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Community and Q&A

Flat unvented roof in Miami

paulmata | Posted in Green Building Techniques on


So I have a gut renovation project coming up that will have an unvented flat roof in Miami.

After reading many articles regarding dry in flat roofs it seems the best option in my climate would be for a reflective white roof with 4” total of ridgid (2” foam board staggered to stop thermal bridging) foam insulation above the roof sheathing.

I then was thinking about having an additional 20R of spray foam insulation sprayed on the underside of the roof from the interior.

Here is where my question arises. Assuming that I can not have a conditioned rafter space 2×10 or some existing 2x8s roof joists. Should I be using closed or open cell for the interior insulation?

From all the research I have read regarding these unvented dry in roofs it would seem that in this particular scenario it would actually be best to use open cell insulation on the interior side of the roof decking and between the joists. What are your thoughts on this any insight would be great.

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  1. GBA Editor
    Martin Holladay | | #1

    Open-cell spray foam makes more sense in this case than closed-cell spray foam, for two reasons:

    1. The open-cell spray foam will allow the roof sheathing to dry toward the interior if it ever gets damp.

    2. Open-cell spray foam has a blowing agent that is more environmentally benign than the blowing agent used in most brands of closed-cell spray foam.

  2. paulmata | | #2

    Thanks Martin, your quick response is appreciated. This is a great site please keep up the good work.

  3. JC72 | | #3

    Reason #3: It will never get so cold that water vapor traveling through the open-cell foam condenses on the underside of the roof sheathing.

  4. GBA Editor
    Martin Holladay | | #4

    Reason #4: Open-cell spray foam costs less than closed-cell spray foam.

  5. Expert Member
    Dana Dorsett | | #5

    Reason #5: Half-pound open cell foam uses only about half the amount of polymer per-R as 2lb closed cell foam.

    Polymer production a real environmental footprint too- it's not ALL about blowing agents. For that reason using reclaimed foam for the 4" up top is far greener than using virgin stock goods, since no new polymer is being produced. It's out there, and it's cheap (sometimes REALLY cheap!)

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