would be a bit smoother than they are, not that a day or two with a diamond cup wheel couldn't solve the problem.

    Alternatively, I wonder if they can be repurposed to hold the insulation in place in until it is backfilled.

    I would use a combination of a primer and self adhered membrabe Rockwool, and a drainage mat over the surface here if you can get it. There are more pieces involved, like the generic "protection board" you see in the diagrams and all that.

    Check out the oak ridge foundation handbook, there are a lot of great details in there.

    As far as the slab goes, I'd throw down a layer of gravel, and 2" of your favorite rigid foam, and that should be plenty, assuming you've insulated the foundation.

  4. scottwoodward | | #4

    Thanks Kyle. Those wall ties are metal bars that suspended the rebar in the forms -- three levels of rebar. They can be bent over fairly easily, but that doesn't solve the problem. A couple broke off when installing the sewer pipe, so they're not hard to break off. But, they would certainly be helpful in holding the insulation. So you're thinking Rockwool instead of XPS?

    The inside walls of the foundation are not insulated. Nothing is insulated at this point. How thick of a course of gravel would you suggest? My dirt guy filled right up to the marks for the bottom of the slab because I hadn't been considering insulating under the slab (based on advice from my foundation guy).

    1. kbentley57 | | #6

      While they would be helpful in holding it, it may interfere with the waterproofing, unless you used a fluid applied.

      I would choose Rockwool for a few of the reasons Bob listed below. It will help shed water, instead of becoming waterlogged like XPS or eps. I would protect it with a simple mat, because it's softer than XPS, and it'll keep the dirt out.

      As far as the stucco goes, it is a pain, I'll admit. I used hardboard screwed through to the walls for lath backing. It's not technically approved for outdoor, but it's really just a backer for the lathe. Durrock and wonderboard, and others will work too, they're all just susceptible to a limited number of freeze thaw cycles. I suppose you could use tyvek or similar behind the lathe just the same. It's only needed on the upper portion though, to about a ft below the ground.

      The gravel is for a capilary break, so that moisture doesn't continuously wick upwards. A few inches of #5 is really all you need, and then the foam, vapor barrier, concrete.

      1. scottwoodward | | #7

        Thanks again Kyle. What are the possible downsides if I don't add the #5, the foam and vapor barrier under the slab? I'm disappointed that my foundation guy didn't recommend this. My dirt guy only did 1 foot lifts of clean fill from the footings up to the bottom of the slab. It's not too late, but I'd have to have my dirt guy scrape off the 7-8 inches of fill to make room for the #5 and the foam.

        For the outside walls -- it would be the mat first (dimple mat?), then the Rockwool, and then the protection board? And the protection board is only for the first foot (or two) below grade?

        1. kbentley57 | | #8

          The vapor barrier could/should be installed regardless of the other two, it's just a large plastic sheet. Code specifies a minimum of 10 mil in some places, but people often use 6 mil that can be found in the big rolls at home Depot or Lowe's, you just have to baby it so that it's not ripped during the concrete work.

          The only real downside to not having them is increased energy usage, and the potential for moisture ingress if the vapor barrier gets ripped. There is such a depth of soil there that I don't really foresee that as a problem in reality. Wether it's worth the hassle is up to you, you only have this one time to get it right though. Code specifies this to be R10 in your climate zone now anyways, if your local municipality has adopted a.more.recent version of the IRC.

          The order of the insulation and dimplemat is debated pretty often. Some people argue that the dimplemat should protect the waterproofing layer, and the insulation goes on the outside. Others argue that the insulation should be firmly placed against what it's insulating, otherwise it looses some of it's r value.

          I fall in the later camp, in that I feel like I sleep better knowing that the insulation and waterproofing mbrane are separated from soil, and to some degree a bulk portion of the water, before it even approaches the wall.

          I'd spend a little bit googling that and reading more opinions, as I doubt there's an absolute best practice for all situations.

          Correct on the protection board. It only needs to extend below grade around a foot, or whatever material size is convenient to use without waste or making unnecessary cuts.

          The products I've used are Henry's db200, and wp200, aside from the insulation, if that gets you started on products. There are many that are similar, and they're all pretty good, so I wouldn't get too hung up on one in particular.

          Hope that helps

  5. user-1072251 | | #5

    I used to insulate the exterior foundations but the stucco you'll need to cover it with is a PIA and the ant, termites and a variety of rodents love to use it for tunneling so that it becomes exterior Swiss cheese, and a tunnel for getting into the house. I now only do the interior foundation walls. I always insulate under the slabs. You'll need a type of foam suitable for the weight of vehicles. Two issues: Exterior damp proofing or waterproofing and a heavy layer of poly under the slab will keep much of the moisture out and keep your space drier and more comfortable. The ground stays around 50; typically colder than interior living space. If you intend the interior to be above 50, a lot of that heat will be absorbed by an uninsulated slab. ("Heat Sink") So interior heat and humidity are best addressed at the level you are at now.

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