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Is it possible to install closed-cell polyurethane foam in a closed cathedral ceiling cavity?

E3zpcpuxd4 | Posted in General Questions on

We have a family room with a cathedral ceiling which gets very hot in the summer. Bay Area climate 3C. The cathedral ceiling has 2×6 rafters with plywood sheathing/asphalt roof on top, drywall on bottom. I removed the eave blocking and pulled out the fiberglass batts which were original insulation.

The cavities are 8 feet long by 22″ wide by 5″ tall and accessible from the both the attic (top of cavity) and the outside (bottom of cavity via removed eave blocking).

I thought I could just have closed cell polyurethane foam sprayed into cavity from top and bottom, being careful not to overfill and pop the drywall on the ceiling side. This would give our cathedral ceiling a much higher R-value and hopefully keep it cooler. I imagined you could use a long wand to spray foam and slowly retract wand as cavity fills with foam. Since cavity is accessible from top and bottom, you could do 4 feet from the top and 4 feet from the bottom using a gun with a 40″ wand. Using closed cell polyurethane, the dew point should be inside the foam, negating the need for a vent channel.

But insulation estimator said they will not spray polyurethane foam (open or closed cell) into a closed cavity as it will not cure due to lack of air/moisture. He said the uncured foam might remain uncured and damage drywall.

I’ve been scanning the finehomebulding forum and haven’t seen anything about curing being a problem for polyurethane foam in closed cavities.

The problems I’ve seen for foam in closed cavities seem to be related to the foam setting up before it has a chance to settle into cavity, leaving voids. Or that foam cannot make it’s way past obstacles in the cavity. But this was for wall cavities, where as I have a nice open cathedral ceiling with cavities which are 22″ wide and 5.5″ tall with no obstructions.

Is the estimator correct about curing for closed cell polyurethane foam?

If so, are there any other foams which would work?

I was originally thinking of sliding 22″ wide x 2″ thick slabs of pink Foamular foam into the cavities. 4″ would give me R-20. But there is no way to seal against air infiltration around the foam slabs, which is why I would prefer to spray in polyurethane foam, which both air seals and insulates.

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  1. GBA Editor
    Martin Holladay | | #1

    Trust your spray foam contractor. You don't want to install spray foam in an 8-foot-long closed cavity.

    Your options include:
    1. Opening the cavity from the bottom and installing spray foam.
    2. Installing dense-packed cellulose in the cavity, supplemented by rigid foam underneath to improve the total R-value.

  2. E3zpcpuxd4 | | #2

    Is it because the foam won't cure?

    What about a pour in place foam?

    Like Tiger Foam's slow rise foam:

    Tiger Foam™ 200 bd. ft. Kit - SR
    Slow Rise Cavity Fill formulation is designed to fill cavities such as existing walls or molds. Great for retrofit - drill and fill behind existing drywall or plaster and lathe or any pour in place, ditch or mold released applications.

  3. Foamer | | #3


    Polyurethane foam will cure just fine in a closed cavity. Just think of freezers, boats and so on that use pour foam in closed cavities.

    But spray foam cannot be applied with a wand. The chemicals mix under high pressure at the tip of the gun and the rapid foaming action starts immediately. That said, there are extension tips available for the professional guns we use, which allow us to spray out five to ten feet in a narrow stream. Assuming that you have good access top and bottom, it should be possible to foam an 8 foot cavity but it will require a steady hand and lots of time so expect to pay a premium. The job will be easier with a lower density foam because it expands more rapidly but you will sacrifice some r-value.

    Low rise foam is another option but I have no experience with them. They are tricky in closed cavities because of the risk of overfilling but that should not be a concern in your case.

    If you are considering dense packing with cellulose as Martin suggested, you should know that it is a controversial subject. Joe Lstiburek put it this way: "As fantastic as the dense pack approach is for walls, it is a pretty dumb approach for unvented cathedral ceilings and flat roofs. And, no matter how hard some of us work to try to stop people from going ahead and doing it, they persist on doing it. First, it is a building code violation. Second, it violates the physics." (Insight # 43, "Don't Be Dense",

    Good luck with your project.

  4. E3zpcpuxd4 | | #4

    Removing the dry wall to spray is okay with me, but my wife vetoed this option.

    I'm glad to hear polyurethane foam will cure in a closed cavity.

    I shouldn't have said "wand". I meant a gun with a long tip. I use a gun with great stuff pro which releases the foam at the tip and I imagined the polyurethane foam was applied with something similar. Here's a touch'n seal link to a long barrel gun with 39" tip:

    I want to use the 2 lb polyurethane for the high R value. Can anyone recommend a bay area contractor who can handle my job?

    If I were to use cellulose in the cavity, I already created a vent using 3/4" foiled back foam. I folded 1" of the edge backward to create a 3/4" high x 20" wide vent and attached this to the top of my cavities. This leaves about 3.5" x 22" x 8 feet cavity which I would like to fill with 2 lb foam. But if I filled with cellulose wouldn't that meet code since there would be a 3/4" vent under the roof sheathing?

    I don't believe I need the vent if I use 2 lb foam since it has low permeability and the dew point will be in the foam. But I wanted more vent area in that part of the house.

    I would be interested in hearing from anyone with experience using the low rise pourable foam. Does it set up slow enough so it will make it to the bottom of an 8 ft cavity slanted at 30 degrees???

  5. Paul Eldrenkamp | | #5

    The pourable foam can work, but you're flying blind when you install it and it's very difficult to know what sort of coverage you're getting. We have given up on using spray foam except in situations where we can see 100% of the surface area it's being applied to (though I'll admit this might be an over-reaction to a few bad experiences). If it's all practical, get access to an IR camera and use it to inspect some closed-cavity spray foam jobs that your preferred contractor has completed; that may give you a better idea of what level of coverage he/she can really achieve in your case. (A really good foam contractor will own an IR camera and will be pleased to show you past projects.)

    That being said, the actual difference between 3.5" closed-cell spray foam and 3.5" of dense-pack cellulose in reducing solar gain through your roof assembly may not not turn out to be be as significant as you're hoping. If the foam is a lot more expensive than cellulose, you may want to forget about it for now, go with the cellulose, and use the money you save to add rigid insulation on top of the roof when you replace the shingles (with a high-albedo roof) in a few years.

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