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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