I have a pool area that I have enclosed in its own building. I'm running
into humidity issues with the roof and am looking for some advice.
I put a directed air vent every other beam but am still having condensation
issues inside the roof during the winter months, resulting in water damage
to the beams and the underroofing.
I'm surprised you are more concenred with roof condensation than condensation
on indoor window surfaces, which are likely to be cooler than a roof in
wintertime, if the roof has any insulation.
Add more roof insulation with a vapor barrier below to raise the ceiling
temp above the dew point of the room air, or raise the ceiling temp with
some fin-tube heaters just under the ceiling, with no air circulation,
and/or reduce the room air dew point temp by circulating dry outdoor air
in wintertime or using a pool cover or a dehumidifier that pumps heat back
into the pool.
For instance, if it's 30 F outdoors and the room air is 80 F and the roof
has R10 insulation, including an R2/3 slow-moving air ceiling surface film
resistance, (80-30)1ft^2/R10 = 5 Btu/h will flow from the room out through
each square foot of roof, making the ceiling temp 80-2/3x5 = 76.7 F. To
avoid ceiling condensation, you might keep the max room air dew point 5 F
lower, ie Tdp < 71.7 F, with a vapor pressure Pa < e^(17.863-9621/(460+Tdp))
= 0.793 "Hg, approximately. The vapor pressure of 80 F air at 100% RH is
P80 = 1.047 "Hg, so you might keep the room RH < 100Pa/P80 = 76% to avoid
condensation. With less roof insulation, the RH would have to be lower.
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