Why water gets behind siding
No siding is waterproof. Wind drives rain through laps, joints and around windows; water runs down behind trim; and in a wet climate the back of the siding can stay damp for weeks. The real waterproofing is the weather-resistive barrier (house wrap or building paper) and the flashing behind the siding. When siding sits tight against that barrier, water that gets behind it is trapped in a thin film, soaks into wood siding from the back, and can find its way through staple holes and laps into the sheathing.
What a rainscreen does
A rainscreen creates a small, continuous space between the back of the siding and the weather barrier. That gap does three things:
- Drainage – water that gets behind the siding runs down and out at the bottom instead of sitting against the wall.
- Drying – air can move behind the siding, so both the siding and the barrier dry faster after rain.
- Capillary break – wood-based siding and paint no longer wick water from a wet barrier, which helps paint last longer.
How much gap is enough?
Surprisingly little. Building science research summarized in the trade press shows that even a small drainage space – a few millimeters – drains water far better than siding pressed tight against the barrier, which is why Oregon’s code settled on a minimum of 1/8 inch. Larger gaps of 3/8 to 3/4 inch, created with furring strips, add ventilation and drying on top of drainage. In very wet or exposed locations, and behind wood siding that must dry from the back, the larger gap is generally preferred.
How it is built
- Furring strips. Vertical wood or plastic strips (often about 3/8 to 3/4 inch thick) are fastened over the barrier at each stud, and the siding is nailed to them. Insect screen or a vented strip closes the gap at the bottom and top.
- Drainage mats. A three-dimensional plastic mesh installed over the house wrap creates a gap of a few millimeters.
- Drainable house wraps. Wraps with built-in texture or spacers create a small drainage space without separate strips.
Vinyl siding already has a built-in air space behind its profile and is loose-hung, so it drains and ventilates on its own; the rainscreen question matters most for fiber cement, engineered wood, wood and other siding that sits flat against the wall.
Oregon made it a code requirement
After a wave of moisture-related wall failures in the late 1990s and early 2000s, Oregon amended section R703.1 of its Residential Specialty Code. According to the Journal of Light Construction, siding on homes built under that code must have a drainage space of at least 1/8 inch, either with a true rainscreen gap or with a weather-resistive barrier tested to drain at least 75% of water (ASTM E2273). Other places rely on the national model code, which requires a weather barrier and flashing but, in most climates, not a drainage gap. Your local code and your contractor can tell you what applies where you live.
When a rainscreen is worth it
- Wet climates – the Pacific Northwest, coastal areas and anywhere with long rainy seasons
- Fiber cement, wood and engineered wood siding, especially in dark colors that heat up and drive moisture inward
- Walls with little overhang, exposed to wind-driven rain
- Homes with a history of rot or peeling paint behind the old siding
- Walls with exterior foam insulation, where the siding cannot dry to the inside
What it costs and what changes
A rainscreen adds materials (strips, screen, trim extensions) and labor. It also pushes the siding out from the wall, so window and door trim has to be deeper, and roof-to-wall and deck flashing details change. Costs vary by method: a drainable house wrap adds little compared with standard wrap; full furring-strip rainscreens add more. In Portland, where many projects use fiber cement over a rainscreen, HomeAdvisor reports wood or fiber cement siding installations averaging about $18,471. Ask your contractor to price the rainscreen as a separate line so you can see the difference.
Details that make or break it
- The gap must be open at the bottom (with insect screen) so water can drain out
- Flashing above windows and doors must direct water into the gap and out, not behind the barrier
- Window trim and sills must be extended to the new siding depth
- The barrier itself must still be lapped correctly – top over bottom – and taped at openings
- Fasteners must be long enough to go through the furring into framing
Common mistakes
- Closed bottom. If the gap is sealed at the bottom, water has nowhere to go. It needs an opening protected by insect screen.
- Furring over torn wrap. A rainscreen does not replace a sound weather barrier; damaged wrap or paper must be repaired first.
- Short window trim. When the siding moves out, windows look recessed and water can get behind trim unless sills and casings are extended.
- Horizontal furring on lap siding. Strips must run vertically (or be notched) so water can drain; horizontal strips create dams.
- Skipped kick-out flashing. Where a roof meets a wall, a kick-out flashing must direct water into the gutter, not into the gap.
Rainscreens and insulation
If you add rigid foam insulation over the sheathing during a re-siding job, the wall can dry less easily to the outside. A rainscreen gap over the foam gives the siding a place to shed water and dry, and gives fasteners a solid base in the furring strips. Walls with exterior foam are among the strongest candidates for a rainscreen.
Is it worth it on a re-siding job?
If you live in a wet climate and are installing fiber cement or wood-based siding, a rainscreen is usually one of the best-value upgrades you can make, because the wall is already open and the cost is a fraction of the total project. In drier climates, a good weather barrier and careful flashing are the priorities; a drainable wrap is an inexpensive middle ground. The worst option is the one many older homes have: siding nailed tight over torn or missing paper, with no flashing at all.