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Spray foam and Roxul for flash and batt?

Steven G | Posted in Energy Efficiency and Durability on

Hey guys,

I am in the process of finishing my attached garage in zone 5a. It will not be heated year round, just when I am working in there. When it will be heated, I will be using a 240v 6k watt electric heater.

I am torn on how to go about insulating my unvented cathedral roof. I ready that fiberglass batts against the sheathing are a no no but what about roxul? If that’s no good either, can I just spray 2″ of closed cell and fill with roxul then sheet rock?

I know the best thing is to foam the entire thing, but I just do not see the value in a space that will not be heated 100 percent of the time. So my options are:

1 – All Roxul – R30
2 – Just 2″ closed cell foam – R14
3 – 2″ closed cell foam + R15 Roxul = R29
4 – 2″ closed cell foam + R30 Roxul = R44
5 – 3″ closed cell foam – R21

Home was built in the 60’s and understand that current code is R49 but not sure if its needed for a space that will not be used all the time.


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  1. Chaubenee | | #1

    I am pretty sure that you are going to hear that 2" of closed cell in that application in that zone is not enough and that you need at least three inches. How about cellulose? Or open cell, both with a vapor retarder and carefully sealed Sheetrock on the ceiling.???

  2. GBA Editor
    Martin Holladay | | #2

    You can probably get away with breaking the rules, for two reasons: (1) Your garage won't be generating much moisture, and (2) Your garage won't be heated for many days each year.

    That said, it's better to follow the rules -- especially because the next owner of your house may convert your attached garage to a bedroom.

    What about doing it right? Among the options you haven't mentioned:

    1. Building a vented, insulated roof (one that includes soffit vents, a ridge vent, and an air channel connecting them).

    2. Installing a flat (horizontal) drywall ceiling, with cellulose insulation above the ceiling.

    For more information on the right way to insulate a sloped roof assembly, see How to Build an Insulated Cathedral Ceiling.

  3. Reid Baldwin | | #3

    How long will you be working in your garage at a time? When you first turn the heater on, most of the heat will go to heating up the stuff in the garage. The insulation level determines your energy usage at steady state. If you turn the heat on Friday night in anticipation of working out there all weekend, it will be steady state most of that time. If you decide after dinner to spend a couple hours working in the garage, the differences in insulation level won't make much difference.

  4. Steven G | | #4

    I wish I had the funds to rebuild the roof or spray foam the whole thing but it's just not possible. The roof is a very low pitch, almost flat so I don't think building a flat ceiling and cove ting is possible.

    Since it will only be heated when I decide to work in there, can I get away with roxul up to the sheathing with a vapor barrier over it? Or should I just shoot it with 2" of foam and call it a day?

  5. GBA Editor
    Martin Holladay | | #5

    The correct answer is that you need a ventilation gap under the roof sheathing if you intend to install mineral wool between the rafters. Can you install soffit vents and a ridge vent?

  6. GBA Editor
    Martin Holladay | | #6

    If you want an unvented roof assembly, then one of your proposals -- to just install 2 inches of closed-cell foam and call it a day -- is a relatively safe approach from a moisture perspective, but not so good from a fire-safety perspective. Any spray foam needs to be protected on the interior with a layer of gypsum wallboard.

  7. Steven G | | #7

    Would I hurt anything by installing r15 roxul on top of the r14 foam? I read that to do this correctly, I need an r20 of foam with an r49 ceiling but was curious if I could get away with just the r14 foam since I will be at r29 ceiling total

  8. GBA Editor
    Martin Holladay | | #8

    As my article ("How to Build an Insulated Cathedral Ceiling") notes, the type of flash-and-batt approach you have chosen for your unvented roof assembly requires that the spray foam layer have a minimum R-value of R-20 in Climate Zone 5. If the foam layer has a lower R-value, the roof assembly is susceptible to moisture accumulation.

    That rule applies to occupied homes. When it comes to a garage that is only occupied intermittently, you don't necessarily need to follow the rules, as long as (a) your local code enforcement official agrees, and (b) as long as you are proceeding with open eyes, aware of the potential risk.

    The risk increases (a) if you start heating the garage for longer and longer periods, and (b) if any moisture-generating activities occur in the garage.

    You still haven't explained why you are wedded to an unvented roof assembly instead of a vented roof assembly.

  9. Steven G | | #9

    the structure is a very low sloped roof with 2x8 rafters and rolled asphalt. From what I read, this will not work vented since so much of the cavity will need to be open.

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