Drywall ceiling on a metal roof
Hi,
I’m in New York and found mold in the ceiling in my one room extension, it was a bummer. So, I ended up pulling the whole ceiling down and exposing the roof.
Its a flat roof made up of steel decking supported by 12 inch steel c-joists with no venting. The ceiling I pulled down had no vapor barrier so condensation was forming on the metal roof decking and dripping down onto the drywall below.
I’m not really sure what all is above the roof decking. I don’t see any thermal breaks where the c-joists are set into the block walls so I’m operating under the assumption that the entire roof has no or very little in terms of insulation.
I’m not in a place to replace the whole roof so looking to get somewhere sane. Current thought is to have the roof decking and joists covered in a few inches of closed cell foam to create a vapor barrier and get some insulation. Assuming that makes sense, whats the best path forward for putting up a drywall ceiling that has appropriate thermal breaks?
-G
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Replies
Hi Geoff.
I'm not too familiar with the steel rafter and roof assembly that you have, but I think you could create a thermal break beneath the rafters in a few ways. The first would be by installing strips of rigid foam (or continuous sheets) along the bottom of the rafters before installing drywall. In an assembly like this, continuous rigid foam would need to be vapor open enough to allow inward drying.
Another method would be to install strapping perpendicular to the rafters to fasten the drywall to, which would work if you were filling the cavity with netted and blown in insulation. You could create as much depth as you wanted depending on what dimension material you used for strapping.
In any case, a few other thoughts: Make sure you have and adequate layer of closed cell foam to control the dew point temperature at the foam's interior surface. Consider installing a bond break between the foam and the metal roof to make future roof work possible without having to tear out the insulation. And make sure that you install an airtight ceiling. More condensation issues are the result of air leaks that vapor diffusion.