Best Material for Rainscreen Battens
I have always used wood (both cedar or fir) to make my rain screen battens for obvious environmental reasons but the allure of products like Cor-A-vent does make me wonder what it would be like to use them. They probably wouldn’t cup and I imagine would be easy to apply. The manufacturer for Cor-A-Vent says they are “ heat resistant” but would this product be as durable as a natural wood product? I have not done my due diligence comparing cost. Perhaps I will be sorry about past decisions when I find these answers? One rain screen I recently put up had a surprising amount of boards cup to varying degrees which had not happened before to such a degree. I attributed this to getting product that was not cured enough. Any thoughts or experience would be appreciated.
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Building "green" doesn't mean just using natural/unprocessed products. You should think about the building lifecycle. As soon as you do ONE replacement cycle, you've probably (maybe even usually) cancelled out any green advantage you originally had. For this reason, I try to build for things to last. That means use polymer materials where necassary, or treated wood products. Often times the modern product will far outlast the "natural" product, and you end up with a better/greener overall structure when you look at the total lifecycle impact. That's why this stuff gets complicated sometimes.
That said, battens in a typical rainscreen should be reasonably well protected from the elements. Cor-a-vent is a good product though, it has been around a while, and is often used. I even see it (or something very much like it) used to ventilate masonry walls on commerical buildings. I'm guessing they are made of polypropylene from the feel of the material, but I've never really checked to be sure. "heat resistant" will likely be similar to the heat resistance of PVC pipe. They'll be OK in typical "the sun made it hot" applications, but they're going to get destroyed in a fire.
I've been generally unimpressed with the quality of most thin lumber (and thick lumber too, actually). Many people use strips of 3/4" plywood to build their rain screen gaps. Plywood will be less likely to cup and warp on you, just be sure to use one that has an exterior rated glue (the "X" in "CDX", for example).
Thanks Bill, does anybody have an idea of the usable life of something like Cor-A-Vent? I imagine it’s got to be a long time if UV light is not degrading them?
If you can get it in black, it'll probably last a long time even exposed to UV light. If you can only get it in white, it probably won't last long if exposed to UV light, unless the've added a UV stabilizer to the resin.
If you can install it where it's NOT exposed to UV light, I would expect it to last something on the order of centuries, as long as it's not subject to physical damage.
The main benefit of proprietary products like Cor-a Vent is that they can be used in horizontal applications where wood battens impede airflow, and as screening at the bottom of the cavity. Otherwise plywood strips function just fine, are a lot cheaper, and provide an additional nailing surface for the cladding fasteners.
Good points. Thank you.
I was about to ask a question about coravent and rainscreen strips of wood. I see this question here so I'll chime in here--I was thinking of using 1/2 inch thick x 1.5 inch wide strips of plywood (left over from sheathing). For the tops of walls, and for above & below windows, I am looking at a coravent or similar product.
1) is this product a reasonable alternative to coravent?
$0.44/foot for the 3/4 inch thick(HDPE is the material)
$0.40/foot for the 3/8 inch thick product
$0.37/lineal foot for 3/8 inch thick, and now I'm also finding it in 7/16 inch thick.
(extruded polypropylene plastic)
I'm assuming I'll have to buy it online and have it shipped? I'm annoyed by all these newfangled expensive plastic components that become imperative to purchase, but I can also see the benefits of using this compared to the alternative which is awkwardly wrapping insect screening around the tops and bottoms of my furring strips. But honestly that would be a lot cheaper.
3/8" plywood (CDX) sheets ripped down to 1 1/2 inch strips would cost about $0.11 per foot.
3/4" CDX ripped the same would be $0.16/foot.
1x4's cost about $0.29 a foot, unless ripped lengthwise then $0.14/foot.
2) The Quarrix furring strips seem to only be available in 3/8 thickness and 3/4 thickness, so my planned 1/2 inch strips won't work. I guess I can go to 3/8 inch plywood??
Not even sure what my question is, just talked to my lumber yard and they can order coravent for me and it's about $0.39/foot. So maybe my question is whether the coravent product itself is superior to the other brand (quarrix) listed above?
3/8" plywood has two issues if you expect to use it to support siding:
1- It's not thick enough for fasteners. 1x4s are usually what people use, and 3/4" plywood is the same thickness as a 1x4. Plywood is a better option than a 1x4 too, in my opionion.
2- 3/8" plywood is often 3 ply. If you cut it with the cuts perpindicular to the grain on the OUTER two plys, you'll get a very weak and bendy plywood strip. You have to be sure to cut PARALLEL to the grain on the outer two plys. Even if you do this, it's still weak, but it is MUCH better than if you cut it the "wrong" way.
I don't really see any particular difference between coravent and the stuff you found. HDPE is a good material too. Both of them are "plastic cardboard" as far as I'm concerned :-) They serve the same basic purpose and I'd expect them to perform similarly as long as they're protected from physical damage, and ideally from sunlight exposure as well.
Thickness for fasteners is not relevant if you can fasten to framing in the wall itself. Even 3/4" isn't considered enough to fasten to for a lot of different siding options and wind conditions. I remember from installing Hardie panel that even my 2x4 furring was technically not quite enough for common nails.
A few random points:
- You get the vast majority of the benefits of a rain-screen without venting the top of the cavity. The only reports I've heard of problems appear to be from very cold climates where some builders have seen condensation form at the eaves.
- Venting above and below windows is also something I'm not convinced adds enough to justify the increased chance of bulk water intrusion. Staggering the furring below, and leaving gaps at the sides, allows air to move around the window and maintains a continuous cavity. The siding above the head-flashing should be kept up 3/32" for drainage, which also allows some air movement.
- The other option for screening the openings is perforated J flashing. Tempting as it is I wouldn't use bug screen. It's pretty fragile and if it ever gets damaged there is no way to repair it.
+1 for not using aluminum insect screen. Use the flashing Malcolm recommends, or use a stainless steel mesh with heavier guage wires than typical bug screen. Aluminum insect screen does not hold up for long, especially in areas with certain soil conditions that rot the aluminum away (that's MOST soil conditions, from what I've seen).
As much as I don't want to be that guy, I guess I am that guy; the more appropriate term here is not batten, but furring strip. Battens almost always refer to pieces that cover gaps in siding. Furring always refers to something that creates a space or evens out the space between two layers.
Quite right. I've edited my post. I'm still fuzzy on whether they should best be described as furring or strapping though.
I don't like being "that guy" either but I do find language fascinating, including the terms we use in construction. I don't see a problem with using "batten." Here's one dictionary definition: "a long, flat strip of squared wood or metal used to hold something in place or as a fastening against a wall." Sounds like rain screen furring to me. In the northeast, the term "strapping" has a range of possible uses but I know it's not as common in the rest of the country.
While off topic, since you mention you like language, I'll give you two interesting examples. English is my wife's third language. She said she used to be confused when first learning english (in Toronto, BTW) when people would say "go get ON the bus", because she would envision sitting ON TOP of the bus, not in it. She had originally learned in her native language to get IN the bus. It's an interesting difference in convention. I think (just my opionion), that this may have originated with the first buses in North America being open seating on top, not enclosed as they are now. Back then, getting ON the bus made more literal sense.
Another is that her other two languages don't use articles, so she often wonders if she should use the word "the" or not. To a native English speaker, this seems easy -- but if you look into it (and she has), it turns out there aren't really a lot of rules about the use of the word "the". I just tell her to use "the" in front of "things", and to ask me otherwise :-)
One last thing, Russian convention is to use "AT" more where we would use "IN". So they have, for something like a concert "artist AT show blablabla" instead of "artist IN show blablabla". They also negate things a little differently, so our "cannot" is their "ne magu", literally "not can". Same meaning, but a different way to do it. The rest of their language primarily uses the English word order, although it gets a lot more complex with tense -- sentences are like word jumbles, with different versions of words giving the tense (basic example, "ya ne magu" is "I can't", literally "I not can", but "T ne mozhet" is "you can't", literally "you not can"). That's what messes me up when I speak it -- the grammar is weird :-)
Bill, syntax differences like that are funny! My wife majored in Russian and Spanish in college and has lived in Russia and various Spanish-speaking countries, picked up Czech when living there and has learned a fair amount of French and Italian along the way. Meanwhile I can barely speak English (due to a speech impediment). We have a lot of language jokes at home.
In terms of the range of meanings of "batten", the origin is from the French bâton meaning "stick", and in different contexts it can mean a wide range of long skinny wooden pieces, so this use seems valid.
In terms of how I hear it used, and how I read the definition Michael quotes, I think of it as something that is directly used to hold something down, as in the familiar phrase "batten down the hatches" which means to secure tarps over hatches in the deck of a ship with strips of wood over the edges. So to me, it feels better to use "batten" for the outer hold-down strips in board and batten siding, and use "furring" for the rainscreen spacers. But I don't think it's wrong to use "batten" beyond that scope.
Speaking of ships, I've heard it suggested that getting ON a bus (and even stranger, getting on a plane) were influenced by the fact that one traditionally boards a boat by first stepping onto the deck.
Enough of this fun diversion--I am going to get on ... with my plans for the day.
Battens are also in sails (as stiffeners). That's actually my first awareness of them.
thank you all. follow up thoughts. my hope is that ill get the furring strips aligned on top of framing (could be tricky but I can dream) so that the hardie plank siding is hanging on framing. so in that case 3/8 is adequate i think?
i really appreciate the feedback on where the rain screen detailing is more critical and where its less so.
i could use coravent at bottom of walls. i havent examined it but is my impression correct that it would keep bugs out if installed at bottom of walls? then i could do as someone suggested and detail around windows and top of walls without coravent, reducing the amount i would need.
it seems that the bigger the rain screen gap, the more complicated the detailing around doors and windows so id like to keep it 3/8 if possible and adequate. maybe the sticking point will be the maddening tedium of getting all those furring strips to land on the studs. id fasten them up with light staples and count on the pneumatic nailer to hang the siding through the strips into the framing. i think i have 2.5 inch galvanized nails for that.
Our code allows rain-screen furring to be attached to just the sheathing as long as the sheathing is 1/2" or thicker. We use 1/2" to 3/4" plywood strips, meaning your cladding fasteners have at least 1" of meat to penetrate. For some siding that may not be enough to satisfy the installation instructions, but in practice it's plenty. Inadequate backing is almost exclusively limited to situations where there is exterior insulation.
Using thin furring risks short-circuiting a couple of the chief benefits of the rain-screen gap. If the WRB doesn't lie entirely flat it can bridge the gap defeating the capillary break, or block air-flow though the cavity. There is a negligible difference in the complexity of a wall with 3/4" rather than 3/8" furring.
Cor-a-Vent or perforated flashing will keep out most bugs. Ones small enough to get through (things like sugar-ants) would make it through other spots anyway.
Thanks Malcolm, I appreciate this response. I see your point about 3/8 rainscreen being potentially compromised by wrinkles in my housewrap. Got it.
So you're saying that if I went with 1/2" or 3/4" strips I could skip worrying about landing my furring strips on the studs?
I guess I could double check with Hardie about that too.
It's the accepted practice her and there have been no issues. However, as you say, it may not meet the manufacturer's installation instructions.
It also depends on what you are doing for your wall layers. Coravent offers a batten /furring rated product but it doesn't hold a fastener for your siding. So if your looking to lay it over insulation, they don't recommend it. If you are using a product like ZipR, you can use the coravent as you will have sheathing to drive into.
I would consider coravent when running vertical siding to prevent the wall from getting too bloated.