- 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
Corp. 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 (800/666-2241,
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.