Download PDF version (130.1k) Log In or Register to view the full article as a PDF document.
Q.I plan to install wood siding over a vented rain screen on a house in Seattle, where we get rain for long periods during the winter. My question is, how can the air cavity dry if the exterior relative humidity is 100 percent? Assuming the air cavity temperature is the same as the exterior temperature, the air coming through the cavity can’t pick up any additional moisture.

A.Corresponding editor Paul Fisette responds: If the relative humidity is in fact 100 percent, then of course there will be no drying. However, since the relative humidity is not 100 percent all the time, some drying will occur during these periods.

Remember, the rain screen provides you with insurance. The siding keeps most of the liquid water out of your wall system, and the screen provides backup protection. The air space allows any water that gets past the siding to drain, and the circulating air helps dry the back of the siding and the wall when they get wet.