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How to ventilate a hip roof

user-1127400 | Posted in Energy Efficiency and Durability on

We are going to be replacing the roof on our house as part of a project to make the house more energy efficient as well as an addition and remodel. I live in NJ – a little bit north of central, so we have pretty good swings in climate between summer and winter. The house has a shallow hip roof, and the house is 42′ wide and 26′ deep. There are nice large overhangs at the eaves, and we will be putting in lots of venting there, but the question is what to do at the top. There is an old existing power fan, two passive vents and a ridge vent. My guess is that the ridge vent is not very effective on it’s own due to it’s small size and the pitch of the roof, but I also think that the power fan is probably being short-circuited in it’s effectiveness because of all the upper passive vents.

The sheathing in the attic is all molded because a number of years ago, someone closed up the vents at the eaves. We’re thinking about replacing the sheathing with ZipSystem roof sheathing. We’re going to do flash and batt in the attic – about 2″ of closed-cell foam and then R38 of batts on top.

I’ve attached a picture so you can see the roof. Should I leave the fan in and close up the other vents, or just do passive venting? What’s the best strategy?


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  1. user-1127400 | | #1

    I should add that there is, and will be no HVAC equipment placed in the attic, nor is there anything stored up there.

  2. GBA Editor
    Martin Holladay | | #2

    The reason that you have mold in your attic is not related to the soffit ventilation. (Attics tend to be very dry, unless the roofing is leaking.) The cause of the mold is almost undoubtedly air leaks through your ceiling. It's also possible that you have open chases that connect the attic to a damp basement or crawl space.

    If your contractor does a good job of air sealing your ceiling, you won't have to worry about ventilation. I certainly advise against the use of a powered attic ventilator (a fan).

  3. wjrobinson | | #3

    Joe L. says soffit vents are good and that smaller ridge vents are fine. Your insulation will due you well, make sure they have done the same thing dozens of times and make sure their best spray guys do your project not a new guy. Weather is critical. Ask them if they adjust for temperature as the day progresses. Ask them if they sample foam and during the spray. Make sure they don't spray near your ceiling joists over 2" as the cure goes bad, smells etc. There really is an art to spraying correctly and it is real easy to be sloppy and spray problems that will never go away. Thickness is very very important, thin is better than thick per spray.

    Next, you need to make sure your windows and doors and cellar rim joist are sealed too at the same time.

    Lastly you need to blower door test and possibly add some Panasonic bath fans that run upon all bathroom uses and run via automated timers for 20 minutes after normal shower times, so say 30 minutes. They use zip energy and make no noise hardly anyways. Nice fans.

    There is more to it than this too. Depends on how far you want to go.

    The Zip on the roof, remember is great externally, but is still OSB on the side you had the mold. High quality plywood may be better though not necessarily needed. Put on a good roof, membrane around an up the sides of skylights, continue it above and below skylights, false chimneys need proper membrane and flashing, 2 runs along eaves or more if your eaves are very wide.

  4. user-1127400 | | #4


    Yes, you are correct - the main cause of the mold is leakage into the attic. We stuck our head up there in Feb, and there was water dripping from the roof nails. Aside from poor insulation and no air barrier, there are at least nine old pot-lights that are not insulation contact rated which are a large contributor, and a non-insulated attic hatch. I'd get rid of the hatch, but with a hip roof, I don't have any other choice for access, so we'll insulate and weather-strip it.

    There's a whole-house humidifier that was being run at a fairly high setting to compensate for the poor envelope of the house, so after all the work we plan on doing, that should be throttled back or eliminated which will also help.

    The plan is to replace the lights with sealed, insulation contact rated units, remove the old, inadequate and soiled fiberglass, vacuum and clean, and then do a flash coat of closed cell foam of .5-1", followed by at least R38 of fiberglass batts. The old cedar shake siding and tar paper on the entire house is going to be removed and then we are sheathing over the old sheathing with ZipSystem R6 panels. This should improve the efficiency of the existing insulation in the walls without disrupting the entire living space (we're going to be living in this thing during construction).

    We'll remove the powered fans, and I'll assume that with the improved venting at the soffits, that the ridge vent will perform adequately, and the improved air sealing and insulation will make this a cozy house year-round.


  5. GBA Editor
    Martin Holladay | | #5

    That sounds like every building performance contractor's nightmare: a house with a leaky ceiling, plenty of recessed can lights, and a humidifier running full speed all winter long. That's a recipe for attic problems, for sure.

    You're on the right track to fix your problems. Put the humidifier on your front lawn -- with a cardboard sign on it that says, "Free."

  6. user-1127400 | | #6

    Thanks for the tips AJ.

    The insulation contractor is one of the most experienced we could find here in central/north NJ. All the existing windows are being removed and re-installed to be flush with the new ZipSystem R6 panels we're installing, so they will get good attention. We have pretty deep/wide eaves, which I find help shade the upstairs windows very nicely, so we were planning on two rows of ice and water shield membrane already.

    We'll also pay attention to the rim/sill. There will be a foam sill gasket, and we'll caulk the rim joist very well. We already spec'ed the Panasonic fans. We're going with the in-line ducted FV-20NLF1 240 CFM unit to handle our two upstairs bathrooms.


  7. wjrobinson | | #7

    Igloo coolers glued over the lights, switch to CFLs. Spray foam over the igloos. Humidifier on the lawn or better yet toss into a lava lake (see you tube).

    More, I use these baffles to retrofit recessed lights to sealed with a CFL and the igloo inexpensive foam coolers. The cooler idea is not mine... thanks GBA for this discussion area.

  8. roofnron | | #8

    Using your numbers of (42x26) / 300, then divided by 2 = 1.82 sqft * 144 = 262 sq inches exhaust ventilation.

    It appears you can get real close to achieving the proper exhaust ventilation with the ridge you have for the upper roof. One type of shingle over ridge vent provides 18 sq inches per foot. You would only need 15 foot.

    Or lets say you don't like ridge vents, you can get basic static vents that provide 65 sq inches each which means you would need only 4 of them. Point is this is not even close to a roof where a power fan would be necessary.

    You need to make sure that you provide more intake then exhaust. The roofer can very easily provide continuous intake at the eave without having to dig into the soffit if you prefer.

    Using the insulation technique you are describing I would still suggest you keep the attic properly ventilated. The picture is of CDX plywood decking on a mansard with no ventilation (intake or exhaust), outside of the building envelope. The owner says the shingles were put on 11 years ago. The plywood is "dry-rotting" on the sun sides of the building. I realize OSB may not be as prone to this damage but all these materials contain some moisture.

    I would also make sure the bath fan is piped out of the roof through a proper dampered vent. Don't let them hang a hose in the soffit or in front of a static exhaust roof vent.

    edited to correct some typos

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