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Community and Q&A

Open- or Closed-Cell Spray Foam

ron0923 | Posted in General Questions on

I’m  currently building a house with 2×4 exterior walls in climate zone 4. the walls are sheathed with 5/8 inch cdx plywood and will have peel and stick WRB [ Henry VP 100 ]and then all the  exterior walls will have a layer of 2and1/8 inch EPS foam sheathing . A vented rain screen and board and batten siding. My question is will open cell spray foam be ok to use in the walls and bandboards?

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  1. Expert Member
    RICHARD EVANS | | #1

    This is a great wall assembly for your climate. OC foam will work just fine but I would use cellulose if you have access to an installer. If your exterior EPS covers the rim board then you won't need spray foam between the floor joists. Netted cellulose will work great there.

    Open cell spray foam is extremely flammable and releases deadly smoke if it ever catches fire. Also, if not applied/mixed properly, repairs can be a nightmare.

    Why risk it? It adds no benefit whatsoever for walls in new construction.

  2. ron0923 | | #2

    thanks Rick,
    Any concerns about settlement from the netted cellulose?
    Here's alittle more info on the build . the foundation is an ICF crawl space with 2inch foam under a concrete slab. The exterior EPS does cover the rim board. would you use netted cellulose in the rim boards of the crawlspace since they will be left open?

    1. Expert Member
      Dana Dorsett | | #6

      >"Any concerns about settlement from the netted cellulose?"

      If installed at 3lbs per cubic foot or higher there is zero concern about settling in the stud wall bays. Almost all netted cellulose is first dense-packed to 3lbs+, then becomes slightly denser when the pillowing is rolled flat to the stud edges.

      In a zone 4A climate the thermal mass of cellulose gives it slight performance edge over open cell foam of the same R-value. The "thermal diffusivity" effect delays (and reduces) the peak (or low) indoor surface temps relative to the outdoor peaks, slightly reducing the peak heating & cooling loads while evening-out the indoor temps a bit. This factor becomes more noticeable in higher-R walls, but still measurable in 2x4 walls.

      From a green building perspective, cellulose has a negative carbon footprint- it's sequestered carbon. Half pound open cell polyurethane foam's carbon footprint is comparable high-density fiberglass or rock wool (but only about half that of HFO blown closed cell foam at any given R-value) , but it's still in positive territory.

      >"...since I have the EPS on the exterior that closed cell would not be permiable and allow any drying that may need to occur."

      With the 2" EPS on the exterior you DEFINITELY want to use open cell rather than closed cell on the interior in the band joist area, but not necessarily near the floor where it can become waterlogged in a minor flood.

  3. ron0923 | | #3

    I was going to use closed cell for the rim boards in the crawl space but don't have enough to make it economical for an installer and I'm under the impression since I have the EPS on the exterior that closed cell would not be permiable and allow any drying that may need to occur.

  4. Expert Member
    BILL WICHERS | | #4

    You can use open cell spray foam in those walls. With 2-1/8" EPS (about R9) on the exterior in your climate zone, you have a lot of flexibility with insulating on the interior since you have enough continuous exterior insulation for dew point control inside the wall. Spray foam's main advantage here over other types of insulation is that it will also air seal the wall. If you air seal the wall in other ways (taped sheathing, caulk, canned foam, etc.), then you can use anything you want inside the wall, such as cellulose or mineral wool batts (I'd probably use mineral wool myself since I like that product).

    You could use open cell spray foam for your rim joist here too for the same reasons, provided your exterior foam covers the exterior of the rim joist. Spray foam is probably your best option here since this can be a tricky area to air seal any other way. You'll get a LITTLE drying with open cell spray foam, but not a whole lot if you're using a 3+ inch thick layer to get up to similar R values to the walls.


  5. ron0923 | | #5

    Thanks Bill, are you saying it would be ok to use closed cell in the rim joist then?
    I am considering mineral wool as well as blown cellulose for the walls.

    1. Expert Member
      BILL WICHERS | | #7

      No, you need open cell to allow for inward drying (Dana beat me to the reply :-).

      In regards to the tolerance for bulk water (the “flood” Dana mentioned), you could potentially use a layer of closed cell spray foam on the bottom of the rim joist, directly on top of the foundation, then fill the rest with open cell. This would leave a drying path through the open cell for most of the rim joist area with only the bottommost part sealed off with closed cell where water protection is needed. That “sealed off” bottom area could still dry through the rim joist wood into the open cell insulated area. I’d keep the closed cell layer around 1” here.

      Better would be to ensure you don’t have bulk water problems that could soak into the open cell foam like a sponge. If you’re high enough above grade you should be ok in that regard, otherwise drain tile and a sump will help. That assumes you’re in an area where water is a concern — some places are dry enough it’s not really an issue.


      1. ron0923 | | #8

        thanks guys, water shouldn't be an issue. Think I am leaning towards the dense pack cellulose for the walls and open cell foam at the rim joist for the first floor. My crawl space is over 4 feet high and I do have a footer perimeter drain to a sump pit plus the crawl space will be a conditioned space. On the exterior grade the lowest distance from the rim joist to grade will be about 10 inches and increases from there. while we are discussing things I have another question. As we all know lumber prices are through the roof right now so my cost have gone up . I am planning on using henry's blue skin VP 100 as the WRB on the exterior walls. I was also going to use it on the lets call it the sub roof sheating because I will have 10 -5/8 inches of EPS foam ontop of that for the insulation and then sleepers then roof sheathing for the shingles. Could I just tape the sub roof sheathing for air sealing and then just use a synthetic rooing felt to save some money. looking for suggestion

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