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Community and Q&A

Roof Ventilation Conundrum

sdaboulder | Posted in Energy Efficiency and Durability on

I have a project with a simple cruciform plan consisting of two gable forms. One of the gables is a cathedral ceiling at 4 1/2 in 12 pitch. The other gable is a scissors truss (same slope top chord, 2 1/2 in 12 bottom chord) which over-frames on top of the cathedral ceiling. This is in climate zone 5, but in Boulder, Colorado which is classified as a dry climate. I’d like to ventilate the roof, but over half of the conditioned floor area falls within the intersection of the two gable forms, which means soffit vents are not possible. My concern with a ventilated roof is that this center section won’t perform well, so I’m tempted to use an unventilated roof with 2-3″ of closed cell up against the roof sheathing and R-30 batts or blown cellulose below. Cost is a major issue, so standard framing solutions are preferred and rigid board insulation is too expensive. I know some asphalt shingle manufacturers won’t warranty installation on hot roofs, but Certainteed appears to. The basic question is whether a hot roof is a better choice when sufficient attic/cathedral ceiling ventilation is questionable. BSC seems to say yes. Any other thoughts out there? Thanks.

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  1. wjrobinson | | #1

    I just did a home a few years back with open cell Icynene unvented in the Adirondacks here in NY. All is fantastic. The home has great Southern exposure, the furnaces stay off all day and sometimes hours into the evening. The homes we built in the 80's never had that kind of performance. The foam encapsulated the trusses which is good and one other thing, my frame was up for quite awhile prior to spraying along with the entire build process not rushed. I am wary of spray foam if used in a fast build process or in homes as a retrofit. Let the off gassing take place without occupants if you have to use spray foam. I like unvented roofs assemblies or as BSC details... building exterior venting above sheet foam placed to the exterior. Interior standard vent chutes and fiberglass should never be done anymore. I won't do it anyway.

  2. jklingel | | #2

    AJ: "Interior standard vent chutes and fiberglass should never be done anymore." No complaint about the fiberglass part, but what is wrong w/ the vent chutes? Isn't that a simpler, yet effective method? Thanks. john

  3. Riversong | | #3

    I would never say that a hot roof is a better choice, but occasionally it's the only practical choice because of overly complicated roof planes. But if there is enough eave at each of the gable extensions to provide sufficient intake venting, and continuous ridge vents with external wind baffles, then it should be possible to ventilate the roof (though it would require baffling in the stick built sections).

    If you go with the hot roof, I would suggest using light colored shingles, though 4½:12 is a pretty low slope for composite shingles. Alternatively , use a partially self-venting ribbed metal roofing and a permeable roofing underlayment such as #30 felt.

  4. wjrobinson | | #4

    John... where I live you can drive around the streets and view thousands of installs of fiberglass with vent chutes. Almost every roof tells me that it is letting out the heat in a huge way. The home I mentioned above is built near unheated cabins. It and the unheated cabins have full roofs of unmelted snow on them with not an icicle to be found nor any patterns of heat showing in the roof snow. To me that is as simple as it gets to know that my Icynene roof is way outperforming the standard. And it shows in the propane use, in the times that the furnaces run, it shows in all respects.

    And as to shingle temperatures... I was concerned before and after spec-ing the unvented roof. So I checked into it... and found in mid summer that the roof temp was 5 degrees higher on the unvented roof verses the porch roofs. And I did use Certainteed shingles. The shingles look brand new to this day and I worry not about shingle temp anymore. By the way the shingles are black as that is what the owners wanted. Lighter shingles and cooler roofs are a plus in cooling climates along with a nice large Magnolia tree for shade with a swing coming from a branch.

  5. wjrobinson | | #5

    Robert... I was about to give you a plus... then I see you dinged me a minus... my answer post is a beaut buddy. Let's trade pluses... I'll start.

  6. sdaboulder | | #6

    AJ... My understanding is that Icynene is not an adequate vapor retarder and therefore is not code approved for use in unvented roof assemblies. Robert... I should have mentioned that the cathedral gable has a foot higher plate. The result is that at something like 10 rafter bays, the bottom of the rafter is essentially within conditioned space with no access to outside air. The only option for intake in these locations is something like the Cor-A-Vent In-vent. I'm not crazy about that solution because it looks vulnerable to water intrusion, especially given that these would be in close proximity to valleys. Thanks for your feedback.

  7. GBA Editor
    Martin Holladay | | #7

    It's possible to use Icynene or other open-cell spray foams in an uninsulated roof assembly, as long as you remember that, in a cold climate, it's necessary to spray vapor-retarder paint on the interior side of the cured foam.

  8. wjrobinson | | #8

    Northeast Sprayfoam did my last project and sprayed a latex vapor retarder on the foam. They and I met with code officials and received permission. Not sure what is what out your way but my guy Doug may know someone in the business out your way that he would recommend. Make sure you go with a foam company with lots of quality work under their belt. Go check sites for happy clientele and do get insurance certificate along with a long list of satisfied customers. Ask their supplier if they have any unresolved complaints. Spray foam is tricky. Done right I like it.

    Oh and last I spoke to Doug they have an intermediate weight foam and a closed cell foam, the two lower weight foams are preferred due to being water blown.

    Another point... an inside point... there are better guns, better weather to spray in, and every crew has better guys with more experience than the new guy. My company has switched to better equipment over the years.

  9. wjrobinson | | #9

    Thank you Martin, I didn't know if things had changed since my last use of it.

  10. sdaboulder | | #10


    Doesn't that vapor retarder on the interior side of the foam prevent the cavity from drying to the inside? I thought one defense against imperfections in the assembly that might allow moisture intrusion from either the interior or exterior was to allow drying of the cavity to the inside. Maybe the Icynene is such a good air barrier that this isn't a concern?

  11. GBA Editor
    Martin Holladay | | #11

    In a cold climate, the vapor drive during the heating season is from the interior to the exterior. That's what it's a good idea to apply vapor-retarder paint to the cured foam.

    Fortunately, vapor-retarder paint is not a vapor barrier, and some drying to the interior is still possible.

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