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Q.I’m considering installing wood siding over a rain screen, but I’m unsure whether the potential benefits justify the extra expense. What is the real benefit of an air space behind wood siding?

A.Joe Lstiburek responds: The main benefit of a rain screen is to increase moisture removal by creating a ventilated cladding. The ventilated cladding allows drying of both the siding (from the back surface) and the wall assembly (through the sheathing and building paper) into the air space behind the siding. Some sidings, including vinyl, aluminum, and brick veneer, are inherently self-ventilating. Other claddings, like wood siding, need help. In a rain-screen installation, the wood siding is installed over vertical battens. Alternatively, some installers vent clapboard siding by using wedges, clips, or oval-headed ("bumpy") nails to separate the siding laps.

What is the real benefit? Well, is it a benefit for the siding not to rot, and the paint not to peel? Before the days of plywood, OSB, foam sheathings, cavity insulation, and interior poly vapor barriers, wet siding could dry towards the interior. Today’s walls have low drying potential, due to the use of impermeable or semi-permeable sheathing, high levels of cavity insulation, and interior vapor barriers.

Finally, research has demonstrated that plastic housewraps and felt building papers can lose their water repellency when they are directly in contact with some types of wood siding, due to tannins and other extractives from the wood. Similar problems may occur due to contact with soaps, detergents, bleaches, dirt, dust, and paint. None of this is a problem when the siding is installed over an air space.

Joe Lstiburek is a principal of Building Science Corp. in Westford, Mass., and is author of the Builder’s Field Guide ( available by calling 978/589-5100).