Ridge vent in an unvented roof
quantumgirl
| Posted in General Questions on
About three years ago we switched from a traditional vented roof with soffit and ridge vents to an unvented roof design. We had open cell spray foam blown into the entire roof right underneath the roof sheathing. All the vents were closed up. Attics and crawl spaces become part of the conditioned living space.
now it’s time to replace the shingles and the roofer tells me I need a ridge vent. Is he mistaken? What good would a ridge vent do if spray foam is air impermeable? He claims to have 30 years of experience but unvented roofs are pretty much unheard of down here in south east Texas (hot humid climate) so I am doubting him.
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Replies
> spray foam is air impermeable
It's air impermeable but very vapor permeable. While it meets code, some have seen moisture problems with unvented open cell foam.
But how would a ridge vent help? What would it do? I don’t have an attic. It’s all conditioned space. What would it even vent?
What would it vent? Vapor?
https://www.buildingscience.com/documents/insights/bsi-088-venting-vapor
Stewart
Yes, vapor. The cited article is exactly the right one to address this question, though I doubt the roofer knows it. Whether he knows it or not, the 2018 IRC allows (requires) the use of vapor diffusion venting for unvented roofs using vapor-permeable insulation. This is a brand new concept in roof ventilation, thanks to Joe L.
The OP doesn't need a fully open traditional ridge vent. He needs a covered vapor diffusion vent per these guidelines, or the prescriptive requirements of the IRC.
Would he also need a vapor diffusion vent for closed-cell foam insulation? I know closed cell is a vapor barrier, but is there still a risk of moisture getting to the roofline via incomplete air sealing?
And would the potential for an exterior water intrusion justify a vapor diffusion vent where the interior has closed cell spray foam? (Not thinking of a big, permanent leak, but maybe a hurricane or high winds that push some water uphill through some cracks.) Seems like the vapor diffusion vent might give the water somewhere to go.
Nola,
I wonder about that too. I understand the intent, but the way BSC has had them detail the framing and shingled ridge is an absolute mess.
Malcolm,
I agree that the cited article is somewhat of a mess, but that's because they were playing with the concept. I'm pretty sure they settled down to saying that you can detail it exactly like a standard ridge vent, just cover the openings in the sheathing with Tyvek or other waterproof membrane with a permeability of greater than 20 perms.
Note sure how much this would help with incidental wetting of the sheathing, but it probably wouldn't hurt. These are not necessary with closed cell foam because there is so little vapor diffusion through the foam.
I'd measure/inspect what's happening with your current roof and make no changes (including type of underlayment) if you don't have a problem (you probably don't).
If you want a vent, you need to build one above the sheathing. I'd make it for air since vapor diffusion ports are only code approved for air permeable insulation. See R806.5.5.2:
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
Jon can you please explain what you mean by building a vent over the sheathing? I’m familiar with traditional ridge vents and I read up on the vapor diffusion vents for air permeable insulation but how would a vent over the sheathing look like and function?
For that I would enter "vented roof" in the search box in the upper right.
https://inspectapedia.com/ventilation/SIP-Roof-Vent-Details-383-Bob.jpg
Quantumgirl,
An un-vented roof with open cell foam may allow some moisture to make its way to the sheathing. Stewart and Peter suggest that the solution to this is to include a vapour-diffusion vent at the ridge to allow this moisture to dissipate. Jon thinks they are only useful when the insulation is air-permeable, not vapour-permeable, and instead thinks venting above the existing roof sheathing would be more useful. To do this you would strap the roof, provide soffit and ridge vents, and add another layer of sheathing before re-shingling.
Thank you for the explanation!
If moisture does make its way to the roof sheathing, would it not dry out through the open cell foam towards the interior side though, instead of escaping through any kind of ridge vent to the outside? I am in Houston so most the time it will be cold and dry on the inside of the house vs hot and humid on the outside. I assumed moisture always moves towards the drier side?
Am i correct in thinking that moisture traveling through the open cell foam to my roof sheathing and creating problems there would only be an issue during the few winter days where it’s necessary to run the heater?
Quantumgirl,
I agree with Jon's post #8. In the absence of any evidence you roof has moisture problems I'd leave it well alone. You will be able to see how the sheathing is doing when the roof is stripped. If there is any moisture damage visible near the ridge it may be worth installing a vapour diffusion vent, but my hunch is you don't need it - and certainly don't need a ridge vent.
A standard ridge vent is going to do nothing useful for this roof. A vapor diffusion vent has at LEAST one or two orders of magnitude more area than standard ridge vent, and would require removing enough roof sheathing the ridge to have enough to matter (or meet code):
https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports
See R806.5, section 5.2 (as Jon R already cited):
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
The min-specs are:
•port area 1:600 of the ceiling area
• vapor permeance greater than 20 perms
• roof slope greater than 3:12
• air supply 50 cfm/1000 ft2 ceiling area
• insulation installed directly under the roof
deck
Got all that?