Moisture under a low-slope metal roof
Hi Martin,
I am a builder in the Seattle area and I’m having problems with moisture under the roof deck on a low slope metal roof. Here are the specifics:
2:12 roof with Titanium UDL 30 underlayment and Taylor snap lock.
2×12 rafters with foam ventilation baffles top to bottom and HD R-38 batts under that.
Sheetrock ceiling (no vapor barrier under sheetrock nor was it primed or painted with a low perm product).
The moisture doesnt seem to be between the insulation batts and back of drywall but between underside of roof deck and foam baffles. I notice the insulators did not seal the edges or the seams of their foam baffles.
Any thoughts?
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Replies
Hi Jim -
First question is about the air tightness of the roof assembly, to the interior of the roof venting. Roof venting enables soffit to ridge air flow of OUTSIDE air when the roof assembly is airtight; if not, those same chutes can convey warm moist air leaking from the building up into the roof assembly.
Peter
A 2:12 pitch is not going to give you much stack effect. There's going to be very little airflow inside those channels to dry things out. Also, depending on where you are in Seattle, there's always relatively high humidity, further slowing the drying process. UDL-30 is a vapor barrier, so there's going to be no drying towards the underside of the roofing. With your conditions and construction details, I think moisture problems are inevitable unless you've got an airtight ceiling assembly and at least a smart vapor retarder (like Intello) under the drywall.
FWIW, airtight baffles would help a little bit, but not much because they don't have much insulating value. If you caulked them airtight, the underside of the baffles will still probably be cold enough to cause condensation. You need to stop the air and vapor flow at the ceiling level with this assembly.
It's all about more drying that wetting. Here are potential sources of wetting:
1) air from the interior (ie, air seal the interior side better using a blower door)
2) diffusion from the interior (vapor retarder paint will help)
3) exterior air contacting sheathing cooled below ambient by radiant cooling
4) leaks
Lowering indoor humidity will help with 1 and 2. Negative building pressure can fix #1 (but try air sealing first).