Rain Screens, 15 Years In: What Two Builders Learned the Hard Way

By Charlotte Mustard, President, IMW Group, with Mike Maines of Mottram & Maines Architecture and Josh Salinger of Birdsmouth Design-Build

I started with a simple question for Mike Maines and Josh Salinger. How long have you been building rain screens?

Mike first read about them in a 2001 Fine Homebuilding article and was using them on the job by 2005. Josh put his first one on a house around 2010, and he thinks it may have started with a manufacturer rep showing up and making a good case. Between the two of them, that’s more than 15 years of building rain screens, much of it while the details were still being worked out.

Mike is a designer and co-owner at Mottram & Maines Architecture in Maine. Josh owns Birdsmouth Design-Build in Portland, Oregon. I brought them together for an IMW Backroom conversation to revisit Mike’s 2019 Fine Homebuilding article, Understanding Rainscreens, and talk through what has changed since. A lot has, and the few things that haven’t are worth talking about too.

What is a rain screen?

A rain screen is a small gap between your siding and the water-resistive barrier (WRB) on the wall behind it. Water that gets past the siding has a clear path to drain out, and air moving through the gap helps the siding and the wall dry.

Mike explained why it clicked for him right away. If you’ve spent time around older homes, you’ve seen where wood rots and where paint peels. “It doesn’t necessarily solve all problems,” he said, “but it certainly helps a lot of things work better if there’s that gap.”

Josh came at it from a different angle. When that rep first showed up with a product, he ran it through the same filter he uses on everything. “I’ve always had a way of interrogating products from a science-based perspective,” he said, “and it just kind of struck me as making sense.”

Where it started: squishy mats and drawing from scratch

Both of them started with a product from Benjamin Obdyke, a yellow plastic mat that goes on the wall behind the siding. Back then it was called Home Slicker and included an attached WRB; today the more common version is the mesh only, sold as Slicker Classic. It works well behind cedar shingles, but it’s a little squishy, which makes it harder to use behind other siding.

Josh remembers that Home Slicker came with the WRB already attached, a two-in-one product that sounded easier than it was. “Trying to put that on was just a hassle,” he said. “When it came to windows and flashing, it just got to be way more complex than it had to be.”

I went into the conversation assuming they both started out building their own gap with wood furring strips, and that purpose-made products came later. It was the other way around. Both of them moved from mats to wood strips around 2013, and Mike said that’s when it stopped feeling like an extra. “That’s where I learned that it’s really an essential part of an assembly,” he said. “Not optional.”

The early mistakes

Mike’s first big rain screen project was a repair job. Around 2007, his firm took all the siding off a leaking house on the Maine coast, added a rain screen, and put the cedar shingles back on. The house sat on a bluff with the ocean on one side and a harbor on the other, which Mike described as “basically an airplane wing shape.” The wind never stopped, and when it rained, water actually traveled up the wall and into the windows.

His research said the rain screen would even out the air pressure and fix the problem, but the windows just weren’t up to the task. That could have soured him on rain screens for good. Instead, he said, “I don’t think I’ve done a project without a rain screen since then.” (And he’s more fastidious about window pressure ratings now as well.)

A few years later, Mike specified a rain screen on a remodel of a house built in the mid-1700s. He drew the vertical strips but didn’t spell out how the rain screen should meet the windows. The foreman was experienced and good at his job, and he did what made sense to him, running the head flashing over a window back to the face of the strapping. Mike saw right away that it would send water into the window, and he saw something bigger too. A rain screen needed far more detail and specification than he had been giving.

“I just realized there’s more to this than just putting up some strips,” Mike said. “There’s a lot of integration here that I need to figure out and then convey, because this is really new and different stuff.”

Josh’s early trouble was on paper. His first full rain screen house had Zip System sheathing, cork exterior insulation, and flat trim around the windows, and all of it was new to his team. He got lucky with an architect who was game to try it and a siding crew who didn’t push back. The hard part was drawing the assemblies. “There wasn’t a lot out there to really draw upon,” he said. “We were doing everything for the first time.”

From cost fights to a seat belt

For years, getting a rain screen into a project meant a fight. “I recall getting pushback from all sides on the extra cost,” Mike said. “Claiming it was going to take so much longer and cost so much more.” Josh had the same experience. He’d walk clients through the science and the details and still couldn’t get them over the hump.

That has flipped. “I haven’t heard complaints about the cost of using a rain screen in years,” Mike said. Josh put it more bluntly. “I would never do a siding project without a rain screen. It would be like driving without a seat belt. You just wouldn’t ever do that.”

For Josh, part of what made it stick is how his company is set up. Birdsmouth self-performs what he calls “wood and water.” His crew does the framing and carpentry, and they also handle everything that touches water: the WRB, the window installs, the rain screen, and the air barrier. Tile, drywall, and flooring can go to subs. “The water is the thing that’s gonna destroy the house,” he said, “so that’s what we keep within our company.” The newest carpenter on his team has never built a wall any other way, so there’s no one left to push back. And in Josh’s view, there’s no gray area. “You can’t say you’re a quality builder if you don’t do that.”

I wondered out loud whether some of this is their reputation doing the work, since clients who hire Mike and Josh already trust them. Mike laughed that off. “We still get pushed on pretty much everything,” he said. Rain screens just aren’t one of those things anymore.

That doesn’t mean the whole industry is there. The day before we talked, Mike spoke with a builder who wanted to know whether rain screens were even necessary, and nobody at that company had heard of them. The same week, I talked with a builder on the Southeast coast whose biggest challenge is getting anyone, architects included, to care about building science at all.

What struck me is how personal this spread has been. For Josh, it may have started with a manufacturer rep who showed up with a good pitch, and for Mike, it was a magazine article that happened to land in front of him. After that, it came down to individual people saying yes, like the architect and siding crew on Josh’s first job, and then to builders like these two writing about it and teaching the next group. That’s how most building science reaches the field, one jobsite and one conversation at a time.

Materials then and now

The early options were squishy mats, two-in-one wraps, and whatever you could bend out of window screen to keep bugs out. Today the list is longer and better.

Mike’s go-to is plain 1×3 spruce because it’s produced locally and it’s affordable. Josh pointed out that purpose-built insect screens like Cor-A-Vent weren’t common when they started, and now they’re everywhere. Perforated flashing made for the bottom of the gap is readily available in the western US, though Josh’s team often bends its own from rolls of stainless steel screen because it costs less. For jobs where wood strips don’t fit, there are rigid plastic products like Benjamin Obdyke’s AeroNet that hold their shape and don’t compress.

In wildfire zones, there are now intumescent ember screens that swell shut when they encounter high heat or flames. They aren’t cheap, but neither is losing a house. On a recent project, Josh’s team put them only at the bottom of the gap. “Most of it’s going up,” he said. “The sucking is happening at the bottom.” He admits you could make the case for top and bottom, especially where embers are wind-driven.

Some things didn’t work. Mike tried ripping OSB sheathing into strips, and the freshly-cut edges swelled when they got wet. He doesn’t recommend ZIP or other OSB for furring anywhere it rains regularly. Josh has a story about a house where fiber cement strips were used as battens, and the screws rusted through until the siding came off by hand. That full story is coming soon on the Birdsmouth blog.

What the research confirmed

For a long time, designers and builders like Mike and Josh were going on field experience. Much of the research published since then has confirmed what they were learning on the job.

John Straube and Jonathan Smegal of RDH Building Science reviewed nearly 400 publications on rain screen performance, going back to 1938, for the Rainscreen Association in North America (RAiNA). How much water gets through varies a lot with location, the type of siding, and how well it was installed. In general, typical siding sheds almost all the rain that hits it. Maybe 1% gets through to the back of the cladding, and about 1% of that makes it back to the WRB.

“It’s surprising how much water gets behind the cladding in the first place,” Mike said. “I think most people assume the cladding is keeping all the water off.”

Josh zeroed in on a part of the research that explains why airflow matters as much as drainage. Even in a wall that drains well, water hangs around. Drops can bridge a small gap, water soaks into the pores of materials, and surface tension holds it in place. “Even though you have a drained assembly, it’s still retaining a whole bunch of water,” he said. “That’s where the ventilated bit comes in.” His overall take on all that research was simple. “They get into this crazy detail just to find out, yeah, they work.”

One more thing stood out to me. Straube has pointed out that code gap sizes were never really settled by science. On the BS + Beer Show, he explained that Canada started at 3/4 inch, settled on 3/8 inch in 2005, and U.S. codes are heading smaller. In his words, “None of it is based on good science.” Those numbers were agreed on in a committee, which is worth knowing the next time someone treats a code minimum as the right answer.

How big should the gap be?

A tiny gap drains water well but doesn’t do much for drying. Ventilation drying becomes effective at around 3/8 inch. Mike prefers about 3/4 inch, open at the top and bottom so air can move through. Past about 1 inch, fire becomes a concern. Mike’s full breakdown of rain screen gap size and furring strips, including his good, better, and best options, is coming soon on the Mottram & Maines blog.

Do furring strips need to be rot resistant?

No, and it’s one of the most common pushbacks Mike hears. “If the rain screen is performing the way it should, then there’s no need for those pieces to be rot resistant, because they’re drying along with everything else,” he said. Josh agrees. “Wood can dry. And it’s got all the opportunity in the world to dry inside of a ventilated rain screen.” Mike’s post on which furring materials work and which to avoid is coming soon on the Mottram & Maines blog.

What’s still hard

After 15 years, two things still don’t have a clean answer.

The first is windows. Mike is still working out the best way to start and stop a rain screen around window openings without creating dead ends where air and water can’t move. The right detail depends on the window, the trim, the flashing, and whether there’s exterior insulation. “We were just talking this morning,” Josh said. “I don’t think we’ve ever done the same thing twice on a building.” Every custom home is different, whether the client lives in a wildfire zone, has a tight budget, or cares most about embodied carbon. What has changed is the range of good answers. “At the beginning, we had no idea,” Josh said. “Now we’ve narrowed it down to a certain range of options we can choose from.”

The second is vertical siding. The old approach was to cross-hatch two layers of strips, but that creates a gap of about 1 1/2 inches, which raises the fire concern. Some builders run strips at 45 degrees instead. Josh has used that approach, but he can’t say how well it dries, and he says it gets “obnoxiously complicated” around windows and doors. His walkthrough of rain screen siding details is coming soon on the Birdsmouth blog, along with the story he tells clients about the day a two-story rain screen blew air at him like a hairdryer.

When Josh runs into a detail he hasn’t seen before, he goes back to one rule. Check your four control layers, which are water, air, vapor, and heat, and make sure each one is continuous through the whole detail. If one isn’t, start over.

Where that leaves us

Rain screens went from a line item builders had to defend to something Mike and Josh wouldn’t build a wall without. Over those same 15 years, the materials and details kept getting better, and the research ended up confirming what they had already learned on the wall.

Some builders are just now asking what a rain screen is. If that’s you, Mike and Josh have already made most of the mistakes, and they’re sharing what they learned.

I’ll admit I was surprised by how much of this they had at their fingertips. Mike was quoting research reviews from memory, and Josh pulled up Straube’s numbers mid-conversation and had the sources in my inbox before we hung up. These two bring the receipts. That kind of recall only comes from people who care a lot about getting this right, and we’re lucky at IMW to get to work with clients like that. It makes our job a lot more fun, and a lot easier, when the people we work with are this invested.

Keep an eye out over the next few days. Mike is publishing his deep dive on rain screen gap size and furring strips on the Mottram & Maines blog, and Josh is following with his post on rain screen siding and what goes wrong on the Birdsmouth blog. Follow Mike and Josh on LinkedIn so you catch both when they go live.