Recap: Insulate shop with polyiso between rafters
(Removed from an old thread with a similar title — I decided to repost this as a separate question)
I am back working on my shop. To recap, it’s a small 12′ x 20′ detached shop, in Mass., vaulted ceiling, well-vented at ridge and soffit, using 2″ cut-and-cobble polyiso installed below plastic vent channels in each rafter bay, it’s a low ceiling with no room to put the polyiso below the rafters.
I have been sealing the larger gaps at the perimeter of each piece of cut-and-cobble rigid foam with Great Stuff. Experience so far says that Great Stuff works well for larger gaps (say 3/16″ or more) but it is a mess — and probably ineffective — for smaller ones.
My inclination is to fill the larger gaps with Great Stuff and then use 3″ or 4″ foil tape bridging from one piece of polyiso across the rafter to the next piece to air seal the whole thing. The ceiling will then be covered with 3/8″ plywood.
I know the air seal will not be perfect and this will cut the insulation value due to convective heat flow. That’s too bad, but it’s also OK — it’s a shop, and in winter it is heated only when in use, not all the time. So I can live with some heat losses.
The big question is, if I do it this way is that imperfect air seal gong to create moisture issues? I don’t mind if it takes a little extra energy to heat the shop due to imperfect sealing, but I do mind a lot if I get condensation in the roof assembly, or anywhere else.
Thanks for any help …
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Replies
Tom,
I don't think you will have any moisture issues -- just a poorly insulated ceiling.
Martin thanks for the note. That was my conclusion but I was only guessing.
If you have time, I am curious as to why one might or might not have moisture issues. Reading the material on cathedral ceilings, it understandably seems to be focused on keeping the warm moist air away from the (in winter) cooler roof deck, either using a tight, unventilated assembly with closed-cell spray foam, or a ventilated roof with careful sealing below the insulation. I am on the latter track but imperfectly so. Am I avoiding moisture issues just because the rate of infiltration of the warm moist air is likely to be low compared to the rate at which it is removed via the ventilation channel between the top of the insulation and the roof? Or is there something else going on?
Tom,
There are two ways that you are avoiding moisture problems:
1. You are paying attention to airtightness -- by taping the seams of your polyiso, and (hopefully) by installing a gypsum drywall ceiling. (The gypsum drywall is necessary for fire safety, but it also provides an air barrier.)
2. You have a ventilation channel directly under your roof sheathing, and the air flow through that channel, however imperfect, will in some seasons of the year (especially on dry, windy days) help to lower the moisture content of the roof sheathing.