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Advice on hip roof venting and underlayment choice

be_wrj | Posted in General Questions on

Zone 6A
Ranch house 50′ x 30′
Hip roof 6/12 pitch
Attic has been air sealed and insulated. Brief details:
All existing fiberglass batting removed.
Air sealing completed.
The existing 2×8 rafters were birdsmouth cut at the top plate leaving about 4″ between the sheathing and top plate. Not enough room for soffit ventilation and proper air sealing and insulation in this critical area. So closed-cell spray foam was installed in lifts building up to 9″ thick.
18″ of cellulose installed elsewhere.

Time now for reroofing before adding PV.
The existing roof has a ridge vent and two “mushroom” vents not far from the ridge. The mushroom vents will go away. The ridge vent will be replaced during reroofing.

I’m thinking that adding ridge vents along the hip rafters will allow some convection and help keep the attic temperature down. The entire length of the 4 hip rafters wouldn’t need to be vented (sheathing cut into), just the lower portions. Enough to pressurize the attic, so slightly more vent area on the lower hips than at the ridge. Of course the ridge vent cap would need to be installed the full length of the hip in order to maintain proper roof flashing details.

At the eaves where there is spray foam, I’m thinking that a vapor permeable roof underlayment is more appropriate here than a traditional vapor impermeable underlayment (ex. ice and water shield) because the risk of ice dams is low and if water does get to the sheathing the thickness of the spray foam at the eaves will greatly limit drying to the inside. So the entire roof could have a vapor permeable underlayment.

I appreciate thoughts, comments, advice, etc.


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  1. GBA Editor
    Martin Holladay | | #1

    You never explained where you installed the 18 inches of cellulose -- up against the roof sheathing or on the attic floor. I'm going to guess that you installed it on the attic floor -- in effect, creating an unvented unconditioned attic (which, as you may know, is a code violation).

    The standard code formula requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.

    These code requirements may or may not concern you. It's possible that your attic will perform perfectly well without any ventilation. The two main worries are damp roof sheathing (which is best avoided by limiting air leaks at your ceiling) and ice dams (which could start near your eaves, where you have only R-24 insulation).

    If price were no object, one way to solve your two problems (insufficient insulation at the eaves, and the risk of ice dams) would be to install R-25 of rigid foam above your existing roof sheathing, followed by another layer of roof sheathing and new roofing. This would create a great roof, but it would be expensive (and it's probably overkill). For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

    Adding various types of roof vents -- either hip vents or mushroom vents, for example -- could help you get close to code requirements for venting. However, hip vents sometimes allow snow to enter an attic, and venting does little to reduce your two worries (damp roof sheathing and ice dams).

    For more information on why attic venting isn't effective at addressing attic moisture problems or ice dams, see All About Attic Venting.

    If this were my house, and I couldn't afford to install rigid foam above the roof sheathing, I would probably just install a ridge vent at the top of the roof and keep an eye on what happens (assuming that there is no local code official who is insisting on meeting code requirements for venting).

    You still need Ice & Water Shield at the eaves, for two reasons: it's a code requirement, and that area is vulnerable to ice dams.

  2. be_wrj | | #2


    Many thanks for your thorough response, much appreciated.

    You are correct, 18" of cellulose was installed on the attic floor. I see now that my use of the word "elsewhere" is unclear, it should read "on the attic floor" and I would edit my original post it but I don't see how.

    I'm not concerned about meeting ventilation code requirements in this case (my own house). I considered making the attic an unvented conditioned space through the exterior rigid foam method you describe as well as other methods, and also considered ways to provide soffit ventilation, but the costs and complications did not justify the benefits in my experience. As you mention and reference in your articles, properly air sealing the attic floor and addressing basement moisture issues are top priority, and both have been accomplished here.

    Your point about hip vents allowing blowing snow to enter the attic is well taken. I will probably just maintain the venting at the ridge and keep an eye on things. My experience suggests the attic will perform much better than when it was not well sealed and only had 7" of fiberglass, and even then there were no ice dam or attic moisture issues.

    Thanks again.

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