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Vented Rain Screens & The Heat Rises Rumor

Will Tinkelenberg | Posted in General Questions on

Okay, I know heat doesn’t rise. Hot air does. So, imagine if you will, a wall with a vented rain screen, above it an unvented “hot” roof (such that for hot air in the eave there is no escape up under the roof), and an eave with a continuous soffit vent. The hot air in the vented rain screen will rise up into the eave. What’s to make that air drop down through the soffit vent, when the air outside the eave is colder and denser?

Thanks, Will Tinkelenberg

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Will,
    Q. "What's to make that air drop down through the soffit vent, when the air outside the eave is colder and denser?"

    A. Wind, for one thing. For another: if warm air from a rainscreen gap enters a soffit, then an equal volume of air must get pushed out of the soffit -- so there will necessarily be some air exchange.

  2. Will Tinkelenberg | | #2

    Wind, yes. The question is probably mostly academic, because it seems even if a vented rain screen has no vent at the top, and only the vent at the bottom, it will allow for pressure equalization on either side of the siding. And, even without a vent, wind will still facilitate drying. So... while I understand that the warm air in the rain screen gap wants to rise and displace the air in the soffit, can it? It seems the warm air in the soffit, especially if it's directly under the roofing, might not be cooling off. Also, there's likely a lot of it, vs the amount in the narrow rain screen gap. I wonder if the air in the gap can actually push the greater amount of warmer air in the soffit down and out against the cooler air below the soffit...

    I wonder if a vent at the top of a rain screen is even necessary at all? It certainly complicates detailing, and potentially adds an unsightly reveal. In my limited experience with detailing masonry veneers, I don't recall any kind of dedicated venting for the airspace; just the weep holes at the bottom necessary for drainage. If we accept that siding, such as clapboard siding, is not air-tight, maybe only the "drainage slot" at the bottom of a "vented" rain screen is all that's necessary...

  3. Will Tinkelenberg | | #3

    Will a drained rain screen work in lieu of a vented rain screen? What I mean by "drained rain screen" is a system that includes a drainage slot at the bottom, but no vent at the top. Not unlike a masonry veneer system, which only has weep holes for drainage of the air space. Thanks, Will

  4. GBA Editor
    Martin Holladay | | #4

    Will,
    If the rainscreen only has openings at the bottom, it will still effectively provide most of the benefits of a rainscreen.

    However, if the sheathing ever gets wet, a rainscreen with openings at the top and the bottom will help the sheathing dry faster than a rainscreen with only openings at the bottom -- because the rate of airflow will be higher in the rainscreen with openings at the top and bottom.

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