Rainscreen Venting
miroland
| Posted in Green Building Techniques on
I think I read in a recent string that rainscreen venting at the bottom, top, and around windows isn’t necessary because the siding will let air in and allow drying of the minimal amount of rain that will make it through. Is this correct, or did I misread?
Michael Roland
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Replies
Michael,
Some people install a rainscreen without venting, and these installations usually work fine. However, if you include screened vent openings at the top and bottom, research shows that damp siding will dry faster than if the rainscreen gaps are not vented.
Michael,
Welcome to Building Science. As with many issues, it depends. Some siding installation instructions do not recommend vented screens; in some areas of the country where fire hazard is high, vented screens are not allowed; and in some other areas of high humidity (coastal or SE), ventilated screens in walls and roof assemblies can bring more humidity in, where it may add problems as well.
Armando, I don't understand your comment about ventilated screens in walls in high humidity areas bringing in more humidity. How would the typical exterior wall keep humidity off the housewrap? Siding's purpose is to shed rain, but it isn't an air barrier or vapor retarder. Can you explain what you meant?
In our climate (mixed humid SE) I would recommend always including the vented rainscreen when the siding is wood and it's installed over insulated sheathing, especially foil-faced. I've seen ten-year old cedar siding cooked to a crisp when installed that way, it comes off in shards. It'd also be wise for any siding in a location regularly exposed to severe wind-blown rain. Otherwise I'd consider the rainscreen optional. Armando's point may be that it's a bad idea to vent the top of the cavity into the roof, and I'd agree. If the roofspace is vented via the soffit boxing a separate vent is needed for the rainscreen at the top of the siding.