In the mid-2000s, the design/build firm I worked for took all the cedar shingles off a new house on the Maine coast, added a rain screen, and put the shingles back on. The house sat between ocean and harbor on a bluff that acted like an airplane wing, turning a light shore breeze into an unrelenting wind. When it rained, water was getting inside and we couldn’t figure out how, though we could watch the raindrops flowing up the wall.
After a lot of research, I thought a rain screen would create a neutral pressure zone behind the siding and solve the problem. It didn't, because the windows weren't up to the task; the constant wind prevented their weep holes from draining. Even though it couldn't fix that house, I came away convinced that rain screen gaps belonged on every wall I designed, and I don't think I've designed a house without one since then.
I got to compare notes on this recently with Josh Salinger of Birdsmouth Design-Build in an IMW Backroom conversation hosted by Charlotte Mustard. We've both been designing and building rain screens for more than 15 years, starting while the details were still being worked out, and it was striking how much of what we learned in the field the research has since confirmed. The biggest lesson for me is that having the gap matters, as does its size, but the details around it probably matter the most. That's the lens I'd suggest for every question below.
What a rain screen does
A rain screen is a gap between the siding and the water-resistive barrier, also called a weather-resistive barrier or WRB. (Its legacy name is housewrap.) The WRB is typically located just outside the wall sheathing and stops liquid water from getting into the wall. The gap gives water that gets past the siding a clear drainage plane, and in a ventilated version, it lets air move through and dry things out.
Siding lets more water through than most people assume. It sheds most rain but lets some through via gaps, cracks, fasteners and transitions. A building science rough rule of thumb is that of the water that hits the siding, perhaps 1% will get through to the back of the cladding and 1% of that can make it back to the WRB. If the assembly includes exterior insulation, only 1% or less of what reaches the face will make it back to the WRB behind the insulation.
A rain screen has two jobs, draining and drying, and they need different amounts of space. Most of the confusion about gap size comes from treating them as the same job.
Draining and drying are two different jobs
Drainage takes almost no space. Straube has noted that measured gaps around 1/32 inch drain well, and by 1/8 inch, drainage is no longer the question. Josh pointed out something from Straube's 2022 work that surprised me: they tried to make the biggest water drop they could, and the largest one they could measure was just under 1/8 inch.
Drying is a different story. A very small gap will drain your wall; it won’t provide ventilation drying. Water bridges small gaps, clings to surfaces, and soaks into the siding and strips. That retained water leaves most quickly when air moves through the cavity; otherwise it has to slowly diffuse to the interior or exterior. Josh summed up the research this way: "keep it over three eighths of an inch, allow for the ventilation to get rid of that retained water."
So when someone asks whether a drainage wrap counts as a rain screen, my answer is that it does one of the two jobs well. Whether that's enough depends on the siding, the climate, and how much risk the project can carry.
Good, better, best
When I wrote Understanding Rainscreens for Fine Homebuilding in 2019, my original title was "Rainscreens, Drainscreens, and Ventilated Cladding," because those are three different things. Here’s how I rank three common approaches to drainage and drying behind cladding.
Good: drainage wraps and thin mats. Products like Tyvek DrainWrap, sometimes called crinkle wrap, and Benjamin Obdyke’s HydroGap drain liquid water effectively. They provide essentially no ventilation. Three-dimensional mats like Benjamin Obdyke's Slicker Classic and Mortairvent create a little more space. Straube describes entangled mats as clearly better than no gap and closer to a 3/8-inch open gap, though not as good for drying. They are good for drainage and for keeping insects out, and they are particularly good behind cedar shingles.
Better: a deeper gap ventilated only at the bottom. This is a clear gap of 3/8 inch or more that is open at the bottom of the wall but not at the top.
Best: a deep gap ventilated at the top and bottom. The gap is open directly to the outdoors at both the bottom and the top of the wall, so air can move through. For me, the preferred range is about 3/4 inch.
How thick does a rain screen need to be?
For drainage, very little. For drying, 3/8 inch is where airflow starts to do real work, and Straube's guidance puts full airflow at gaps over 3/4 inch. That's why I usually land at 3/4 inch with vents at the top and bottom.
Going deeper doesn't buy much in a wall. On the BS + Beer Show, Straube was asked whether any assembly needs more than 3/4 inch. His answer: "Not in a wall, Ben, but absolutely in roofs." Roofing acts as a vapor barrier and the airflow path is much longer, so roofs are their own conversation.
There are exceptions on the other side too. Painted wood siding is the one case where Straube says he would always use at least a 1/2-inch gap, ventilated at the top and bottom. Anyone who has spent time around older homes in New England has seen why; paint holds up far longer on siding that can dry from both sides.
The upper limit is fire
Bigger gaps have a real downside. Straube traces the concern to National Research Council of Canada tests in the 1960s and 70s, where a 2-inch gap behind foam insulation acted like a chimney. Canada then capped the gap at about 1 inch when foam is used, and his current guidance puts fire risk rising above about 1 inch.
In wildfire country, Straube suggests fiber cement or metal siding on a 3/8-inch gap over mineral wool. That's a reasonable starting point; the details at the bottom of the gap, including ember screens, matter as much as the depth.
Details matter more than depth
This is the point I'd most like readers to take away. Straube put it plainly: "The details matter more than whether the gap's an eighth of an inch or three quarters." His 2026 talk closes on the same idea, focusing on slope and continuity over gap size.
A deep, well-ventilated gap with dead ends at the windows still fails; a modest gap with continuous drainage and good flashing usually performs. I learned that on a remodel of a house dating to 1705, where I drew vertical strips but didn't detail the windows. The builder ran the head flashing back to the face of the strapping, which would have allowed water that got behind the cladding to reach the window frame. There's more to a rain screen than putting up strips. As a designer, I think the integration is best figured out and conveyed on the drawings.
I'm still refining how to best start and stop a rain screen around window openings without creating dead ends. The answer depends on the exterior casing, how it's flashed, how thick the exterior insulation is, and the order of operations. It's complicated, even though it's just vertical strips.
Do furring strips for rainscreen need to be rot resistant?
No. This is a question I hear often from builders and clients. If the rain screen is performing the way it should, there's no need for the strips to be rot resistant, because they're drying along with everything else.
The reasonable follow-up is whether water gets trapped where the strip touches the WRB. It could, but only by horizontal capillary action, and there usually isn't enough water there to make that happen. Straube says he can "almost never imagine" needing treated furring; his team has torn apart 20-year-old walls and found the strips in good shape, because three sides of each strip can dry. Josh put it simply: "Wood can dry. And it's got all the opportunity in the world to dry inside of a ventilated rain screen."
Our go-to is 1x3 spruce. It's produced locally, it's affordable, and it's easy for crews to work with. There's also a benefit that doesn't get discussed much: the siding nails go into the furring instead of through bare housewrap, which cuts way down on the number of holes in the WRB.
What to avoid for furring
When I say furring strips, I mean solid lumber. Ripped sheet goods are a different story. I've tried strips of ZIP sheathing; it's a great material when used as intended, but ripping it leaves freshly cut edges exposed, and in a place where it rains, those edges swell. I'd avoid ripped OSB of any kind for the same reason.
Over thick exterior insulation, battens secured through the insulation to the framing support the cladding and help hold the insulation in place. They also create a rain screen gap. Using rot-resistant lumber won’t hurt anything, as long as all fasteners will remain in good condition over decades, which is often not the case with pressure-treated lumber. But as with battens installed on solid walls, there is no need for these to be treated lumber either.
Horizontal strapping for vertical siding is another tradeoff. I sometimes use strips of 3D mat behind horizontal strapping to provide a capillary break and drainage; it works, but it doesn't allow enough airflow to be ideal. Rigid products like Benjamin Obdyke's AeroNet stand off the wall without compressing, which is a benefit. They also use more material and cost more than some other options.
Climate changes the calculation
Where you build affects how much margin you need. A dry climate like Colorado is far more forgiving than Josh’s Pacific Northwest location, which is generally mild but damp, or our climate here in the Northeast, which can be hot or cold and is often wet.
That's part of why I treat a rain screen as a default rather than an upgrade. In our climate, the cost is modest compared with the cost of a wall that stays wet.
Where to start
Get the details right first: continuous drainage, good flashing at every opening, and no dead ends. Then aim for a ventilated gap of 3/8 inch or more, with 3/4 inch as a sensible target in most walls. After that, use plain lumber for the strips and put the savings toward better windows.
For the bigger picture of how rain screens went from a hard sell to standard practice, read Rain Screens, 15 Years In on the IMW blog. Josh's post on rain screen siding and what goes wrong in the field is coming soon on the Birdsmouth blog.