September 9, 2016

Vapor Venting An Unvented Roof: In Praise of Belts and Suspenders

"Classic Ridge Rot" documented by the Building Science Corporation in Alaska

“Classic Ridge Rot” documented by the Building Science Corporation in Alaska

Planes can fly safely with one engine. And you can keep your pants up with just a belt. But if you’re looking to be protected from fools, then adding suspenders to the belt might be what sets you free. So while we’ve previously outlined the essential importance of inboard airtightness and vapor control and have specifically written on The 8 Golden Rules for Foam-Free Unvented Asphalt Shingled Roofs – you’re still looking for that vent. And admittedly, we really should vent all our walls and roofs on the exterior as described in our blog post A high performance roof should be vented – how to do this properly, for maximum protection. Yet sometimes venting isn’t in the cards – due to cost or existing conditions for example …and you’re still looking for that vent.

Interestingly, as Joe Lstiburek of the Building Science Corporation has pointed out in BSI-088: Venting Vapor, the moisture damage found in unvented roofs is often at the ridges – using SIP panels in Alaska, and cellulose roof in Northern California in his Building America Report 1511 with Kohta Ueno, as illustrations.

Again, tested and verified inboard airtightness with smart vapor control, following the 8 Golden Rules, will eliminate this problem. But when you’re still looking for the suspenders, Joe identifies a great set: the ridge vapor vent.

ridge vent cad dwg

Ridge Vapor Vent (adapted from BSC)

Like draining an infection, the ridge vapor vent is a release valve at the point of potential highest moisture concentration.   The ridge vapor vent is formed by holding/cutting back the vapor retarding OSB/plywd sheathing and replacing it with vapor open GUTEX Multiplex or exterior grade gypsum board.  Over the GWB provide vapor open waterproof and airtight SOLITEX Mento 1000.   Then install the ridge roofing on furring strips – forming a mini back vented roof.

Also Read:

Make Better Choices: Toward A Less Toxic World

Is Foam Evil? A New Paradigm of Foam: Less is Best

The Ten Golden Rules for Foam-Free Flat Roofs

New 475 eBook: High Performance 2x Framing

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3 Responses to Vapor Venting An Unvented Roof: In Praise of Belts and Suspenders

  1. calebcrawford September 15, 2016 at 9:07 am #

    I would add an insect screen to the edges of the vent, and a metal drip edge to the ends of the OSB cap.

  2. Andy Shapiro September 28, 2016 at 10:07 am #

    From Building Science Corp BSI-088 on this approach:

    “Can this work for a cathedral ceiling yes. Check out Figure 8. Does this work everywhere? Sure about DOE/IECCClimate Zones 1, 2 and 3 (Figure 9). Pretty sure about Climate Zone 4, but need more work to be sure. Climate Zone 5 and 6 need a lot more work.”

    Some caveats needed here?

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