Rainscreens: minimum depth
matt2021
| Posted in General Questions on
On this forum, I’ve often read that, ideally at least, a rainscreen should consist of a gap no less than a 1/2” deep. This video calls that a “myth,” and claims that 1/4” is the minimum, that only a gap less than 1/4” would be too small. What are folks’ opinions on this? (For the room I’m building, I’ll see if I can get away with 3/8”; knowing that even 1/4” is acceptable, of course, would make me feel much better about that.)
The video covers other “myths,” including the claim that you need an insect barrier. Personally, I don’t buy his argument; not adding a barrier seems too risky to me.
Thanks!
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Rainscreens can do 2 things, provide a capillary break, or in addition provide ventilation.
In many situations (for example a stucco wall in a dry climate) a capillary break is all that is needed. This allows any moisture that gets behind the stucco to drain or evaporate away without damage to the underlying substrate. A capillary break can be very thin, even 1/16". For example, you can use Dupont's Homewrap, which is smooth, and add a second layer of Stuccowrap. The texture of the stuccowrap creates a gap between the 2 wraps and that's enough to provide drainage behind the stucco.
But if you want more ventilation, a "ventilated rainscreen", then you want to go with a larger gap. Even then, though, 1/4" is adequate. I shoot for 3/8" in my designs. I believe some wetter localities like Portland, OR may require gaps as large as 3/4". But truthfully, if you have a 1/4" gap you will get plenty of ventilation... IF ... you provide a gap at the bottom AND the top of the wall. You want the air to be able to flow in and out at the bottom and the top.
Thanks! That’s very reassuring. I’m in New Jersey. Maybe I’ll go for 1/4” after all, as I can use every fraction of inch. My sheathing is ZIP (R6); so, no house wrap. The siding will be cedar wood cladding (beveled horizontal boards), primed and painted.
I will definitely have air circulation from bottom to top. I’m still studying the details around the windows (which are all tightly placed one next to the other. I’m not sure I’ll be able to have a gap between the windows; maybe I will make the rainscreen’s strips very thin for those areas, so as to leave a vertical air gap between them, between the windows.
Hi Matt. Good luck with the project.
Cor-a-vent's SV3 is an easy way to take care of venting the bottom and top, while providing a barrier to bugs.
https://www.cor-a-vent.com/siding-vent-sv-3.cfm
You can cut sheets of 3/8" plywood for the furring. That's a cheap and reliable way to go. Or of course you can use similar sized lumber. 3/8" furring works well with the SV3. Otherwise, if you go down to 1/4" you may have to use bug screens, which can be a hassle to install and not be as reliable.
The ZIP-R should be fully taped and you'll want to tape the bottom of the panel to the foundation as well. Alternatively you can use a fluid applied product. But I think those are usually more expensive.
Flanged windows? - You might be able to run the furring 1-2" out from the edge of the window frame, so as not to be over the flanges. Then your trim would go over that. There are a bunch of different ways to handle flashing and trim details around windows. I'll see if I can come up with something and post it here later.
Thanks, for everything! Leaning very strongly towards 3/8” now, as I was planning to use Cor-A-Vent, but did not yet know it would not come as 1/4”.
I’m attaching some pictures of the space between the windows. The measurements are: 4” between window frames; nail fins (now covered with tape) are about an inch (so, I have 2” of sheathing—now covered by the tape—to which maybe attach the furring strips, so as to leave air channels on the immediate sides of the window frames, if that’s one of the things you were thinking about; I guess I was thinking of having the strips of plywood against the window frames, and a gap between the two adjacent strips; but that way the strips would have to be nailed through the windows’ nailing flanges, and that does not seem like a good idea).
To the outside, the window frames are 1” deep. That creates a potential problem for trimming:
- if the outside trim is not a bit thicker of the thickness of the rainscreen and siding combined, the siding would not look good (it might even end up sticking out here and there); if I use thicker trim, I need to find the right thickness (I’m planning to use cedar wood), and the trim might go past the window (will make the window look, very slightly, like an “innie”? I don’t think I would mind that, though from an aesthetic point of view. My house’s existing windows actually have trim—wrapped in aluminum—that protrudes well past the windows). Perhaps none of this is a real problem. I guess I’ll need to find the lumber in the right thickness, so that it does the job with the siding and does not look too beefy for the windows, assuming that the latter is a potential problem., which it might not be.
> if I use thicker trim, I need to find the right thickness (I’m planning to use cedar wood), and the trim might go past the window (will make the window look, very slightly, like an “innie”? I don’t think I would mind that, though from an aesthetic point of view.
Keeping things in plane is more problematic than creating a reveal between them. Typically, having a small reveal between the plane of the trim and the face of the window frame is just fine. Err on the side of the trim protruding more from the building because the siding should not be proud of the trim; it should "die" into the trim.
Your flashing tape comes onto the window frame and not just the flange. This is a good thing. However, aesthetically, you may want to trim it a bit as it seems to be turning the corner and is visible on the side of the frame parallel to the house.
matt2021,
I agree with you. 1/4" to 3/8" works fine as long as you aren't using house-wrap as your WRB. I don't buy his logic on not needing insect screening either. It may work where he is, it wouldn't here.
Venting the top is a trade off between the increased drying capacity it adds, set against the added complexity of detailing, and the possibility of bulk water intrusion into that top opening from both weather and washing your siding. Rain-screens work fine both ways.
Try and keep all your furring off the window flanges. It leaves you a vented gap against the window frame, and the flanges add depth, taking the siding out of plane.
Thanks, Malcolm!
I had no idea that rain-screens could be NOT vented at the top, and still work (if not as much).
The advice on keeping the furring strips away from flanges is really important. I fear I might have a real problem there, as the flanges are thick (because of the sealant) and irregular, AND are VERY close to each other. Did you have a chance of looking at the pictures I enclosed? Perhaps I should run 2" strips between the windows, hoping to hit only flat material (and working on flattening where needed, as even the ZIP flashing tape adds thickness and irregularities). Tomorrow, I'll check more carefully, but at the moment I am quite worried. The workers did not pay attention to those details, and the flanges are very irregular unfortunately.
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Malcolm, I’m just reading this now. Can you elaborate on why 1/4 inch to 3/8 inch gap is ok except if using house wrap as the wrb?
Mike,
Most sheet WRBs, like Tyvek, don't sit entirely flat against the wall. With a narrow cavity you risk having the WRB bridge the gap, which defeats the capillary break, or they can end up blocking the cavity altogether.
Matt2021,
Our code requires rain-screens here in coastal BC, but not that they be open at the top, and they work well. The only problem I've ever heard was some reports that in very cold climates you can sometimes get moisture accumulating at the top of the cavity.
Don't stress about the flanges. It's a small difference in depth. many builders put the strapping onto the flanges, it's just something to try and avoid if you have the choice.
Malcom, can you expand on cold climate moisture accumulating at the top? Seems like that could only be from interior air leakage since the exterior cold air carries so little moisture?
tdbaugha,
I've only heard of it in a comment here on GBA by a New England builder. Maybe Michael Maines remembers who it was and the context?
Edit: Found it in Peter Yost's comments at the end of this article: https://www.greenbuildingadvisor.com/article/how-to-vent-a-rainscreen
Regarding trim and siding -
Set the furring strips clear of the window flanges. In the 4" space between 2 adjacent windows center a furring strip that is 1-1/2" or 2" wide. This will leave a small drainage gap behind the trim, around the window frames. Make sure to leave this gap open at the bottom for drainage.
I would use 5/4 material for the trim. This gives you a full inch of thickness. The clapboards typically project out about 3/4". So they would neatly project less than the trim. You will want to caulk the jamb joints where the clapboards butt against the trim. Optionally you can caulk the joint at the top of the header trim between it and the clapboard above. But don't caulk the joints between the window trim and the window frame. Before installing the header trim you should have a header flashing above the window frame and taped to the WRB.
There's more than one schema for what gets caulked and what gets left open but the key concept is to make sure there is a path out for water at the top and the bottom of the window frame.
> You will want to caulk the jamb joints where the clapboards butt against the trim.
Not necessarily. I prefer painting the cut edge of trim, and leaving a gap between siding and trim for drying.
Patrick,
I do a bit of a hybrid. I run a bead of caulking on the furring about 1/2" in, and bed the siding in it. The ends can dry, but there is no horizontal movement of water.