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Spray Foam vs. Rigid Foam / Cellulose Combination

orangutan_librarian | Posted in General Questions on

I would do more research, but we are pretty far into the design process of a build and I was thrown a bit of a curve ball.

I am in climate zone 5b

My architects have designed a wall with 2×6 24″ OC studs, 1.5″ foam board and an airtight house wrap with cellulose insulation. I am planning on trying to hit .75ACH for the blower door test. My builder is more familiar and comfortable with using spray from (specifically, Demilec Heatlok).

I am leaning toward letting him do what he does best and go with the spray foam. I suspect it will be easier to hit the blower door test targets this way. With either method, I may use AeroBarrier if the house is too leaky. What am I missing?


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  1. user-2310254 | | #1

    I'm assuming the builder wants to delete the exterior rigid foam, correct?

    The global warming potential for Demilec Heatlok High Lift or Pro is 1, so pretty good from an environment standpoint if they are are proposing either product. That said, spray foam can be more expensive and really doesn't deliver much advantage in R-value (at least based on what I've read here). And the builder will still need to pay attention to air sealing if you are targeting .75 ACH/50.

    If it were my house, I would prefer your architect's plan but also could see letting the builder (and the sub) use the method they are more comfortable with. At the same time, I'd wouldn't want to absorb a big budget hit or end up with a wall that delivered lower performance.

    Be sure to ask how much foam they intend to install. With closed cell, they will want to stay below the edge of the framing.

    1. Expert Member
      Michael Maines | | #4

      Steve, the global warming potential for the BLOWING AGENT is about 1-1.7, but the total GWP of the foam includes the embodied carbon of the other ingredients. I don't know the exact GWP of the overall HFO foam formulation but it's in the neighborhood of 200-300. Much better than the 1000+ GWP for conventional HFC-blown foam, but still much higher than alternatives. I call it less bad, but still bad, and only use it when nothing else will reasonably work.

      1. user-2310254 | | #5

        Hey Mike,

        Thanks for the additional insight. I'm with you on avoiding foam (especially in walls). I guess my point was that he should use the least damaging foam product, and Demilec Heatlok seems to fit the bill.

        If he is stuck with using foam in the walls, open-cell foam makes more sense don't you think? A lot less plastic and can be trimmed flush with the framing.

        1. Expert Member
          Michael Maines | | #6

          Steve, I agree that he should use the least-damaging product that fits his needs, and budget. I never use open cell foam in walls or roofs, because it has no benefit compared to a relatively low-impact insulation such as cellulose that has separate air control and vapor control layers. But I agree that if he is determined to use foam, open cell is a better choice in walls than a flash-and-fill system: Either way, the wall will perform significantly worse than it would with continuous exterior insulation.

  2. orangutan_librarian | | #2

    Thanks for the breakdown! Yes, it's the High Lift product.

    Any thoughts on long term performance of spray foam? I see some concerns with it handling expansion and contraction cycles and forming cracks- but because it hasn't been used in houses for too long this may be an unknown?

    1. Expert Member
      Michael Maines | | #7

      My first project with polyurethane spray foamed walls was around 1996; 25 years is enough to have a good idea of long-term performance. From what I've seen, you will likely have areas where the foam has separated from the framing--not enough to really impact insulating ability. A bigger concern is improper cure. It's rare, but when it happens it's a disaster. You never have a cellulose or fiberglass installation render a house uninhabitable due to improper cure, but it happens with closed cell foam sometimes--I know of several projects personally, and have read about many more online.

      One change over time is guaranteed--the initially high R per inch will drop, as air displaces the chemical gasses in the foam cells, eventually leaving foam that is R-5.6/in. How long that process takes will vary, but don't trust the industry long term R-value (LTTR), which is an estimate of what the R-value will be after 5 years, based on 18 months of accelerated aging in carefully controlled conditions.

      To be clear, I'm not anti-foam; it's an amazing product in the few situations where nothing else will work properly. But I am anti-lying to sell your product.

  3. JC72 | | #3

    The thing with spray foam is that it must be applied using the correct mix within a specific temperature range. If not then you can have some serious issues (google is your friend).

    They'll have an easier time hitting the air-tightness with spray foam. Demilec Heatlok closed cell is obviously not as "green" as exterior rigid w/cellulose but you'll hit your ACH.

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