Flat roof condensation


#1

i replaced my old BUR flat roof with a PVC membrane. Under the membrane there is supposed to be 2 layers of 1 1/2 inch ISO foamboard substrate above the deck. The roofer took out some of these foamboards in an attempt to drain ponds caused by a wavy deck. Now I have some areas where there is very little insulation between the membrane and the deck. I thought I could compensate for this by stuffing more fibreglass batts from underneath between the joists. But now I’m hearing that may not work because interior moisture could condense on the cold deck from underneath and soak the batts. Ventilation is not possible. What can be done here?


#2

Icynene insulation, may solve the problem.
Getting it into all the cavities could be challenging though.


#3

Have you had any condensation problems yet?

What you will be dealing with if you have condensation is relative humidity and dew points. Where the dew point falls within the roof sandwich will determine where moisture will condensate. So what that means is that given the right conditions, you might have a condensation problem. Still, unless you have seen a problem, I would just keep an eye open for it and just let it be.

With that said, your roofing contractor should have filled the low spots on your deflected roof deck instead of taking away from the high spots; or he should have replaced deflected and warped plywood. I would obviously have to see the roof to properly comment on what you have, and if you post photos someone on this site should be able to help you if I don’t see them posted.


#4

axciom - yes, spray foam seems to be the best way out of this although, as you say, its a pain to get it in there and its also the most expensive.

cerberus - you are absolutely right about filling in the high spots. I told the contractor not to chop out the high spots but he ignored me and kept on doing it. When I cornered him again on this, he just shrugged and said that he didnt want to get more material and he didnt want to get longer fasteners - which I was willing to pay more for anyway. I’ll probably watch those thin areas for the time being but its on mostly on the west side of the building where the wind and weather hits the hardest so I’m not so hopeful about it.


#5

glad i live in florida.

sorry ranger88.

gweedo.


#6

[quote=“ranger88”]axciom - yes, spray foam seems to be the best way out of this although, as you say, its a pain to get it in there and its also the most expensive.

cerberus - you are absolutely right about filling in the high spots. I told the contractor not to chop out the high spots but he ignored me and kept on doing it. When I cornered him again on this, he just shrugged and said that he didnt want to get more material and he didnt want to get longer fasteners - which I was willing to pay more for anyway. I’ll probably watch those thin areas for the time being but its on mostly on the west side of the building where the wind and weather hits the hardest so I’m not so hopeful about it.[/quote]

I didn’t notice where you live, but if you have a lot of cold weather and freezing, a little thinner insulation around roof drains helps keep them from freezing up. Don’t know if you have drains or gutters, but I thought I’d give you some possible good news.