Rainscreen details

A homeowner building a new house asked about how to install a rainscreen, a small air gap behind siding that lets water drain and walls dry out, under a somewhat unusual siding plan: corrugated metal run vertically on the bottom 10 feet of the house, transitioning to wood lap siding above, with 2x8 walls and no exterior foam insulation. Since the metal siding runs vertically, it needs horizontal furring strips behind it to hold it off the wall. The homeowner couldn't find Coravent or similar drainage mesh products locally and asked whether routing small drainage notches into the furring strips would work, or whether the ridges in the corrugated metal already create enough of a gap to skip a rainscreen there entirely.

A building science expert replied that this level of drainage detailing likely isn't necessary. Corrugated metal siding is stiff enough that the furring only needs to be spaced 32 to 48 inches apart, so there aren't many strips to worry about. Rather than routing notches, the simpler fix is to bevel the top edge of the horizontal strapping so water sheds off it, or to mount the strips on dimpled house-wrap, a textured underlayment that naturally creates drainage channels. The reasoning is that very little water is likely to reach that gap in the first place, so it doesn't need to function as a true drainage plane.

For anyone planning a similar wall assembly, the takeaway is that a full rainscreen system with dedicated drainage matting may not be needed everywhere on a house, particularly under stiff metal panels with limited exposure.