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Community and Q&A

Closed cell spray foam/cellulose combination

cabinflyer | Posted in Green Building Techniques on

To all,

Background: 1500 Sq Ft. single level home, Northern Minnesota Zone 7, almost 10,000 HDD.

I am getting ready to insulate my double studded walls. The wall cavity including 2 2×4 24″OC studs separated by a 1″ gap for a total cavity of 8″. The ceiling has 2′ energy heel trusses with 24+” of blown in cellulose

The original plan was to insulate the whole wall in cellulose, and at R3.9/inch the goal was 31 with no thermal bridging. Inside the cavity there was going to be a smart membrane (Intello Plus)

Recently my insulation contractor approached me with the idea of spraying the outside 2×4 wall with 3.5″ of closed cell spray foam. He would also encapsulate the exposed ends of the studs. Then he would then fill in the remaining 4″ cavity with cellulose. The positives are the Rvalue jumps to 40.1or a 23% increase. I would also save the costs of a very expensive air barrier I would have to meticulously detail. The downside is the added costs of the spray foam. I am getting a tremendous discount on the closed cell foam.

Please don’t overthink the costs of global warming etc…Just looking into the potential downside of this application as it relates to the wall.



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  1. user-2310254 | | #1

    Do you know what type of closed cell foam? Older products use blowing agents that are very damaging to the environment.

  2. Expert Member
    Michael Maines | | #2

    Steve, is the Intello also supposed to be an air barrier? Closed cell spray foam is not a great air barrier on its own, despite what salespeople may tell you. If you don't have a good air barrier somewhere in your assembly you are at risk of moisture issues.

    Cellulose dense-packed at 3.5 lbs/ft³ is generally about R-3.6/in, or R-29 at 8" deep. (Only one manufacturer I know of claims R-3.9, but I've asked them if they do anything different than other manufacturers and the answer is no, so I trust what most other manufacturers report.) In deep cavities the density typically goes up a bit, which can reduce the R-value per inch a bit.

    Once you add your wall studs into the mix, you're actually looking about about R-28 for the cellulose-only wall and R-35.5 for the flash-and-fill wall. The IRC code in your zone, for your situation requires 42% of the R-value to be in the spray foam, so you're in the clear in terms of moisture accumulation in that regard, with R-20 on the cold side of the wall and R-15.5 on the warm side.

    If your goal is strictly to save yourself money on energy, I'd be surprised if the upcharge for the foam is worth it. A rough guess would be that in your case the foam would save something on the order of $25 per year in energy, maybe $50 at the upper end. ($1000/yer energy costs x 10% loss through walls x 25% reduction in heat loss through walls.)

    I know you don't want to hear about global warming, but if any part of your goal is to do your part to help the environment for future generations, it's worth considering that most closed cell spray foam is over 400X worse for global warming than cellulose. HFO-blown foam, which is much better for the environment, is finally starting to hit the market--here in Maine, anyway--but it's still not common.

  3. GBA Editor
    Martin Holladay | | #3

    Michael Maines gave you a fairly complete and accurate analysis. The spray foam won't create any moisture problems in the wall. However, it costs more than cellulose, and most brands of closed-cell spray foam are bad for the atmosphere.

    For more information, see Next Generation Spray Foams Trickle into the Market.

  4. cabinflyer | | #4

    Answer to Steve Knapp. Gaco Western is the manufacturer of the closed cell spray foam. Their website says they are one of the companies using the newer isocyanate-free blowing agents.

  5. Expert Member
    Michael Maines | | #5

    It looks like Gaco Western's One-Pass closed cell foam probably has the low-GWP blowing agent, but they don't specify that anywhere that I can see--they are just touting thick layers in a single pass. There's nothing to suggest that their other spray foam products have anything but conventional blowing agents.

  6. Expert Member
    Dana Dorsett | | #6

    Closed cell foam thermally bridged by studs is a waste. The thermal bridging robs it of it's full potential. While the center-cavity R may be getting a 23% increase, the whole assembly performance will be improved by less than 5% in your case.


    From a cost/performance perspective the money is better spent elsewhere.

    The words "... with no thermal bridging..." are cringe worthy in the context of an studwall. Unless all the insulation is a continuous layer, with none of it in the cavities best you can do is "thermally broken framing". But if the thermal break is a mere 1" of cellulose between the courses of studs it's providing at best a ~30% reduction in the thermal bridging of the framing. If you cut some 1" polyiso into 1.5-2" strips and glue them into the 1" gaps between the studs you'd do somewhat better, but even there the money might be better spent elsewhere.

  7. cabinflyer | | #7

    Without debating too much the article you sent a link to doesn't accurately reflect what we are accomplishing. As your article points out, closed cell foam is difficult to trim hence the reason they only spray 3" in a 3.5" bay. What we are doing is spraying the whole bay as well as the exposed ends of the studs. top, and bottom plates. All wood to wood joints and seams have been sealed with acoustic sealant. in both the top and bottom plates.
    I don't know where you came up with only a 30% reduction in thermal bridging. Our WUFI modeling shows the walls...including the most important side of the wall...the one I'm going to be living at will all preform quite nicely if it was ALL cellulose with a proper air barrier...even at -50F. (yes it gets THAT cold here).

    The point of my original question was if we substituted closed cell foam for the cellulose in the "outer" wall and replaced the air barrier with spray foam to include the wall plates, but still kept the cellulose in the "inner" wall plus the 1" cavity, would we run into any unforeseen issues.

  8. Expert Member
    Dana Dorsett | | #8

    Wood is still as thermally conductive when surrounded by R7/inch foam as it is when surrounded by R3.9/inch cellulose. The laws of physics are self-enforcing.

    Without doing 3-D modeling of the problem to get more significant digits of accuracy...

    With 3.5" + 3.5" of R1.2/inch framing timber separated by 1" of R3.9/inch cellulose, you have R8.4 for the wood, R3.9 for the cellulose, R12.3 total for the framing fraction. Without the R3.9 thermal break the framing would be R8.4. The change in heat moving through the thermal bridge is then roughly 1-(R8.4/R12.3)= 0.32, or 32% as a first order of approximation.

    With R7/inch foam instead of cellulose in the 1" gap between inner and outer framing the total R at the framing fraction is R8.4 +R7= R15.4, to the reduction in heat moving through the thermal bridge is then 1-(R8.4/R15.4)= 0.45, or 45%. That's a real improvement from the 32% reduction, but it's still only improving the framing fraction by 45%-32%= 13%.

    There's just not a lot of heat moving through the 7" (R27) of cellulose at center cavity in the first place, so improving the R value at center cavity by 23% isn't as a big change in the total heat flow as improving the R-value at the thermal bridge. At a 20% framing fraction (typical for 24" o.c. framing) the R12.4 thermal bridge is moving a disproportionate fraction of the heat relative to it's proportion of the face area than the R27 cellulose. The 1/R12.4 is a bit more than twice 1/R27, so even though it's 20% of the face area it
    t's moving as much heat as 40% of the center-cavity area, so roughly 1/3 of the heat flow is still through the framing fraction.

    Due to the disproportionate heat flow per face unit of area, improving that framing fraction by 13% does about as much reduction is heat flow as improving the center-cavity R by 23%, which is why inserting strips of R6.5 polyiso for the 20% framing fraction is a much better bang/buck. It's roughly half the thermal improvement at a small fraction of the cost.

    Nobody is suggesting that this a low performance wall, only that the performance INCREASE from changing the fill for the outer 3.5" with closed cell foam is small, and probably not the best performance return on investment, given the high cost.

    A WUFI simulation would answer the question as to whether changing the outer 3.5" to closed cell foam would create unforeseen issues. But simulating it both ways would also demonstrate the (small) magnitude of the net thermal improvement.

  9. cabinflyer | | #9

    We opted to not go the closed cell spray foam route for several reasons, costs being one of them.
    Thanks for all the comments.

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