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If slab edge thermal break is inside the stem wall except for at door openings, how far past the opening should I extend an exterior thermal break in those door locations?

rocket190 | Posted in Energy Efficiency and Durability on

I have a 4′ deep stem wall with a independent concrete slab floor. I have a 2″ XPS foam thermal break around the entire slab edge except for at door openings where the frost wall was dropped to accommodate a thicker slab.

I want to add an exterior thermal break at the door opening locations. How far past the door openings should I run the thermal break in order to prevent heat loss from “sneaking past”? Or will this not be a very effective thermal break since I’m switching from an inside to outside location at these spots?

Secondly, what do you think is a better thermal break…..1″ cellular PVC trim board with a foam concrete expansion strip. Or, two layers of foam concrete expansion strip. I can’t find an r-value for the foam product. It looks like a similar foam as the sill sealer product, but is probably 3-4 times thicker.

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  1. GBA Editor
    Martin Holladay | | #1

    Concrete is a good conductor. That means that you want to insulate your frost wall either entirely on the interior side of the frost wall, or entirely on the exterior side of the frost wall. If you try to switch from interior insulation on part of your foundation to exterior insulation on another part, the concrete frost wall will ask as a thermal bridge, bypassing your insulation.

    Cellular PVC trim boards are dense, like hardwood. They have a low R-value per inch, and should not be used as insulation. To insulate the perimeter of a concrete slab or a vertical frost wall, you need either rigid foam insulation or (in some cases) mineral wool insulation.

  2. rocket190 | | #2

    Martin, hanks for the information. I should have mentioned that these slab edges are in an insulated but unheated portion of my building. Even though it's not heated, I insulated the perimeter wall to R-10, and was planning on R-10 under the slab. I am not doing in floor heat, but I wanted to prevent floor condensation in the summer months, and also provide a "just in case" I desire to add heat in the future. There is a heated part of the building that will also be insulated to R-10 for the slab and R-10 for the frost wall. I will separate those two areas with a strip of 2" foam in the slab that will be hidden with a studded partition wall.

    The reason I went with interior foam against the frost walls is because I didn't want to worry about exterior foam being damaged by insects or weed wackers. Also, the products made to protect the exterior foam at grade level do a poor job in my opinion. The fiberglass "ground breaker" product turns from gray to clearish yellow over time (the natural color of fiberglass) and the stucco finish makes the foundation look like an old rubble stone foundation that someone tried to spruce up. In addition, you still would have foam exposed along the outside of the foundation at the door opening locations.

    That said, the interior foam works great, except for at the service doors and garage door openings. If I brought an inside thermal break to the top of the slab there would be a visible strip of foam in the floor, and I would have an 8" unbonded strip of concrete along the door opening that would probably break off. That's a really poor option

    I was hoping that I could lap an exterior thermal break far enough past the door openings that it would be effective, but if I understand you correctly, this probably won't work? Any other ideas?

    FYI, cellular pvc is specified at r-2.1/inch based on info I ca find on the web.

    When people build a house with attached garage, and it's on a shallow frost protected foundation, they must have the same issue. What do they do?

    From what I've been told a product like this won't work because frost will damage a driveway apron mechanically connected to a building floor. Either the apron will bust, or it will bust off the slab edge. Seems like frost usually wins....

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