More Q&A Spotlight
A GBA reader named Matt2021 has a building project going in New Jersey where every fraction of an inch counts. So when it comes to detailing a vented rainscreen on the exterior, he’s wondering just how small he can make the gap between the sheathing and the siding and still have the rainscreen do its job.
“On this forum, I’ve often read that, ideally at least, a rainscreen should consist of a gap no less than a 1/2 in. deep,” Matt writes in this post in the Q&A forum. “This video calls that a ‘myth,’ and claims that 1/4 in. is the minimum, that only a gap less than 1/4 in. would be too small.”
“This video” is a YouTube explainer posted on the Build Show Network by Jake Bruton of Aarow Building. In it, Bruton offers some tips on installing rainscreens. One of them is that a gap of between 1/4 in. and 3/8 in. is enough. Another is that insect screening to keep bugs out of the air space is unnecessary as long as the gap is fully ventilated. The stack effect powering upward air movement behind the siding, he says, is sufficient to keep insects out.
“Personally, I don’t buy his argument,” Matt says of Bruton’s insect-screen claims. “Not adding a barrier seems too risky to me.”
So we have two issues in this Q&A Spotlight: what’s the minimum gap for an effective rainscreen, and will bugs really be thwarted by air currents?
Rainscreens do two things
Jollygreenshortguy points out that rainscreens have two purposes: they provide a capillary break so that rain water is not wicked into the building, and they provide ventilation so that moisture from either the inside or outside has a means to dissipate.
“In many situations (for example…
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11 Comments
If you do decide to ventilate (as opposed to vent) a rain-screen, the most vulnerable point for bulk water intrusion is under the windows. Considering that this area is already usually open to the sill-pan behind, to me it makes more sense to not introduce an opening in the cladding there, but rather keep the cavity furring down from the sill so you still have a clear ventilated channel moving air around the window, and exhaust it at the top of the wall where an opening is better protected.
Great discussion! We are hearing requests for more use of sheathing/adhered insulation panel, like ZipWalls R-sheathing. The siding installation instructions should be reviewed as the manufacturer may dictate how thick the furring needs to be to provide enough nailing depth to meet the siding warranty. We've found that 3/4" furring fastened thru Zipwall's R-12 sheathing, directly into the studs, does provide enough thickness that you do not have to use fasteners long enough to fasten the siding into the studs, based on a few manufacturers. Always review the siding manufacturers requirements for fastening depth, especially when using R-sheathing like product.
Thanks for this article. I discovered years ago that many cladding product manufactures (e.g., fiber cement siding) require a minimum 3/8" gap to warranty their products. They also have a lot of related recommendations about this relative to use in (low risk) single-family residential verses multi- or commercial, proximity to a body of water, climate zone, and cladding height.
We design all our HVAC systems under slight positive pressure in our area of the country. Positive pressure helps eliminate insects and moisture issues.
synergytodd,,
I'm curious how positive pressure would help with moisture issues. Wouldn't it drive more moisture into the walls from the interior?
Heads up; here comes a long comment!
Notable JLC article says vinyl siding walls NEED a proper rain screen.
A notable article in the July 2022 issue caught my attention. It is titled: “Installing PVC type sidings in cold climates. Beware of impermeable sidings without vented rainscreens”. It is written by George Tsongas who is a “consulting engineer, building scientist with specialization in moisture problems in buildings, and professor emeritus of mechanical engineering at Portland State University (OR)”.
Based on his work of studying building envelope failure in the field, Tsongas finds that vapor migrating from the inside to the outside of a house can cause serious damage (sheathing mold and rot) in houses covered in vinyl siding and he has explicit pictures to show for it. He has observed significant issues in Wisconsin and the Pacific NorthWest. According to the article, vinyl siding doesn’t allow for much drying of the sheathing behind it. This goes against some of the advice I have read over the years (and relied on) including today in GBA. The article mentions vapor diffusion and air leak as troublesome sources of water vapor accumulation but doesn’t go into details about which one may be most responsible for the issue -several GBA articles have attributed the vast majority of issues caused by water vapor issues to buildings air leaks.
Tsongas makes a distinction between single and multifamily with greater issues found in multi given the lesser amount of wall per sqft of living space. Also worth noting; “Most damage in walls has been found to be in walls adjacent to bedrooms where relative humidities are typically higher than in other locations”. Tsongas found that "vapor-permeable fiber-cement" is less prone to problems but still seems to advocate for a rain screen in all circumstances. There is an interesting quote about drainable house wraps; “Many contractors are now selecting crinkled WRB or other “drainable” WRB’s, but that is not sufficient to prevent damage in walls with impermeable siding”.
By contrast, it’s been a common tread in GBA articles I’ve read that vinyl siding doesn’t require a rainscreen, summed up in statements like this one by Michael Maines (see link below); “Standard vinyl siding is essentially its own rainscreen because the back is open, which minimizes capillary action, and the panels fit together loosely enough to allow for some airflow, so an additional rainscreen is not necessary.” Not assigning blame; so far it just seemed like sensible advice to me as well.
There are also points of agreement; Tsongas' favorable opinion of continuous exterior insulation matches what's regularly written in GBA.
The primary advantage of vinyl is its low cost of installation and low maintenance. Continuous exterior insulation and/or a properly detailed rainscreen can easily adds $thousands to any new construction project. Replacing failed sheathing and siding on a house easily gets in the $tens of thousands. And then there is the significant environmental impact of disposing prematurely of tons of building materials and replacing it with new sheathing and new siding.
Vinyl is not the preferred siding material of most GBA readers but the reality is that it's the default siding of affordable housing and it's being used in a quarter of new single-family homes nationwide. It is likely even more prevalent in cold climates and I’ve never seen a vinyl siding install involving the use of a rainscreen.
We may not have a final consensus on this topic yet.
Link to Tsongas's JLC article -it may or may not require subscription
https://lsc-pagepro.mydigitalpublication.com/publication/?m=23705&i=751715&p=48&ver=html5
Some of the previous GBA article covering the topic
https://www.greenbuildingadvisor.com/article/understanding-rainscreens
https://www.greenbuildingadvisor.com/article/all-about-rainscreens
I am surprised to read that contractors are using plywood for furring strips, as mentioned in this article above, "3/8-in. furring for the rainscreen (ripped strips of 3/8-in. plywood, for instance)." There likely won't be lots of water behind the cladding, but to my understanding plywood is so much more likely to swell, deteriorate, and rot if it experiences moisture. I'm sure you all are using exterior glue ply, but I wouldn't even use marine ply, untreated, in a ventilated cavity as furring. Am I missing something? I am working in Michigan where there is plenty of rain, and plenty of freezing weather all winter.
jjmc,
Rain-screen cavities have been mandated in our building code here in the marine climate of coastal BC for almost two decades. All lumber yards sell here them in a several thicknesses of plywood, either treated or not.
Neither show any deterioration when you renovate the walls later on. I think this is because the assembly is by it's nature designed to dry things - including the furring strips that form it.
That's remarkable-- and good long-term experience. Thanks Malcolm. The yards there sell 1 1/2" or wider precut plywood furring strips?
Jerry
Jerry,
Most stock 2" wide strips in 1/2" 5/8" and 3/4". The real beauty is they also have matching perforated J flashing for the bottom.
I wonder that would happen a section of wall experienced a long term leak and an area of the plywood did deteriorate? Given it's really just a spacer, I doubt there would be much effect.
A photo of both being used on a renovation I did last summer
When I worked for a Swiss manufacturer of premium H/ERVs I had a few instances of wasps establishing a nest in the intake or exhaust duct of the system. The vermin screen behind the louvered grilles is fine enough to prevent rodent entry, but not always entry from smaller insects. I am certain the airflow in the ducts is greater than the airflow in a ventilated cavity, so I'd never, ever omit a bug screen in a ventilated drainage cavity.
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