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Community and Q&A

Foam over roof questions

joenorm | Posted in General Questions on

First, I’m in Zone 4C, the chart says I need R-10 XPS but then says 2.5 inch.

Isn’t 2 inch XPS R-10 already? Is the 2.5 just to play it safe because its on the edge? 

If I used 2inch PolyIso foil faced would that be better?

Second, Do people typically foam all the way out to the eaves or just the to the exterior wall intersection?


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  1. Expert Member
    Dana Dorsett | | #1

    The IRC doesn't specify the insulation type, only the R-value, and XPS would be the LEAST green choice due to the HFC blowing agents used and the high polymer weight per R.

    XPS that's labeled R10 is only warranteed to be R9 (90% of labeled performance), since it loses performance as the HFCs diffuse out over a few decades, ergo 2.5" is necessary to be reliably above R10 over the lifecycle of a roof.

    Yes, 2" polyiso would be better, due to the much more benign hydrocarbon blowing agents used, and the more environmentally benign polymer type.

    A presumption behind the R10 prescriptive is an R49 total R. If there is more than R39 insulation under the roof deck there needs to be proportionally more above the roof deck to keep the average temperature at the roof deck the same. It's the average temperature of the roof deck that ultimately determines it's moisture risk from vapor drives.

    1. Jon_R | | #5

      > It's the average temperature of the roof deck that ultimately determines it's moisture risk

      But don't apply this generally. For example, a steady state, always right at the average temp roof deck would never be below the dew point (no moisture accumulation) and some other roof (same average annual temp) might frequently be below the dew point with offsetting drying hot periods many months later. So lots of time with high sheathing moisture and mold friendly temperatures.

      Lstiburek writes "the average outdoor temperature for the location during the coldest three months of the year", but this is a very rough approximation.

      A friend has mold underneath the roof deck in a tropical country.

  2. joenorm | | #2


    Ideally there would be 12 inches of blown in cellulose between the rafter bays

  3. GBA Editor
    Martin Holladay | | #3

    I'm guessing that the chart you are referring to is the table I created for this article: "Combining Exterior Rigid Foam With Fluffy Insulation."

    Note that the table you're talking about has a note at the head of the "inches" column that explains that the chart assumes the use of EPS. If you are installing XPS or polyiso, the thickness of the rigid foam will differ from the thickness shown in that table.

    For more information on this issue, see this article: "Choosing Rigid Foam."

    1. joenorm | | #4

      I am wondering if I can get away with just one course of 2 inch foam here. It seems like the answer is yes, especially if I use PolyIso.

  4. seabornman | | #6

    If you use one layer of insulation pay a lot attention to the joints. I had a bad experience using just one layer on a commercial building. Air movement at the joints created condensation below. Better to install 2 layers and lap joints.

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