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Community and Q&A

Insulating Three Different Slabs

nynick | Posted in Energy Efficiency and Durability on

Slab #1 will be in a 3 car garage that will have an apartment upstairs. I plan on conditioning the garage portion “lightly” so it doesn’t freeze or get too humid. Should I insulate this slab?

Slab #2 will be in an addition to our home. The addition will have a full conditioned foundation and be connected to our basement that we plan on conditioning as well. The inside walls of the new foundation and the old basement walls will most likely be spray foamed. Should I also insulate this new slab?

Slab #3 will be in another new addition that will have a crawl space foundation. The interior walls of this crawl space will also be spray foamed. We plan on connecting this crawl space to the conditioned basement so it would be conditioned as well. Should I insulate this slab?

All the foundation walls will be or already are concrete.



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  1. DC_Contrarian_ | | #1

    What climate zone are you in?

    1. nynick | | #2

      Zone 5, Coastal CT.

  2. Expert Member
    BILL WICHERS | | #3

    I would absolutely put in subslab insulation on slabs #2 and #3, the basement and the crawlspace.

    I would check with your engineer about slab #1. The reason I might not insulate under this slab is that if you plan to park cars in that garage, the insulation might increase the risk of cracking if you don't do anything fancy with the slab (extra reinforcement, thicker pour, etc.). If you assume a car weighs 3,000 pounds, and you assume each tire contact area is about 4" square, that means the car is putting about 47 PSI worth of force on those areas where the tires are. That's more than the normal 25 PSI rated foam. I'm thinking that a thicker 6+ inch slab and some rerod might spread out the load enough that that won't be a problem, or special order some higher PSI rated foam. If your engineer works out a system that will work, then I'd insulate this garage slab too. You just want to make sure you won't have any issues with compression of the foam first since the garage slab is going to carry a lot more weight with cars parked on it compared to the other "normal" slabs.


    1. nynick | | #8

      Thank you. 2 inches of foam?
      I've heard spray foam works well for this application as well.

  3. Expert Member
    Michael Maines | | #4

    Under garage slabs I've used 25 psi foam and a 5" slab without problems. On one project the engineer required 40 psi foam. Loads transfer through concrete at about 45° so the 4" square load area at the top of the slab is about 12" square by the time it reaches the foam.

    1. Expert Member
      BILL WICHERS | | #5

      Did you put any reinforcement in those slabs? I usually spec remesh in slabs regardless of what the slabs are for, since it helps with cracking. I'm curious if your engineer required anything specific IN the slab instead of just beefier foam.


      1. Expert Member
        Michael Maines | | #6

        Bill, I had to review the drawings to answer your question, and realized I made a mistake--under the garage slab we had 25 psi foam; it was only under a center post with a 20,000 lb point load that he wanted 40 psi foam. The slab was 5", 4000 psi mix with #4 rebar 16" o.c. in both directions.

        I usually do my own engineering, though I'm using licensed pros more these days, and when I'm worried about cracking--typically exposed slabs in living spaces or garages--I spec a grid of #4 bar with extra-fine micro-fibers. Sometimes at contractor request I'll do 6" WWM but it's so rare for it to get installed properly that it's not my first choice. I usually spec 2500 or 3000 psi concrete because it shrinks less than higher-strength concrete, and a 7-day damp-cure, and I almost never get obvious cracks.

        1. Expert Member
          BILL WICHERS | | #7

          I know what you mean about the improperly installed mesh. Many times I've had to core floor slabs in commercial warehouse-type buildings, and the mesh is UNDER the slab. When I write my spec, I specify that the mesh will be installed at the centerline of the slab, and then I check that it's on chairs or other support prior to the pour. All too often the mesh is just laid on the ground, or it's supported every 20 feet or some other ridiculously insufficient amount to keep it where it's supposed to be during a pour.

          Thanks for looking into your garage slab detail for me.


  4. charlie_sullivan | | #9

    I'd insulate under all three. The garage won't benefit much thermally if you don't condition much beyond keeping it above freezing in the winter, but it will help with avoiding high garage humidity in the summer.

    You seem to be planning a lot of spray foam. That's an expensive choice with high to very high climate impact that doesn't have real benefits over other choices in new construction.

    1. nynick | | #10

      Thanks Charlie. Coincidentally I have a meeting today with an insulation contractor, a GC and an energy consultant to determine the best approach for this old pun intended.

      At 150 years old there are multiple additions and construction methods. The walls are mostly nominal 2x4's and the rafters seem to be nominal 2x6. I would love to minimize the usage of foam everywhere. Zoning R values are not easy to achieve with these thicknesses, but they are doable, especially with spray foam.

      So we will see. The more cellulose and Rockwool I can use, the better, especially in the new portions, but demolition and site conditions for the old sections are limiting.

      Thanks for your input.

      1. Expert Member
        BILL WICHERS | | #11

        I would avoid using spray foam in walls -- it really doesn't gain you anything there. Rated R per inch for open cell spray foam doesn't beat mineral wool, and effective R per inch for a studwall with closed cell spray foam doesn't either since you can't get a full depth fill. You're much better off using mineral wool in the stud bays (good R per inch, and easy to install well), then putting rigid foam on the exterior for some continuous insulation that will do wonders for your whole-wall R value, especially with 2x4 framed exterior walls.

        Don't buy into anyone telling you that spray foam performs better than it's rated R value because of air sealing, either. R value is R value, period. Air sealing can be done other ways. The R value of spray foam BEHAVES EXACTLY THE SAME as the R value for other insulation products, assuming things are installed equally well. I usually assume about R 6 per inch for closed cell spray foam, and a max fill of 3" in a studwall which would be about R18 best case. In practice, the achievable fill is around 3" at the edges, but something less in the middle, so your effective R value for closed cell spray foam in a 2x4 studwall is usually going to be right around R15, the same as a mineral wool batt.


        1. nynick | | #12

          The real world is a very difficult place for energy efficiency nerds like me and you. I just finished my meeting with no fewer than 7 people in my house. 2 Energy consultants, 2 GC people, 1 large Insulation company representative and the architect.

          The 2x4 exterior walls present a problem because of the cost of adding 2" of foam etc. which then creates overhang issues for the gables, eaves and window framing and doors. Lots of labor, lots of material cost etc. The proposed solution was to spray foam the 2x4's with 3" with a good WRB on the exterior, maybe even a drainable WRB.

          Don't shoot the messenger.

          The attics were also proposed to be spray foamed to 7", covering the 2x6 rafters with 1" of foam to slow thermal bridging. This also removes the "need" for rigid foam on the roof. Again, lots of cost to rebuild a roof for 2" of foam.

          I might add that the GC was trying to do his best to weigh construction cost to efficiency while the Insulation Rep was open to discussion. The energy consultants will be modeling all these decisions and even the architect seemed to be convinced this was the best way to go after all was said and done.

          Having not seen any budget numbers on the entire project, I'm in no position to discuss alternatives until I have a baseline on overall costs. Once I see what it might cost, I can back peddle or move forward as I see fit.

          I will take a hard look at having only one attic foamed/conditioned and the other 2 vented with blown-in insulation to see the money delta.

          I'm sure it'll only get harder as we start ripping open walls.

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