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Community and Q&A

Insulating a workshop roof

user-5458135 | Posted in Green Building Techniques on

Hey all,

I’m in the process of rebuilding a back-yard workshop in climate zone 3, and I’m at the point where I’m considering roof insulation. I’ve read many of the articles on here by Martin and others, including:
– How to build an insulated cathedral ceiling
– Cut and cobble

I’m in a pretty similar situation to those last two threads, but do have some options. At this point, I have the exterior roof in place over attic trusses, and it’s vented with a ridge vent and soffit vents. Like that user, I’m not planning to heat this continually – just occasionally – but would still like to do the best job I can; that means I’m ok if my r-value is not as great as it could be (particularly since the top chord of the trusses are 2x4s). Ideally, I’d really prefer to not have to put a fire barrier over insulation, due to the truss construction and the difficulties of adding plywood or drywall with that. I realize, however, that that may be unavoidable. I’d like to keep the insulation up at the top chord – have the ability to store things up in the truss area with easy access.

Based on my reading, these appear to be my options:
– Baffles, plastic sheeting, blown-in insulation (vented)
– Cut and cobble with polyiso (but this seems to require a fire barrier) (vented)
– Polyiso underneath the top chord (but this now means I have two layers I have to cut around the trusswork)
– Flash and batt, ignoring my venting – does this need a fire barrier? (unvented)
– Closed-cell spray foam (unvented) – is there any that doesn’t need a fire barrier? That energyvanguard article seems to suggest that there might be some.

Other suggestions? Being honest, I principally just don’t want to have to figure out how to sheath the underside with something rigid.

Thanks so much!

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    You described your first option this way: "Baffles, plastic sheeting, blown-in insulation (vented)." I'm not sure what you mean by "plastic sheeting" -- but I don't recommend polyethylene anywhere in this type of assembly. The usual fabric used for installing blown-in insulation is an air-permeable fabric like InsulWeb.

    In general, you don't seem to want to install any drywall on the interior side of your insulation. If you care about fire safety, that means that you shouldn't install any rigid foam or spray foam. (Or perhaps you should develop a better attitude about drywall. Installing drywall isn't that hard.)

    Even if you choose to install a blown-in insulation, drywall is a good idea. It helps contain the insulation, and it's an air barrier. So I strongly urge you to install drywall to protect your insulation.

  2. user-5458135 | | #2

    Thanks so much for the quick reply, Martin! I'm very glad I found this site.

    Re: plastic sheeting, I'd seen one solution involving a sheeting such as this, but sounds like that's not a preferred solution.

    Yes, my desire has been to avoid adding drywall or any kind of sheathing, really - - with storage trusses, that means working around ~3 extra penetrations per rafter, so quite a bit of cutting. I can do it; I've just been working on this thing most weekends over the last couple of years, so I'm itching to be done! I'll consider this, though - thanks for encouraging me in that regard.

    On a related note, 1/4" plywood seems that it might be easier to install due to weight alone - is there a reason you suggest drywall instead?

    Following that, if I were to use plywood or drywall on the interior, what would be your suggestion for insulation? I'd prefer to not lose additional interior roof height, so perhaps the flash and batt or spray foam methods would give me the best r-value? Would that cause any problems with the venting I'd installed? (Really wish I'd found these pages before I decided to do that, but hindsight is always 20/20...)

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    If you want to know more about thermal barriers and ignition barriers, read this article: Thermal Barriers and Ignition Barriers for Spray Foam.

    An ignition barrier (for example, 1/4-inch plywood) is a less stringent barrier than a thermal barrier (for example, 1/2-inch drywall) -- so whether or not you want to go with 1/4-inch plywood or 1/2-inch drywall depends on (a) your appetite for risk, and (b) local code requirements.

    Either flash-and-batt or spray foam insulation can work with a ventilation channel between the insulation and the roof sheathing.

    Here is a link to an article on the flash-and-batt approach: Flash-and-Batt Insulation.

    Here is a link to an article from the GBA Encyclopedia on spray polyurethane foam: Spray Foam Insulation: Open and Closed Cell.

  4. user-5458135 | | #4

    Thanks, Martin, that helps greatly.

    Final question at the moment: I've not yet installed any baffles; just ridge and soffit vents. I could close those off if I need to in order to have an unvented roof assembly with a higher r-value (while keeping the roofline as high as possible). Would that be an approach you'd recommend, or should I stick with the vent channels?

    One other approach I'm considering, given your advice, would be to cut-and-cobble 2" of polyiso between trusses, then add a layer of 1" continuous under the trusses, followed by 1/4" ply or drywall. That would give me R-19 at best (in truss bays) and about R-6 at worst (at trusses with thermal bridging). I'm willing to give up a little headroom for this, but that's more like 1.5" rather than 3-4" if everything's below the trusses.

    Thoughts there?


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