A rain screen assembly is a siding installation method that leaves a small air gap, typically 3/8 to 3/4 inch, between the back of the siding and the water-resistive barrier wrapped around the wall sheathing. That gap gives any water that gets past the siding somewhere to go, so it drains out the bottom and dries with airflow instead of sitting against the wood.
Traditional "face-sealed" siding relies on caulk, laps, and the siding material itself to keep water out entirely. That works fine in a dry climate, or for a while anywhere, but every seal eventually fails somewhere — a nail hole, a butt joint, a corner where two trades' work meets. Once water gets behind face-sealed siding with no gap, it has nowhere to go. It sits against the sheathing, and over months or years that's how rot starts.
A rain screen assembly assumes water will get behind the siding eventually and plans for it. Vertical furring strips (or a manufactured drainage mat) are installed over the water-resistive barrier, creating a continuous gap behind the siding. Vents at the bottom of the wall let air and any bulk water out; vents near the top, often at the soffit, let air move up through the cavity by natural convection. The siding is fastened through the furring into the studs, so it's still solidly attached — it's just standing off the wall on a drainage plane instead of pressed directly against it.
The upside isn't just moisture control. That air gap also helps siding dry faster after rain, which matters for anything wood-based, and it improves the wall's overall thermal performance slightly by breaking direct contact between the siding and the sheathing.
Whatcom County doesn't get hurricanes or hail the way some markets do, but it gets something arguably harder on a house over time: months of low-intensity, persistent rain. Bellingham and Ferndale see rain on a large share of days from October through April, and it's rarely a hard downpour that runs off fast — it's steady, saturating drizzle that sits on a wall for hours. That kind of exposure is exactly what face-sealed siding struggles with, because there's no single dramatic failure point, just years of slow saturation at the weak spots.
Wind direction matters here too. Homes closer to Bellingham Bay, Birch Bay, and the open water near Blaine take wind-driven rain that gets pushed sideways into wall assemblies, not just straight down. That drives more water past laps and trim than a calm rain would, which is precisely the scenario a rain screen's drainage gap is designed to handle.
Moss and algae growth is another local reality. Constant moisture plus shade from mature trees, common on lots throughout Lynden and the county's more wooded areas, gives organic growth a foothold on siding and in gaps that don't dry out. A ventilated rain screen dries faster than a sealed assembly, which gives moss and algae less to work with, though it won't stop growth on its own — that's still a maintenance item.
Washington's state energy code also plays a role. Current code pushes toward continuous exterior insulation on many wall assemblies, and a rain screen is the natural way to install siding over that rigid foam layer while still giving the wall a drainage path. In practice, on a lot of Whatcom County remodels and new builds, the rain screen and the energy code upgrade get solved together rather than as separate decisions.
If you're re-siding, ask directly whether the installer plans to include a rain screen gap or install face-sealed. Not every contractor defaults to it, mainly because it adds labor and material cost — furring strips or drainage mat, extra fasteners, careful detailing at windows and trim so the gap doesn't get blocked. It's a real added cost, typically more noticeable on smaller jobs, and it's fair to ask for that line item spelled out rather than assumed.
It's also fair to know the limits. A rain screen manages water that gets past the siding — it doesn't fix a bad roof-to-wall flashing detail, a missing kick-out flashing over a deck, or window flashing that was never lapped correctly in the first place. Those details still have to be right, or the drainage gap is draining water that shouldn't have gotten in that deep to begin with. If you're getting quotes, ask how flashing at windows, doors, and roof intersections integrates with the water-resistive barrier, not just whether there's a rain screen.
For an existing home, a full rain screen retrofit usually only makes sense when the siding is coming off anyway — during a re-side, not as a standalone project. If your current siding is showing early rot at the bottom courses or around windows, that's worth a look before deciding whether repair or a full re-side with a proper drainage assembly is the better call. Alpine Exteriors walks Whatcom County homeowners through that assessment on-site rather than quoting blind, because the right answer depends on what's actually happening behind the existing siding, not just its age.
No, but it helps most materials that are vulnerable to sustained moisture, including fiber cement, wood, and some engineered products. Vinyl siding already has some incidental airflow behind it by the way it's installed, though a true rain screen still improves on that. Ask your installer what's appropriate for the specific siding you're choosing.
It reduces the moisture that feeds growth by drying faster, but it doesn't eliminate it. Shaded, north-facing walls in wooded parts of the county will still need periodic cleaning regardless of the wall assembly behind the siding.
Usually somewhere around half an inch to just under an inch, depending on the furring or drainage mat used. That difference has to be accounted for at window and door trim, roof edges, and any place siding meets another material, which is part of why it needs to be planned before installation starts, not adjusted after.
Not universally required for every wall type, but Washington's energy code requirements around continuous insulation push many assemblies toward a rain screen approach as the practical way to install siding over rigid foam. Requirements vary by wall assembly and jurisdiction, so it's worth confirming specifics for your project rather than assuming.
If you're planning a re-side anywhere from Bellingham to Blaine and want to know whether a rain screen assembly makes sense for your walls, Alpine Exteriors can look at your current siding and flashing details in person and lay out the options plainly, including the added cost and where it does and doesn't matter for your specific house. Reach out to Alpine Exteriors to schedule a look at your home.