recommendations for solar attic vent fan?

[mole]

I have a 150yr old house with about a foot of moldy fiberglass & cellulose insulation in the attic along with the wonderful debris from re-roofing and providing a home for a red squirrel family or two over the years. I suspect there's some major air leaks up there too, so I'm gonna take a 1 week attic vacation this spring, rip all that crap out, try seal it right, and then reinsulate. The mold was likely caused by a lack of ventillation. I have a gable vent, maybe 18" by 24" on the east side of the attic which is open to the outside with a window screen. There's no vent on the west side. It gets wickedly hot up there in the summer. It looks like an easy and effective way to fix the ventillation might be to install a solar attic fan on west end peak of the roof. The attic area is about 800sqft. I see a lot of models of solar fans out there at a wide range of prices. I'd welcome any thoughts/ experiences on solar attic fan sizing, selection, reliability, value, etc.
Thx

 

[woodgeek]

Best practice is **no fan** just an appropriate amount of passive ventilation. Ideally 1 sq ft of free area per sq ft of attic floor. Ideally that much high (near the ridge or top of gable) and a similar amount low (i.e. soffit), to allow stack effect to drive some flow. It will still get hot as blazes up there in the summer, but if the floor is airsealed and insulated, that won't matter.

The mold is due to the air infiltration from below hitting the cold attic. Airseal and problem solved.

Why no fan?....with imperfect airsealing attic fans draw conditioned air from the house, increasing energy bills. Folks 'swear' that their upstairs got cooler (yup, cooler air was drawn up from lower in the house) or that their attic was loads cooler (same reason) but their energy bills actually went up. If the attic floor is airsealed, then this is not a problem, so why not? Well, the modest amount of cooling, combined with the high insulation level yields tiny energy savings in the house AC bill....less than the juice to run a grid powered fan, and if the fan is solar the payback time in energy saving is several times longer than the expected life of the fan.

 

[TradEddie]

The best piece of DIY advice I've ever been given was to do any attic work before mid-Spring. You'll be amazed how hot an attic can be even on a sunny Winter day. Take that day off as soon as you can.

FWIW and on a related note, the worst DIY advice I ever got was to use dilute vinegar to soothe the itchiness from working with fiberglass....I truly wish I could remember who told me that, because it was undoubtedly the most long-delayed practical joke ever, and deserves recognition as the masterpiece it was.

TE

 

[mole]

Woodgeek,
Thanks for the insight. I hadn't really considered the effect of the fan on sucking the heated or cooled air out of the house. I guess I'll just add a gravity flow vent near the peak and focus my energy on air sealing and reinsulating.

TE,
My 20yr old helper returns from college early May, which was when I was planning to do the work (I was going to have the fan up and running first!) Maybe I should move the job up to spring break instead. I love the vinergar advice! That's priceless!