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Previous code for unvented cathederal ceiling insulation

Steve Gabrielidi | Posted in Building Code Questions on

Hey All,

Just curious if anyone knows the previous insulation codes for a low pitched, unvented cathederal ceiling.

My home was built in the 60s and I am finally getting around to insulating and finishing the garage with the above described roof. I know current code is r49 with at least r20 of spray foam for dew point and that’s my only option. I just don’t see how the rest of my house is all still ok since it was built/insulated way before spray foam existing. Now if I want to just fill the garage bags with roxul or cellulose I hear my roof will mold and collapse.

I am just stuck on what to do since it will be in unheated space 98% of the time and will have a small electric heater for the days I need to tinker in there in the winter.

Please help

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  1. GBA Editor
    Martin Holladay | | #1

    In 1960, many U.S. jurisdictions had no building codes. Even jurisdictions with building codes usually had no insulation requirements; in those days, building codes focused mostly on life safety issues, not energy efficiency.

    The three most common model building codes used back then were the BOCA code, the SBCCI code, and the ICBO code. To answer your question, we'd need to know your precise location, and a historian would be required to perform the necessary research.

    I have no idea why your house is OK, because you haven't described the insulation in your home's roof. I'm glad that your house is OK, however.

    To determine the current building requirements in your local area, contact your local building department. It's possible that you may be allowed to insulate your garage roof to a lower standard than R-49 -- but there is no way to determine that without asking.

    If you do want to insulate your low-slope garage roof in a way that meets modern standards, this article will explain how to do the work: Insulating Low-Slope Residential Roofs.

  2. Steve Gabrielidi | | #2

    Thanks Martin.

    The rest of my roof seems to be insulated with just white fiberglass insulation. It just doesn't make sense to me that I have to spend thousands of dollars to insulate a garage that will barely see heat with spray foam but my entire living space doesn't.

  3. Expert Member
    Malcolm Taylor | | #3

    As Martin suggests do talk to your building Inspector. Our code has a separate section and requirements for renovations. Depending on the extent of the work, you may not be expected to bring the building up to current code standards.

  4. D Dorsett | | #4

    There's nothing magic about spray foam or R20 for roof deck protection.

    If you are assuming you need R20 because that's the prescriptive level for US climate zone 5, note that the R20 is predicated upon an R49 total- R, and a heated conditioned space. If it is going to be a heated garage, maintaining that ~40% ratio of foam-R to total-R would be important. It's unlikely that you have sufficient rafter depth to achieve R49, but as long as the ratio is met, you won't have moisture accumulation.

    If it's an unheated garage with no internal moisture sources, the dew point of the garage air will be far colder than the fully conditioned house air, making the risk even lower (by quite a large amount!)

    If you're not insulating to R49 and have 2x6 rafters, an inch of RIGID foam (any type) above the roof deck, and unfaced or kraft faced (but not aluminum faced) R20-R23 batts in the rafter bays is almost certainly enough, but a layer of Certainteed MemBrain between the fiber insulation and the ceiling gypsum would reduce the risk even further. Alternatively, a flash-inch of closed cell foam on the underside of the roof deck is more than sufficient protection for the roof deck (even for 2x8s), and simply compressing low density batts to fill the cavity works just fine (with or without MemBrain). MemBrain is 2-mil nylon, and fairly inexpensive. It has the advantage of having variable vapor permeance- it's a class-II vapor retarder when the air next to it is under ~40% relative humidity (as it can be during winter when the air is drier), but becomes vapor open when the RH is over 60% (which it would be in the spring as the roof assembly heats up and is cooking out the winter moisture uptake.)

  5. Steve Gabrielidi | | #5

    Wow great info D

    The structure is built in 2x8s if that makes a difference.

  6. Steve Gabrielidi | | #6

    So if I just wanted to use roxul insulation and skip the foam, would I push it up against the sheathing or leave a little space?

    Also, there is a soffit letting air into the garage, would I block that off or leave it?

  7. GBA Editor
    Martin Holladay | | #7

    You can't install Roxul mineral wool in a roof assembly unless you include a ventilation channel between the top of your insulation and the underside of your roof sheathing.

    This type of ventilation channel works best in a steeply sloped roof. Low-pitched roofs are hard to vent. (You won't be happy to hear it, but it's true.) The best way to insulate a low-sloped roof is with rigid foam above the roof sheathing or spray-foam insulation on the interior side of the roof sheathing.

    If you have your heart set on using mineral wool, you need a very deep air space above the insulation, and a vented cupola (doghouse) in the center of your roof. More information is provided in this article: Insulating Low-Slope Residential Roofs.

  8. Steve Gabrielidi | | #8

    Your right Martin, I din't want to hear that! Ha!

    Unfortunately, I refuse to spend thousands of dollars on insulating a 30x30 unheated, unconditioned garage when the rest of my 3400 sq ft house is insulated with batts. It's just a matter of zero ROI.

    So I thought using mineral wool would be safer than fiberglass since it does not absorb moisture and is mold resistant.

  9. GBA Editor
    Martin Holladay | | #9

    If you want to reduce heat loss through your garage roof, you can install a continuous layer of foil-faced polyisocyanurate on the underside of your rafters. Then tape the seams with a high-quality tape. Ideally, you would also install gypsum drywall on the interior side of the rigid foam, for fire safety.

    This won't provide you much R-value -- 1 inch of polyiso is about R-5.5 or R-6, while 2 inches is about R-11 or R-12 -- but it will slow the heat loss without much risk of moisture accumulation.

  10. Steve Gabrielidi | | #10

    Spoke to the local building department.

    He states since it's inhabitable space, there are no specific code requirements.

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