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Ceiling insulation in the desert

patdb | Posted in General Questions on

Greetings Green builders! Newbie home buyer here..

Just entered escrow on a property out in the High Desert in CA.. 1000 sq ft house. Original house (built in late 1950s) is 500 ft and has two additions, one on each side which are both 250 sq ft. There is almost no insulation in the ceiling, just bits of R-13(!) right now. Going to re-do the whole thing.

Trying to figure out which insulation solution would work best. The original cabin has 2×8 joists while the additions have 2×6 joists.. 

Obviously I wouldn’t be able to fit R-38 in these ceilings, would spray foam be a practical solution for this? 

Thanks in advance for your suggestions!


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  1. GBA Editor
    Martin Holladay | | #1

    There are lots of issues here. Are you insulating a flat (horizontal) ceiling with an attic above? Or do you intend to install insulation that will follow the sloped roofline (as is done with an insulated cathedral ceiling)?

    It's almost always possible to install rigid foam above the existing roof sheathing if there isn't enough room between your rafters for insulation. It's also possible to lower your ceiling.

    There are lots of ways to do this, and lots of different types of insulation will work.

    For more information, see these two articles:

    "How to Install Rigid Foam On Top of Roof Sheathing"

    "How to Build an Insulated Cathedral Ceiling"

  2. Expert Member
    Dana Dorsett | | #2

    It's usually possible to install R38 blown cellulose (11") in 2x8 joists, burying the joists, which is a LOT cheaper than insulating at the roof deck. It's fine to leave the fiberglass in place and just blow over it, but be sure to air seal all of the penetrations first.

    The thin wedges where the roof drops at the eaves may be hard to get to R38 while still maintaining the 1" clearance to the roof deck without cutting and stacking ~7" of polyiso wherever the roof deck is less than 12" from the ceiling gypsum below. Inexpensive used 2lb roofing polyiso is perfect for that application, and really cheap if it's the beat up & broken sheets (which doesn't much matter if you're cutting it up.)

    The 2x6 raftered sections may be a bit harder to get R38 all the way out over the top plates of the exterior walls, but 5.5" of roofing iso is about R31, which is way better than nothing.

  3. patdb | | #3

    Thanks for the comments! It is a flat ceiling with no attic.

    I've attached a pic of the ceiling section that has 2x8's, maybe this will provide a better idea of what I'm trying to do.

  4. GBA Editor
    Martin Holladay | | #4

    Here is a link to an article that describes all of your options: "Insulating Low-Slope Residential Roofs."

  5. patdb | | #5

    Excellent, thanks Martin!

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