In an old house with no ridge vent, would an attic fan work better than vents?
We are going to get a new roof on our old house in the Northwest this summer. Our current roof doesn’t have any snow and ice shield, and we had some ice dam issues this winter near the valleys (like many people). Our roof needed to be replaced anyways but we had some solar attic fans installed in each attic to help with ventilation, though this year we realized we don’t get enough sun for the fans to be effective, so we were planning on getting electric powered fans installed in the same cutout where the solar fans currently are. We have the attic eaves baffled and open, we had insulation added two years ago but our upstairs is a finished attic with knee walls so there is no ridgecap vent, it’s plugged with insulation. In an ideal situation we’d raise the roof and fur the trusses (the house is framed with old 2×4’s) and the gable ends of the house aren’t accessible due to how the attic is built out so we don’t have gable end venting. I know you’re not an advocate of attic fans so would passive roof vents be a better option? We also decided to join the masses of people in our town and buy heat tape for the rooftop eaves and valleys which did help though we’re hoping the snow and ice shield will ensure we don’t get any moisture inside the house (it wasn’t “attic rain” or condensation buildup in the attic but most definitely large chunks of ice building and backing under the shingles). Anyways any advice is appreciated, thank you in advance.
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Replies
Shmac,
Your current situation is a mess. You apparently have 2x4 rafters, and the attic has been finished in a way that blocks ventilation. You don't describe the airtightness of the house or the level of insulation, but clearly something is very wrong, because you've had ice dams.
As my article on the topic (Fans in the Attic) points out, a powered attic ventilator is likely to make the situation worse.
The best solution to this problem, by far, would be to install an adequately thick layer of rigid foam above the roof sheathing in conjunction with your re-roofing job. Here is a link to an article that describes the work: How to Install Rigid Foam On Top of Roof Sheathing.
The good news is that you are about to embark on a re-roofing job -- the perfect time to do this work.
The bad news is that you are tempted to adopt cheap "fixes" (powered attic ventilators and electric heat tape) that will make the situation worse.
For more on the causes and cures for ice dams, see these articles:
Prevent Ice Dams With Air Sealing and Insulation
Ice Dam Basics
He's in a high humidity area, foam and he won't have any sheathing left in a few years time. All rotted away. What he should do is what the local roofer recommends.
Anon3,
You've been posting opinionated comments on GBA for a few weeks. I think it's time for you to update your user profile with a real name. Here is a link to an article that tells you how to do it: How the GBA Site Displays Readers’ Names.
Installing an adequately thick layer of rigid foam on the exterior side of roof sheathing will lower the chance of moisture accumulation, not increase it, and will make the sheathing less likely to rot. A great many field studies have confirmed this fact.
With all due respect, and speaking as a former roofer, Shmac is far more likely to get good building science advice by reading articles on GBA than by listening to the advice of local roofers.
The pacific northwest is NOT a high humidity area, with average outdoor dew point temperatures in the mid-40s F even in summer. Asphalt shingled roofs don't dry toward the exterior (much) in any climate zone, given that a typical #30 felt + shingle roof runs about 0.1 perms.
Powered attic ventilation is likely to cause as many problems as it might solve. In a zone 4C climate an unvented solution installing 3" of polyiso above the roof deck and 6" of open cell foam or damp sprayed cellulose on the under side of the roof deck would meet code on a U-factor basis. If reclaimed foam (rather than virgin stock brand new polyiso) is used it should run $5-6 per square foot, but expect it to be more if the installers aren't familiar with how that's done.
Yup, the powered attic fan is your best bang for the buck. It will improve your indoor air quality as well.
Annon3,
Can you flesh-out how running a fan in an attic outside the conditioned area of the house will improve indoor air quality?
Yes anon3, I would be most interested in how iaq would be improved. Maybe by dramatically increasing the ach it would provide "fresh air" but in an uncontrollable manner and with unpredictable energy penalties beyond the consumption of the fan. And with due respect, I feel that more reading and less posting would be the best course of action.