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Should I vent an unconditioned roof?

Brad_S | Posted in General Questions on

Home Built: 1915
Climate Zone: Marine 4 (Seattle)
Unconditioned Attic, no ductwork

Roof Joists: 2×4
Sheathing: OSB
Pitch: 9/12
Reroofed: Not sure, maybe 20 years ago, asphalt shingles

Ceiling Joists: 2×4
Ceiling Type: Flat, no vaults.
Square Feet: 900
Existing Insulation: Cedar roof shingles thrown on top of fiberglass bat.

Venting at Eaves: None. Zero. Zip.
Venting on Roof: 4 black roof vents (galvanized steel?)

Future Insulation: R-49 or R-60 blown (cellulose or fiberglass)
Future Modification: Whole house fan. (Maybe?)

I intend to insulate my ceiling with either blown cellulose or fiberglass. Watched the advertisements for each and remain confused. Code in my region is R-49 but not sure if R-60 would be worth the extra cost as long as I’m already doing the job.

At present, I have mold growth on the topmost ridge at least. I haven’t thoroughly inspected the attic with a good light yet.

I was told I should add ventilation to the eaves (drill holes) and insulation baffles. But then I read this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-attic-venting

Now I’m wondering if roof was originally unvented (1915) and if the 4 roofing vents were a later addition during reroofing jobs.

I’m a little confused about which direction to head. If I add a whole house fan then I’d need attic venting. However, I could also stick a fan in front of an open window in the back of the house and open my front windows. There aren’t too many hot days during the summer up here. On the other hand, winters have the highest energy costs and least comfort. I will definitely air seal the ceiling to prevent humidity from reaching the attic through air leaks (not sure if I should add an actual vapor barrier but the article sounded like that was unnecessary.)

If I decide not to install a whole house fan should I add venting and baffles along the eaves (no soffits on house)?

Additionally, should I remove the four existing roof vents on the roof?

I almost wonder if venting a whole house fan out of a wall might be better in my climate zone than doing it the traditional way through the attic. If I lived in a hotter climate like the Sacramento Valley in CA I’d definitely want to evacuate the attic, but up here I’m not sure.

Thanks for any advice. And if there are too many questions, my primary question is: should I add venting along the eaves before blowing in R-49 insulation?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    My usual answer to a question like yours, about a 100-year-old house, is something like, "If you don't see any signs of mold or rot up there, you don't have anything to worry about."

    But you have noticed signs of mold.

    The correct answer is, "This type of attic needs soffit vents and a ridge vent," and that's the answer I'm going with in your case.

    Note, however, these two facts:

    1. If the source of the moisture in your attic is the interior air, the most important thing you can do is to perform air sealing work to prevent interior air from entering the attic. Here is a link to a relevant article: Air Sealing an Attic.

    2. In the Pacific Northwest, outdoor air is humid, and it's possible that introducing outdoor air to an attic via soffit vents may introduce enough moisture into the attic to support mold growth. This is a local phenomenon. Ultimately, this type of mold is mostly harmless -- but it's worth knowing about the phenomenon so you don't freak out if you see some mold.

  2. Brad_S | | #2

    Thanks, Martin, I'm certain moisture is coming from inside the house. I saw light penetrating from the ceiling light cutouts when I was in the attic. With single pane windows and little to no wall insulation we get mold around the internal window sills too if we don't constantly wipe down the condensation. The Air Sealing an Attic article will be helpful.

    Seattle outdoor air humidity right now is 92%, but temp is only 47F with a 46F dew point. Indoor humidity is currently 69% at 55F. Just getting to the point where I might turn on the heater and bring it up to 62F.

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