Cathedral Ceiling "Sweating"


Issue has been fixed. However, I am trying to learn the reasoning and why it happened. I have five years of roofing experience but feel like i learn and understand better if i know why.

There was a cathedral ceiling in the house. Roof and attic space are completely cut off from said room. No soffit vents or ridge vents on previous roof. Previous roof had architectural shingles and asphalt felt. No issues. No leaks.
New roof was installed with architectural shingles and synthetic felt. The roof then began to show signs of sweating on the ceiling. Installed soffit vents and ridge vents, solved the issue.

My question is that once the shingles are installed then you have a vapor barrier. But the only difference between new and old roof was installation of synthetic instead of asphalt felt. Does the permeability of the felt play a role once the shingles are installed? I


Likely wasn’t sweating, more likely condensation was forming.


I’m curious why the ventilation system wasn’t installed to begin with?



I’ve never seen it personally, but 15 lb felt has the somewhat unique property of becoming a bit vapour permeable when it gets wet; from condensation for instance.

So it could be that with the old roof there was some vapour drive out, allowing condensation to end up in between the felt and shingles, rather than underneath. Synthetic definitely won’t do this.

Doesn’t seem like that would have been a healthy situation in the first place though, it’s better with the ventilation.