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Community and Q&A

Open cell vs. closed cell for hot roof in Florida

PaulT6300 | Posted in Green Products and Materials on

I just bought a 25 year old home in Jacksonville. 2 Story framed home off grade. We are considering replacing the fiberglass insulation in the attic with spray foam on the underside of the roof sheathing. My goal is to move make the attic more of conditioned space because 1) the air handler and all the A/C ducts are in the attic 2) We’re just finishing install a number of recessed cans in second floor ceiling.

I’ve been reading the recent articles that show open cell can lead to roof decking deterioration from humidify coming up through the attic. Locally the spray foam installers all recommend open-cell. Their arguments are that when the roof leaks you can tell with open cell. Some even claim 5.5″ of open cell is a air/vapor barrier but I don’t think that’s true.

I should say also that this house had a high humidify problem. When we moved in November with no A/C or heat on (yes it can happen in FL) we measured indoor humidify of 70-80%. We’ve tackled that with re-grading certain areas, gutters, french drains, whole house dehumidifier and finally 2″ of closed cell under the first floor decking.

What would you recommend for my goals in the attic?


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  1. user-2310254 | | #1

    Paul. Can you tell us more about how your attic is constructed? Is the roof in good condition or due for replacement?

  2. GBA Editor
    Martin Holladay | | #2

    As Steve is implying, it's possible to create an unvented conditioned attic (as you want to do) by installing one or more layers of rigid foam above the roof sheathing. This article explains the steps: How to Install Rigid Foam On Top of Roof Sheathing.

    Here is a link to an article that gives several other options for this work: Creating a Conditioned Attic.

    If you want to use spray foam, here's my advice: there are fewer reports of problems with closed-cell spray foam than with open-cell spray foam. Since you have already arranged to have 2 inches of closed-cell spray foam installed in your floor assembly, it seems clear that there is at least one insulation contractor in your area who is willing to install closed-cell spray foam.

    If you end up installing open-cell spray foam in your attic, in spite of my advice, you should (a) make sure that there is a forced-air register in your attic to introduce cool air in the summer and warm air in the winter, and (b) install a hygrometer in your attic to monitor the indoor relative humidity (RH).

    -- Martin Holladay

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