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Insulation for Standing-Seam Metal Roof in Hot Climate

merlinm | Posted in General Questions on

We’re in the design phase for two SFRs in central Texas between San Antonio and Austin. Sites are on the border of IECC climate zones 2 and 3. Conventional frame construction. Roofs will be standing seam metal. Would like to have ≥ R38 for the roof insulation.

The attics will be inside the conditioned space. Conditioning will include dehumidification. In terms of air sealing, we plan to achieve ≤ 3 ACH50.

None of the framing or roofing subs we’ve talked with have done exterior (to the roof sheathing) insulation.

Looking for ideas to get to the insulation levels described above. Both closed and open foam have been suggested by various potential subs.

Any thoughts/recommendations on alternatives to get to or above the R38 would be much appreciated. Thanks.

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Replies

  1. Expert Member
    Akos | | #1

    SPF can work but expensive. If you do go with open cell make sure you have enough conditioned airflow in the attic to control humidity.

    You are in a climate where you can also go with a simple batt assembly with a vapor diffusion port:

    https://web.ornl.gov/sci/buildings/2016/docs/presentations/practices/practices-01/Practices01_Paper99_Ueno.pdf

    https://www.jlconline.com/how-to/insulation/avoiding-wet-roofs-part-ii_o

    Going much above code for insulation is probably not worth it in your climate, you get more energy savings with a lighter color roof.

  2. GBA Editor
    Kiley Jacques | | #2

    Hi John,

    Here is a related Q&A Spotlight article in which Martin Holladay stresses—no matter which spray-foam insulation you decide on—the importance of dry sheathing the day of application. He says guaranteeing a drying mechanism for the sheathing is important, and recommends installing a ventilation channel above the sheathing along with a second layer of plywood. A few readers also weigh in with ideas for other approaches and product recommendations to ensure drying potential.

  3. JC72 | | #3

    You could try contacting Matt Risinger (Austin area builder). He's has done a couple of unvented metal roofs (A few YouTube vidoes on it). Perhaps they can point you to a roof company?

  4. Jon_R | | #4

    Whatever code compliant un-vented option you use, consider also using vapor permeable underlayment and eave to peak foam backer rod to add some additional resilience. No data on this, but it makes sense to me.

    > we plan to achieve ≤ 3 ACH50
    You can do better. I'd plan for ≤ 1.5.

  5. merlinm | | #5

    All, many thanks for your extremely helpful comments. Much appreciated. The SOLITEX UM looks very interesting as this could be installed over the sheathing or nail base.

    Jon, we are planning for well less than 3. Still have details to work out in the plans in terms of getting the air tightness desired.

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