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Is 3″ open-cell foam enougth?

dchaplin | Posted in Energy Efficiency and Durability on

I live in northeast Indiana and want to insulate my 30x40x10 pole barn shop, which will be heated with radiant floors using a propane boiler.  I’ll probably keep the barn between 55-60.  I know air infiltration is the biggest challenge for a pole barn, so the foam is high on the list if the budget allows it.  

The foam sales guy claims that because it’s so air-tight that  3″ of open-cell foam (R12) is plenty and that taking it up to R20 would never recoup the additional costs.  I understand there are dimensioning returns for continuing to add more R-Value,  but I’m not sure if R12 is enough.    How can that be calculated to find out a break-even point?

My house is pretty high-performance, with R40 SIPS and an ACH of 0.6.   I hate to spend more to heat that shop than my house!  lol

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  1. Expert Member
    BILL WICHERS | | #1

    Are you required to hit any particular R value here per code? If not, then you can go with the "anything is better than nothing" technique. If you do have to meet code, then you'll likely need something more, since I'm assuming you're in climate zone 5, where R12 is not enough for the walls or the roof. Spray foam applied evenly over the entire surface acts like continuous insulation, so it does perform better than batts in this case, but that "air sealing makes it better than something else of the same R value" is some marketing BS. R value is R value, and thermal bridging will knock down the effective R value of spray foam just as it will with any other insulating material.

    If you consider that the spray foam is likely to be installed in a continuous layer without thermal bridges, and you work out the R value of spray foam required to meet the U factor requirements in the code (which apply to the overall assembly, which is how continuous insulation works too), then you need about R16.7 on the walls, and about R38.5 on the roof. Note that this applies in this case not because spray foam is magic or air seals well, but rather because the sprayfoam covers everything in a single layer, with no thermal bridges protruding (such as how studs are between batts in a regular stud wall).

    I think you'll probably be spending more to heat your shop than your house if you only go with 3" of open cell in your pole barn.


    1. dchaplin | | #6

      There are no Code requirements, just a conscious effort to build the above code in an affordable manner. I agree that it's most likely a high BS factor for only trying to sell airtightness.

  2. Expert Member
    Michael Maines | | #2

    Foam salespeople and manufacturers just love to exaggerate and outright lie about their products. Any insulation can be made to be airtight; there's nothing special about foam's R-value other than it provides air sealing in addition to slowing heat flow. But it doesn't help with air sealing where most air leaks happen, at the edges and transitions between different planes and materials.

    Northern Indiana is in climate zone 5; the 2021 IRC requires R-20 cavity insulation plus R-5 continuous insulation, or R-13 cavity plus R-10 continuous, or R-20 continuous-only. 3" of open-cell foam is about R-11. Accounting for framing, your whole-wall value would be around R-8. In other words, less than half of what the building code requires.

    While increasing R-values does have diminishing returns, I almost always find a decent return on investment to go above code-minimum. Doubling the R-value slows heat loss by 50%. I use BEopt to model different options: It's free and fairly easy to learn how to use. This is another free program:

    1. dchaplin | | #7

      thanks for those links! I'll dig into them.

  3. 1869farmhouse | | #3

    Can I ask why you’re going with open cell? Especially in a pole barn, closed cell seems to be a better fit in my opinion.

    1. Expert Member
      Michael Maines | | #4

      Why do you say that? Closed-cell uses significantly more up-front carbon emissions to get the same R-value as open-cell foam, and it's just as airtight; it just doesn't block water vapor movement. I would rather see one of the many low-carbon insulation products available.

      1. dchaplin | | #5

        My Sip house is closed-cell is incredibly air-tight. This sales guy likes Open-cell causes it's 60% cheaper.

  4. Expert Member
    Akos | | #8

    You can calculate this. Your 30x40x10 barn would have 1400sqft of walls and say 1200sqft of ceiling.

    Assuming you are somewhere around 8000 heating degree days, at base 50F or 60F it would be less but for now we can use this for rough calculations.

    Heat loss through an R12 roof/wall through the heating season:

    8000 HDD *24h * 1400sqft walls /R12= 224 therms
    8000 HDD *24h * 1200sqft roof /R12= 192 therms.

    At 50/60F it will be less but none the less at my local propane costs, that is real money.

    About 1.5x to 2x as much insulation is a good place to aim for. One cell is fine for walls but might be problematic on the roof, safer option is to use a flash of closed cell against the roof deck with open cell for the rest.

    You might be able to get an all open cell roof to work if you include a diffusion vent at the ridge. This is something that works in warmer climates but since there should be no major moisture source in your building, it might work in colder climate.

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