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Community and Q&A

Poor man’s spray foam help

Ferrell Brown | Posted in GBA Pro Help on

Newbie here:

I am rehabbing a stickbuilt cottage in FL. It was torn back to studs by the previous owner. I obtained permits and am actively getting through the inspections and such. Next step is insulation. I discussed the process of foaming in cut foamboards and the county seemed ok with it. I bought 1 1/2 inch polyiso board from a recycle group that is in great shape. I have close to 200 4′ x 8′ boards.

The county is concerned as the commercial boards I purchased do not have a labeled/stamped R-value. They want documentation on polyiso itself. Where could I find that? I assume there is not documentation for the exact purpose that I am using it for (poor man’s) but if anyone knows of that please let me know.

The construction of my cottage is solely 2 x 4s (yes, I am serious even the floors, I think this was a kit home from the 70’s). I will be foaming in two layers (3″ total) with staggered joints. This takes me well beyond the minimums required by the power form I orderd for the permits. I am comfortable with the process except that I do not know if I should leave gaps anywhere. I am using this technique in attic, walls and crawlspace. I am clear that I will have to provide fire barriers, but do I completely leave no areas with gaps?

I appreciate all responses.

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  1. GBA Editor
    Martin Holladay | | #1

    If your polyiso is stamped or labeled with the name of the manufacturer, you could visit that manufacturer's web site and look up the product's R-value. If there are no marks on the insulation, you can refer your building inspector to standard reference works that list the R-values for different materials -- for example, this web page maintained by the U.S. Department of Energy. (A conservative R-value for polyiso is R-6 per inch.)

    For more information on your method of insulation, which is usually called "cut-and-cobble," see this article: Cut-and-Cobble Insulation.

  2. Ferrell Brown | | #2

    Wow. Missed that article. I am kind of committed and there is no way I can get around the thermal bridge.

    The dry side of the sheathing is now concerning me. There is a specific membrane in the brand new roof that I am not accustomed to (I live in Atlanta but this cottage is obviously following FL standards). So that side is not going to allow wet sheathing to dry. My panels are covered with the thin fiberglass that was mentioned. I assume I should seal it right against the sheathing still? Actually, except where some repair has taken place, the roof is made of planks not sheathing. Thoughts?

  3. GBA Editor
    Martin Holladay | | #3

    Most roofs are unable to dry upward. There are a few exceptions -- for example, concrete tile roofs, wood shingle roofs, and slate roofs -- but as a generalization, most roof assemblies are not designed to dry through the roofing.

    I don't know what you mean by a "specific membrane," but I'm guessing that you are talking about synthetic roofing underlayment.

    I don't know what you mean by "my panels are covered by thin fiberglass," but I'm guessing that sentence is a reference to the facing on your polyiso.

    So, if my interpretation is correct, you are asking whether it is a good idea to create an unvented roof assembly (an unvented cathedral ceiling) using the cut-and-cobble method. My answer is that this is a method associated with failures. I wrote that in the article.

    Since you are in Florida, not the cold north, my guess is that the cut-and-cobble method is safer than it would be in a cold climate. One way to improve your roof assembly's performance is to include a continuous layer of rigid foam on the interior side of the rafters, with the seams between the rigid foam panels carefully sealed with a high-quality tape. You should only do this work when the roof sheathing and the rafters are relatively dry.

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