Wood furring strips make room for drainage and ventilation,—IF you keep the channels open
This video features Steve Baczek, one of the best building science architects in the business. He is pointing out some do's and don'ts of using wooden furring strips for a rainscreen wall system. Actually, he focuses on a few "don'ts," but in so doing, he does the do's, too.
Here is a transcript of the video:
Video transcript
Coming to you from the Vibe Studio. Yeah, I got the Vibe board behind me, got it fired up, and I'm pretty fired up about this photo. I know I was pretty steaming the day I took it, and it just kind of reignites every time I look at this photo. Yeah, so today, unfortunately, we're not going to talk about what to do—we're going to talk about what not to do. Yeah, today it's all about, "No, no, no, no, no! That's wrong."
The first mistake: corners that don't drain
So, let's take a look at it. First up—corners. And I might make a note of this—that this project here was a homeowner who decided to be a general contractor, which, in some cases, has worked out. In this case, I don't think it worked out as well. He was a gentleman who just didn't—didn't know what he didn't know and thought he could handle and take on everything. He figured, "Oh, building science is not that hard. Oh, building a house, it's not that hard."
We went through three framers. I think it was the second one that walked off the job and said, "Don't even bother paying me, just don't call me ever again," and him and his guys left after a few days. But anyway, I would typically give them instructions. They had the drawings. He obviously couldn't read the drawings very well, and I would show up occasionally, and we would see some problems.
Let corners drain and dry
So, the first problem is this corner board. Yeah, we do have a corner board going here, and then we have one here, and then the siding is going to come into that. So we want some spaces to nail to, obviously, or some boards to nail to if we're doing that furring system. But what we don't want is boards ganged up with those joints.
I told them that anytime you put any two pieces of furring strip together, you have a 3/4-inch space between them. The issue there is if I look at the boards in section—and I'm going to draw those here—right? If I have a board here, and I have a board here at 3/4 of an inch sitting on the sheathing, well, water can drain down through there. If, in fact, I put a board right next to it, now water can be held in tension there, right? And water held in tension means that I can accumulate water. Things can get wetter, right? Instead of just the water falling down in a joint, now it's held there, getting soaked up by the wood, and my wetting potential increases significantly, which means my drying potential has to increase significantly.
Wood stays dry by not getting wet (duh)
The best thing we can do is not let things get wet. So what does that mean? It means shedding water with a roof, but the little residual water that does make it to the wall—I want to get rid of it as fast as possible. And so having these joints butt tight like that gives us that situation so that we could be holding water in there. And then if I put a corner board on here, right, then what am I really doing there? I'm putting all this wood together laterally, and then I'm putting a wood on top of it. It's basically becoming a little water reservoir. I have no draining.
Oh, by the way, the reason we talked about this being a minimum of 3/4 of an inch—when I put the siding there, that is also a little vent space, right? So here, I get the benefit of venting air going up. I also get the benefit of water draining down. So that space is actually doing double duty for me. It's draining the water away. What water does get held a little bit by the wood? I can then take and ventilate that space and dry it out. So, double duty is happening there. This is the first "no" in that photo. Yes, I said, "No, no, no!"
The second mistake: flashing errors
So, the second "no" is in the photo. Let's move down here. So we flash the window. We get a down-and-out scenario there. We have a nice pan and a back dam. Yes, we install it right—the window—and then I show up a few days later when they're putting all the furring on and getting ready to side. And what do they do? They put a piece of horizontal furring across the top of that! Why would I go through all the effort of having that space drain and then put a dam across the draining space? No, no, no! No dams!
These strips should have just continued up just like that. No horizontal—none of it, right? Just leave it open. Let that water come out of the sill, drain right down into that space, and provide continuity—one of the most famous words in building science.
The third mistake: water trapping
All right, let's talk about the third "no." Water is draining through this system, and you can see it here. All right, we have that again. Big old dam there. Where does that water go? Right? It's pretty simple when you're dealing with water—step back, look at the detail, look at what you're doing, and say, "Where does the water go?" Right? In this case here, it's pretty easy. Well, it goes down—but then it stops. Yeah, that's no good.
All right, this board shouldn't have been in here. All of these should have come down. But more importantly, we missed the piece of flashing that should be there above the window to kick the water out over the top of the window. Then we bring our siding in, and that siding should sit up above that flashing—you know, I don't know, a good quarter of an inch or so, right? It'll create a little shadow line. You'll never see it. It'll blend into the whole system there. But more importantly, water can come out.
But also remember, like we talked about over there—we want to be able to ventilate that. So I need to be able to push air up into that. So having that little air space gets me some air up and into that rain screen system, right? That's how a rain screen with a closed joint-type system works. We need to provide inlets and outlets so that not only are we draining water, but we're adding air to the inside.
So yeah, anyway, there you have it—a few no-no-nos on that project. So stay away from the no-no-no's; we just want projects with yeses. So you know it—I'm Steve Baczek, Architect. That's what not to do. Until next time—long live our buildings!