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Community and Q&A

How should I insulate a small shed/studio with a single-pitch shed roof?

jimcrook | Posted in General Questions on

I live in the maritime PNW (rainy Western Washington lowlands) and plan to insulate a recently-built backyard studio/”fancy shed” ala the recent tiny house craze. 8′ x 14′ building with long shed roof about 15 feet long. Metal roof over plywood and 2×6 rafters. I would like to install a tongue and groove ceiling. In this environment, I’ll probably be running a dehumidifier part of the time.

Should I vent each rafter bay (round soffit at bottom and top blocking) and do cut-and-cobble rigid styrofoam insulation sealed with spray foam? Then a 6-mil. vapor barrier and my T&G?

Or should I not vent each rafter bay at all and use batt insulation and call it good?

Thanks!

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Replies

  1. Eric Padgett | | #1

    I'm sure that somebody will come along shortly to give you a better answer. What I would do... I would make the shed roof unvented and put poly-iso rigid foam on the top of the roof. Based on your description (Marine Zone 4) minimum R10 , but I would go R20 (3 inches) and then fill then interior cavity with roxul R23 pressed to the roof deck and then Sheetrock it in. You may want to read https://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-low-slope-residential-roofs

  2. Steve Knapp CZ 3A Georgia | | #2

    Are you planning to occupy the shed or simply use it for storage and projects? Any plans to use heat or AC?

  3. GBA Editor
    Martin Holladay | | #3

    There are two possibilities: (1) You want to do this right (with insulation levels that are close to code requirements), or (2) You don't really care -- you just want to make this shed a little better than it would be if it were totally uninsulated.

    If you want to do it right, you should aim for code-minimum levels of insulation (R-49). The best way to proceed would be to remove the steel roofing -- hopefully, the roofing was installed with screws, so it's easy to remove -- and to install an adequately thick layer of continuous rigid foam above the existing roof sheathing. Then you can install 1x4 or 2x4 purlins on top of the rigid foam, and re-install the metal roofing.

    For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

    If you want to try the second approach -- the fast-and-dirty approach -- your best bet is to install a continuous layer of rigid foam on the interior side of your rafters (leaving the rafter bays empty). Install the foam in an airtight manner (with taped seams), and protect the interior side of the rigid foam with a layer of 1/2-inch drywall for fire safety. This is a much better approach than cutting the rigid foam into narrow rectangles and inserting the foam between the rafters.

    -- Martin Holladay

  4. jimcrook | | #4

    Thanks all. If I had asked this question before installing my roof, I would've gone with the recommendation to install rigid foam atop the roof sheathing. Hopefully those considering something similar will read this and be helped in creating the most energy-efficient space. But realistically, I don't have the time or energy to take off the roof and redo (plus, my roof flashing would then look kind of bad/wrong-sized). I am going to have electric heat inside and spend a fair amount of time in it for projects (and guests will probably sleep in it occasionally)—so there will be a large amount of respiration, but no cooking or bathing or anything like that.

    I will go with Martin's fast-and-dirty approach since I am primarily interested in a good-enough solution. Martin, are you saying that the rafter bays should be vented with your fast-and-dirty approach?

    Thanks!

  5. jimcrook | | #5

    Oh, and is 3-in-12 considered a low-pitch roof? Because those are straight bays and easy enough to vent...assuming there was enough what, buoyancy, that the air actually moved?

  6. Expert Member
    Dana Dorsett | | #6

    A 3:12 pitch is above the minimum 2:12 pitch required by some metal roofing companies, but it's a "pretty low" pitch for providing much convective force to purge the vent space with outdoor air. A code-minimum 1" air gap meets code, but a pitch that low desperately calls for much more, which is why the rafter bays need be left empty in the Q & D fix.

    To meet IRC 2015 code-minimum with that stackup on a U-factor basis (=U0.026, which is a "whole-assembly-R" of R38.5) would take about 6" /R36 of continuous rigid polyisocyanurate. The roof decking, roofing and interior finish ceiling add up, as does the interior & exterior air flims, plus the additional air films in the empty cavities.

  7. GBA Editor
    Martin Holladay | | #7

    Jim,
    You can vent the rafter bays or not, as you prefer. If you detail the rigid foam carefully (install it in an airtight manner), the rafter bays should stay dry. That said, a few vent openings wouldn't hurt.

    A roof with a 3-in-12 pitch doesn't meet the technical definition of a low-slope roof. You can vent a 3-in-12 pitch roof if you want.

    -- Martin Holladay

  8. jimcrook | | #8

    This has been really helpful. Thank you!

    One more question: What if I were to go unvented, use batt insulation, and not even try to seal my ceiling (install T&G right over the batts)? I would assume that I could "dry to the inside" using the dessicant dehumidifier I plan to run continuously during the wet months. It's a pretty small space (112 square foot). Possible? Or would I end up with moldy/rotten roof sheathing?

  9. GBA Editor
    Martin Holladay | | #9

    Jim,
    If you install batts in your rafter bays, the building code requires that either (a) you include a ventilation channel between the top of your insulation and the underside of your roof sheathing, or (b) you install an adequately thick layer of rigid insulation on the exterior side of your roof sheathing.

    If you disobey these rules, you risk moisture accumulation, mold, and rot.

    For more information, read this article: How to Build an Insulated Cathedral Ceiling.

    -- Martin Holladay

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