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Community and Q&A

Thermal Break Between Garage and House Foundation?

Lance Peters | Posted in Energy Efficiency and Durability on

CZ6A, Ottawa ON

So I’ve been Googling and finding nothing.

I have a pretty standard foundation design for a two story with attached garage. I’m planning to do a capillary break between footings and foundation walls, exterior membrane and dimple mat, exterior above grade insulation, all the usual tricks to keep the basement warm and dry.

With an attached garage foundation as well as a poured front porch extension, both of which connect to the house foundation, how do I keep heat in and moisture out where the garage and porch meet the house foundation? I guess I could treat those parts of the foundation the same way as the house section, but that’s a lot of work and expense.

Is there a way to thermally and hygroscopically separate the two? I was wondering if separating the two sections with high density foam and having multiple penetrations of rebar would work? This would still transfer some heat but would hopefully keep the water from migrating while allowing a structural connection.

Thoughts? Has anyone seen something like this tried before?

There are also high density thermal breaks designed for steel member attachments in commercial buildings. If foam is a terrible idea, perhaps something like that could work?

Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Lance,

    I will give you a response to bump this post. If you are insulating under the slab and around the perimeter, I wouldn't think you would incur much of an energy penalty. (Just guessing here.) On the porch, my biggest concern with a thermal break would be differential settlement.

  2. User avatar
    Michael Maines | | #2

    Lance, it sounds like you're talking about a house with a full foundation, and a garage with a frost wall, as typically required in cold zones where an unheated structure joins a heated structure? I'm not sure what you mean by a "poured front porch extension."

    You specify "exterior above grade insulation." I imagine that insulation also extends below grade, with no interior insulation?

    I have used exterior foam between a house foundation and garage foundation, as a thermal break, though this is one of many reasons I prefer interior insulation. If you want to tie two foundations together in a thermally broken way, products like this can help: https://www.schock-na.com/en-us/home, but I'm not aware of any made specifically for this application.

  3. Malcolm Taylor | | #3

    Lance,

    There has been several discussions here on just that, and as I recall one resulted in a thermal-break filled with foam much as you describe. The GBA search function should turn them up, but I'm not sure what terms to use.

    What as unclear to me, and seems like a problem, is how you handle the part of the garage foundation at the insulated gap from grade to the bottom of the framed walls above,

  4. Lance Peters | | #4

    To simplify how to visualize what I'm thinking, picture a foundation with a complete footing to sill layer of exterior foam insulation. Now consider that the footings are common with an attached garage and a formed porch back-filled with poured slab. The idea is separating the house foundation from the porch and garage both thermally and hygroscopically.

    Steve,

    Differential settlement is what I would think to be the biggest concern as well. Loading the footings with rebar could perhaps help keep continuity between the sections should cracking occur?

    Michael,

    Yes, garage frost wall attached to house foundation wall. Yes, above grade insulation that goes roughly 18" below grade. Those Schock products look super interesting, some of the balcony products are designed to handle tension and shear loads. I'll have to look into that!

    Malcolm,

    Not sure about your concerns regarding the above grade section of the wall? Are you thinking about relative motion causing problems with finishes and/or wall fitment?

    Funny you should mention using the GBA search. In an effort to find a picture of what I was talking about I Googled "foundation with porch" and looked at images, shortly after I found this build blog referencing thermal bridges in foundations:

    https://kimchiandkraut.net/2017/01/02/foundation-details/

    And towards the bottom of the page they reference the Flatrock Passive project by David Goodyear covered on GBA, and he did pretty much exactly what I was proposing to do (A Tale of Two Foundations). He doesn't have any pictures of the foam installation, but he has a very good description of the process:

    http://flatrockpassivehouse.blogspot.ca/2017/05/a-tale-of-two-foundations.html?spref=tw

    He also has some rough calculations of the heat loss and cost savings benefits. I'll have to look up his cross sections and see how similar it is to what I'm planning in order to judge how applicable his math is to my situation.

  5. User avatar
    Michael Maines | | #5

    Lance, for discussion purposes, here is one I did recently, a frost protected grade beam design for a timber-framed structure. I didn't worry about tying the two foundations together, and neither did the licensed engineer who checked my design. Nothing is labeled but hopefully the hatch patterns are clear enough. Basically two separate foundation systems.

    Do you have interior insulation? I'm just not understanding why you would be concerned with thermal bridging etc. if you only have insulation extending 18" below grade in a cold climate.

  6. Lance Peters | | #6

    Michael, thanks for the picture. I'm seeing two footings with insulation on both sides and bottom sitting on a bed of crushed stone, is that correct?

    Yes, I will have an insulated slab (probably R12) as well as full interior wall insulation on the foundation walls, R20-R22 to meet code. The exterior insulation is to increase the overall effectiveness of the basement insulation: by insulating the above grade foundation walls from the extreme outdoor air temps, the concrete should stay closer to the ground temperature which means my code-minimum interior insulation will be performing at roughly half the temperature delta during our coldest weather, which can be in the neighborhood of -25C to -30C (-13F to -22F) night time lows for a month or so Dec. - Jan.

    Grade is to be 30" below the top of the foundation walls, so 48" wide insulation panels will sit about 18" below grade. Below that it will be a waterproofing membrane and dimple mat decoupling the walls from the soil.

  7. User avatar
    Michael Maines | | #7

    Lance, thanks for the additional information; now the insulation only 18" below grade makes sense.

    You're correct, we used compacted crushed stone footings on this project. Where the house and garage don't meet we have a wide foam frost wing.

    Unless you're going for very high levels of energy efficiency, if you have interior insulation on your foundation walls, I probably wouldn't worry about thermal bridging between the house and garage.

  8. Malcolm Taylor | | #8

    Lance,

    it's a minor problem, but the foam (and whatever is protecting it) between the house and garage foundations from grade to the underside of the garage wall framing needs to be somehow detailed to stop water from intruding. When the two foundations are connected this isn't a concern.

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