Flat Roof Insulation
In a flat roof where there is no attic (cockloft), I am installing new decking and insulation above the roof and installing new insulation above the ceiling of the top floor. I can’t vent the roof because there is no attic and the roof joists supporting the decking would prevent air movement anyway. Do I…
In a flat roof where there is no attic (cockloft), I am installing new decking and insulation above the roof and installing new insulation above the ceiling of the top floor. I can’t vent the roof because there is no attic and the roof joists supporting the decking would prevent air movement anyway. Do I have to worry about condensation below the roof or ceiling? Do I have to find a way to have air circulate below the roof but above the ceiling insulation?
It’s not so much condensation that I’d be worried about, but extreme temperatures frying the roofing material.
You do not need to vent a flat roof, period.
Condensation won’t be an issue — in a more extreme climate than NY you might consider installing a vapor barrier under the ceiling joists and above the ceiling gypsum, but a properly installed gypsum board ceiling will be an effective enough barrier that you really don’t need it.
I had the same situation and what I did was use 3/4″ furring channels perpendicular to the joists to allow air circulation among the joist bays and left the two existing roof vents. I also attached 2″ of blue foam board to the furring strips, taped and sealed all joints to act as a vapor barrier from warm moist air from the living space. This adds r-10 to the insulation above. Then one layer of 5/8″ densglas, waterproof wallboard just in case the roof leaks.
You need to comply with manuf specs if you ever need to claim.
Lots of answers here, but the short one easy one is yes, you should vent that space. In the original construction of houses like you are describing, the builders would lower the ceiling height in the back room so that there would be some air circulation, using different lumber for ceiling joists and roof rafters, off-setting the two to leave an air channel in between. New construction materials make this less important, things like Iso board keep the space below cooler, so your roofing is less likely to fail, and some makers of roofing are now saying that you can get away without venting, using products specifically made to address this situation, but if you really want to be safe, you should probably install a couple of roof vents. If you raised the back ceiling and or beefed up the joists or rafters, then this may not be an option, in which case you’re going to need to go with a roofer who knows his/her stuff and pay a little extra for material. The problem is that roofers are notorious for cutting corners, it’s one of the trades that just seems to attract fly-by-night guys. Obviously not every roofer is shady, but if you find a good guy, stick with him.