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How do you tell closed-cell foam from open-cell foam?

user-2895420 | Posted in General Questions on

Our home is all closed in, and we have spent literally hundreds of hours caulking, foaming and sealing our house inside and out.   Our insulation contractor just finished filling all our wall cavities with 5-1/2” of rock wool, and they sprayed (most of) the band joist with foam.   We’ll have blow-in cellulose in the attic which has to wait for drywall.

We had spec’d out 5-1/2” of open cell foam for the band joist, as we also have Zip-R sheathing that adds another R-6.  Unfortunately, when we inspected the house the next day, we found that the spray foam company hadn’t sprayed the band joist in the stairwell, and they had missed several other small spots in the basement band joist.  So we had them come back.

After they left the second time, we started measuring the foam thickness, and found that they had only sprayed an average of 3” of foam in the band joist.  The foam ranges from 2-1/2” to 3” deep.

We pulled our spec sheet, and the quote we received and pointed out that they were 2-1/2” shy of what we had asked for.  The response from the foam company was “we had closed cell foam in the truck, so that’s what we sprayed, and you ended up with the same R value”.

Can someone tell us; how can we tell if we did indeed receive closed cell foam, instead of the open cell foam we spec’d out”.   Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    Open cell is easily dented/penetrated with a finger, and has very lumpy popcorn surface appearance due to the very high expansion ratio.

    Closed cell can be dented with a fingernail but you can't push your finger into it, and has a somewhat smoother, often shiny appearance, not nearly as lumpy. A true closed cell foam guru can make it almost flat, installing it in one steady path leaving it very smooth, but most installers would leave some amount of wave/lumpiness to it from using a back & forth motion or multiple short passes.

  2. Expert Member
    BILL WICHERS | | #2

    If you have some leftover chunks of foam to test, try snapping a piece in half. Closed cell foam tends to break with a loud SNAP leaving a relatively straight, clean edge on the broken side. Open cell takes much less force to break, and makes a pretty ragged edge on the broken side.

    Closed cell foam does have quite a bit higher R value per inch, and vastly less vapor permeability. I don’t think you’d have a problem with lower vapor permiability, but it depends on your application so it’s worth mentioning.


  3. Stockwell | | #3

    Hi Nancy--I am pretty sure I live a couple streets over from you. I could tell you if it is closed cell or not, plus would love to see all your air-sealing efforts(we are still in framing).

    1. user-2895420 | | #4

      Would be happy to do that Kevin. What street are you building on/how do we get ahold of you?
      Based on the feedback, our foam does appear to be closed cell.

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