Does the Size of a Rain-Screen Gap Matter?

Q: Rain-screen wall assemblies seem like a good idea for protecting wood siding and paint, but they can create headaches when it comes to resolving door, window, and other trim details. Wider gaps mean that doors and windows would need jamb extensions, as well as screening to keep bugs out. But would a 1/4-inch or 1/8-inch gap — which might not require any additional detailing, or even screening — be equally effective?

A: Bill Robinson, a long-time general contractor and moderator of the JLC Online exterior-details forum, responds: To avoid the possibility of capillary action, researchers like John Straube of Building Science Corporation generally agree that rain-screen drainage gaps should be at least 1/4 inch wide. To provide ventilation so that wet siding can dry to the back, the size of this gap should increase as the average annual rainfall increases. Rain screens are probably unnecessary in mild climates with less than 20 inches of rain per year, while in an extreme coastal climate with more than 60 inches of rain annually and a lot of wind, the gap should be as large as possible and pressure-equalized, with ventilation at both the top and the bottom of the wall. In an average climate where 20 to 60 inches of rain falls annually, a 3/8-inch gap is typically recommended, though I'd think that a 1/4-inch gap would be acceptable in most cases.

Battens can be ripped from decay-resistant wood, but I've had good luck using battens cut from 4-foot-by-8-foot sheets of 6-mm-thick (1/4-inch-thick) Coroplast (800-361-5150, www.coroplast.com), a corrugated plastic that won't absorb moisture and is sold at most sign shops for about $50 per sheet. For more ventilation and drying capacity, I use El Dorado's 3/8-inch-thick plastic battens (530/620-5287, www.eldoradobattens.com).

Any time you create a gap, you'll need to use screening to keep out the bugs. But if you establish the drainage plane on the sheathing (in the usual place) and flash window and door openings to that, it shouldn't be necessary to fur out the windows and doors, even when using a wider 3/8-inch rain-screen detail.