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Hip roof venting

HN2018 | Posted in General Questions on
Hi,
Property is in San Jose, CA (Bay Area) so no snow but home has frost near eaves.  Some neighbor homes have frost too.
It has a hip roof that is a basic U-shape with two gables, 4/12 pitch (but not all the pitch are the same; some rafters I measure comes out about 4.5/12 pitch).
Roof area is about 33 squares.
There are box vents inside the U-shape as the outside of the U-shape faces the streets the home is on a corner lot.
Open eave construction with exhaust box vents and intake eave vents.
Current roof is asphalt 3-tab shingles over wood shake or wood shingles, under the wood shingles are skip sheating.
So I have started on the inside of the U-shape where there is a valley.
I am removing the old roof, fascia, repair rafter tails, repair termite damage, vacuum out old cellulose insulation, disinfect and deordorize because of rodent damage, use termite foam where there are active drywood termites, treat attic with Bora Care as a termite preventative, air seal the open part of attic, fix ducting if needed, reroute exhaust vents if needed, (add one or two ridgid Velux sun tunnels), put down new R38 fiberglass insulation batts, replace old eave vents and add new eave vents, put down 1/2 in OSB radiant barrier.
At the end of the work day, I tarped the areas and weighed them down with sand bags.
I have three sections of the roof that are opened, three sections has OSB put down, installed fascia boards.
Questions:

I am using half-dormer vents.   I have calculated the ventilation using the 150 rule.  Can I install half-dormer vents on both sides of the hip roof to increase air exhaust?  If I do, will that cause a short circuit in the venting?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    HN2018,
    First of all, can you tell us your name? (I'm Martin.)

    I'm guessing that this is a vented unconditioned attic with insulation on the floor of the attic. It also sounds as if there are HVAC ducts in the attic -- a situation which is less than ideal, but which is unfortunately common.

    Here's the deal: In your mild climate, as long as your ceiling is airtight, you don't have to worry too much about how much venting your attic gets, because your climate isn't cold enough for major problems (in spite of your mention of frost).

    For more information on the research that supports my statement, see this article: "All About Attic Venting."

    So you can install half-dormer vents if you want (I didn't know what they were, but I Googled the term, and saw the images), or not. The number of vents isn't very important.

    The key to everything is air sealing. Spend most of your time performing air sealing work at the ceiling plane, and almost everything else will take care of itself. For more information, see "Air Sealing an Attic."

  2. HN2018 | | #2

    Hi Martin,

    My name is Hoang and thank you for your reply.

    Sorry, I believe the vent is called half-round dormer vent or one supplier listed it as eyebrow vent.

    And yes, there are HVAC ducts in attic and insulation on attic floor.

    Also thank you for the link to your article.

    It was on this site that I learned about air sealing. There are also recessed can lights (some ICAT and some IC) through out the house. I have retrofitted some with LED lights and also air seal them with either foam spray or foam box or both.

    Most likely I will use the half-round dormer vents and add more intake eave vents.

    Can I install some dormer vents on opposite sides of the hip? Will that cause a short circuit in the ventilation?

    Thank you for your time.

  3. GBA Editor
    Martin Holladay | | #3

    Hoang,
    I wasn't implying that these vents aren't called half-dormer vents -- as far as I know, they are. (I was just unfamiliar with the term.)

    Put in a few vents -- it doesn't really matter where you put them, frankly. Just make sure that you understand basic flashing principles, so that the vents don't leak when it rains.

    As I said in my first response, attic ventilation in your climate isn't very important. Attics with very few vents work. So do attics with lots of vents. Just make sure your ceiling is as airtight as possible.

  4. HN2018 | | #4

    Great, thank you.

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