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Community and Q&A

Best unvented shop ceiling option

Kail_Z | Posted in Energy Efficiency and Durability on

I’m building my metal roofed work shop in climate zone C4. Because of the pole barn and shed roof purlins I won’t be able to vent the ceiling assembly. I’m planning on using spray foam directly against the underside of my roof sheathing because if this. I have a choice of either using 3” of spray foam or for less money, I can use 2” of spray foam and a R13 fiberglass bat. Which is best? Code for living space in our area in a ceiling is at least 3” of spray foam, but this is a shop and the 2” plus bat option is cheaper and has a higher overall R value. I am concerned about moisture as I could be doing some wet work in the shop. Any advice would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, does this building have continuous roof sheathing (plywood or OSB), or just purlins and metal roofing?

    I'm going to assume that your roof has continuous roof sheathing (plywood or OSB).

    The short answer is that the R-value of 2 inches of spray foam plus an R-13 fiberglass batt is higher than the R-value of 3 inches of spray foam.

    But we need to know a lot more to give you good advice. The most important question you need to answer is this one: Are you talking about open-cell spray foam or closed-cell spray foam?

    Q. "Code for living space in our area in a ceiling is at least 3 inches of spray foam."

    A. You're wrong. Building codes require homes in Climate Zone 4 to have ceiling insulation or roof insulation with a minimum R-value of R-49.

    If you are using closed-cell spray foam, you would need 7.5 inches or 8 inches of spray foam to meet minimum code requirements.

    If you are using open-cell spray foam, you would need 13 inches or 13.5 inches of spray foam to meet minimum code requirements.

  2. GBA Editor
    Martin Holladay | | #2

    For more information on the technique you are interested in (the flash-and-batt technique), see this article: Flash-and-Batt Insulation.

  3. Kail_Z | | #3

    Martin, thanks for the response. I do have continues OSB Sheathing. The total total R-value code for a ceiling is far above the 3" of sprayfoam, but we have a code that I do not fully understand, that if you are using sprayfoam in a unvented ceiling in a living space then you must use at least 3". I believe this is to push the condensation point outside of the assemble. We still need to use additional means of insulation to bring your total R-value up to code. This is just my work shop and not a living space. I wasn't sure if you or someone else may understand our 3" rule better then myself and know if this is important in my application, or if I would be better of saving money and having a higher total R-value by just using 2" of sprayfoam and a R-13 bat. I could be doing wet grinding in the space and do not want to set myself up for moisture problems. Do you have a better understanding then myself?

  4. GBA Editor
    Martin Holladay | | #4

    If you want to know the code-mandated minimum R-values for the spray foam layer in a flash-and-batt job, you should read the article I provided a link to in Comment #2. Here is the link again: "Flash-and-Batt Insulation."

    You've got the right idea -- for a flash-and-batt job, the code specifies the minimum R-value of the spray foam layer, with the aim of avoiding possible condensation on the interior side of the spray foam layer in winter. If you obey these rules, you'll stay out of trouble.

    For a flash-and-batt roof in Climate Zone 4C, the minimum R-value for the spray foam layer is R-10. Using 2 inches of closed-cell spray foam would put you in the safe zone.

    Of course, adding some fluffy insulation (fiberglass, mineral wool, or cellulose) on the interior side of the spray foam, and in direct contact with the spray foam, would be mandatory for homes -- and would be a good idea, even for a shop.

  5. brendanalbano | | #5

    Kail, do you happen to be in Oregon? Martin's comments are correct per the IRC, but Oregon has some funny variations. For one, a cathedral ceiling in a residential project in Oregon can usual be R-38 in a lot of contexts.

    R806. in the Oregon Residential Specialty Code is probably the passage giving you the R-20 (about 3") of spray-foam:

    Best I've been able to figure out, the R-20 number is because Oregon includes both climate zone 4C and climate zone 5. In climate zone 5, with an R-49 roof, R-20 is the correct number to control condensation. In climate zone 4C you technically don't need as much closed-cell foam. And in climate zone 4C with only an R-38 roof you need even less closed-cell foam. But the 2017 ORSC doesn't have any provisions that I've been able to find to account for this nuance. It's just a blanket R-20 across the state.

    If you don't have to comply with this bit of code, then the typical recommendation for hybrid insulation roofs in climate zone 4C is that 20% of your R-value should be in the closed-cell spray foam, and the remaining 80% can be in fiberglass. I'm not sure if the work you are doing in your shop would necessitate that you adjust that ratio at all. More closed-cell foam is more conservative and more expensive.

    And make sure to choose an HFO blown closed-cell foam like Lapolla Foam-lok 2000 4G, Demilec Heatlok HFO High Lift, or ICYNENE PROSEAL HFO.

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