Rainscreen behind fiber cement siding: when the extra gap is worth paying for
Architects often tell owners building in wet climates to add a vented gap, such as Cor-A-Vent's system, behind fiber cement siding. The IRC requires only a water-resistive barrier, and James Hardie does not require a gap behind its lap siding, so the gap is a judgment call with a price per square foot.
Since the 2006 edition, the International Residential Code has required a water-resistive barrier behind siding (section R703.2), the Pennsylvania Housing Research Center at Penn State notes. A rainscreen, a drained air space between that barrier and the cladding, is an above-code option in most places. Some jurisdictions demand one: British Columbia's Lower Mainland requires a rainscreen behind most siding, according to Bjorn Delblanc, a former James Hardie technical specialist and now an Edmonton siding contractor. He writes that Hardie does not require a gap behind HardiePlank lap siding on any building type.
PHRC's case for the gap starts with how fiber cement is hung: tight against the barrier, which limits how well bulk water drains down it. A gap adds a capillary break, a clear drainage path and, if the wall is vented at the top and bottom, drying by air movement. PHRC puts the minimum effective depth at 1/4 inch, with 1/4 to 3/4 inch common. Delblanc calls a full rainscreen behind lap siding overkill, since each plank touches the wall along a narrow strip and Hardie's boards carry drip edges in its HZ5 markets.
There are three ways to build the gap. Furring strips go on first and fasten through to the studs: Cor-A-Vent sells 3/8-inch plastic Sturdi-Strips in 4-foot lengths, about 1 linear foot per square foot of wall at 16 inches on center, plus its insect-screened SV-3 vent strip at the top and bottom. Rolled mesh or mat products are the second route. A drainable housewrap is the third; Hardie accepts one with more than 90% drainage efficiency under ASTM E2273 behind its panel siding, Delblanc writes. PHRC says wood furring does not need to be treated, while contractor Synapse Construction calls for pressure-treated or borate-treated strips.
Synapse Construction lists furring materials at $0.40 to $1.50 per square foot, drainage mat at $0.60 to $1.70 and labor at $0.60 to $1.75. By those figures, furring plus labor on 2,000 square feet of wall comes to roughly $2,000 to $6,500.
Why it matters: Put the rainscreen in the bid as its own line, priced per square foot, so the owner can weigh it against the wall's exposure. If the owner declines, a drainable housewrap still gives the wall a drainage path.
Sources
- Rainscreen Gaps in Exterior WallsPennsylvania Housing Research Center (Penn State), Jun 1, 2016
- Does James Hardie Siding Require a Rainscreen?Blue Jay Exterior Renovations, Jul 4, 2020
- Sturdi-Strips Rainscreen Furring StripsCor-A-Vent, Nov 3
- 5 Things You Need to Know When Installing Fiber Cement SidingSynapse Construction, Nov 3