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Q.I plan to install clapboard siding using the rain screen method. The siding will be fastened to 3/4-inch vertical battens, nailed over each stud. At the top, I plan to ventilate the rain screen into the soffit area. What is the best detail to ventilate the bottom of the rain screen, while still excluding insects?

A.Luis De Miguel, senior researcher at the Canada Mortgage and Housing Corporation, responds: Venting the top of rain screen cavities is not recommended. Research has shown that when wind pressurizes one side of a house and depressurizes the opposite side, vents at the top of a rain screen or curtain wall can allow moisture to be sucked into the cavity. Vents at the top of a rain screen do not contribute appreciably to helping the cavities dry out. If the cavities are closed at the top, quicker pressure equalization is possible, and less water will enter the cavity.

The bottom of the rain screen cavities can be screened with insect screen or thin aluminum soffit vent, bent into a U-shape. The screen should be stapled to the sheathing prior to installing the vertical battens, and then later folded over the battens and stapled again.

For more information on rain screen details, see the Canada Mortgage and Housing Corporation’s Best Practice Guide: Wood Frame Envelopes, available for $89 Canadian or about $60 U.S. (613/748-2003; www.cmhc-schl.gc.ca).