High Humidity Attic

Attic moisture usually appears because of condensation.

High humidity attic.

Jason advani has insulated his attic with spray polyurethane foam and while his heating bills have dropped high humidity remains a problem. At 140 f 80 humidity corresponds to 120 torr or 16 of atmospheric pressure. If there is excessive moisture accumulation in your attic it will condense on cold areas mainly on the underside of your roof deck. The dew point and rh stay below 70 f and 70 at their peak the peak of humidity not the peak height in the attic.

If you have high temperatures and high humidity in your attic then you may also have a problem with mold. Jason advani jason advani thought he was on the right track when he insulated the bottom of the roof deck in his walk up attic with closed cell spray foam. That s way too high. During the winter months the recommended relative humidity inside a house is anywhere between 30 50 percent.

If the attic is at 80 f and 80 humidity that s about a 21 torr vapor pressure of water. Ventilation is another key factor. But the humidity near the attic floor never gets out of control. As planned the r 40 worth of insulation has drastically reduced his heating bill but now he faces another unexpected problem.

In contrast the humidity near the ridge in the attic gets very high. So compared to that hot humid attic the vapor pressure at the other side of the foam is a factor of six higher. By scott gibson march 18 2019.