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Old 06-21-2020, 02:39 PM   #1 (permalink)
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What's the verdict on attic fans?

When I purchased my home, the inspector commented that the attic ventilation was inadequate.

Using some online calculators suggests that my soffit (intake) venting is large enough, but the home does not have a ridge vent and the gable vents add up to only 20% of what is recommended for a home which may not have a proper vapor barrier in the ceiling. I'm scratching my head on just how to get 560 square inches of Net Free Area with just gable vents, which are typically around ~60 square inches of Net Free.

So my question is, what about attic ventilation fans, on a thermostat / humidistat?

There seem to be plenty of builders and DIYers still recommending these, and plenty more that suggest to proceed with caution. Alarming to me is that Georgia has banned attic fans state-wide, and logic of this is that fans create negative pressure in the attic, and the make-up air is pulled from conditioned space in the home, often causing more harm than good.

My home seems to be relatively air-tight, and there's a minimum of 12-14 inches of blown in cellulose insulating the ceilings. My issue instead is the accumulation of hot air in summer and moisture in winter. I'm not so much concerned with reducing air conditioning load (there's relatively little) but rather, to improve the longevity of my roof.

What's the verdict?




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Old 06-21-2020, 03:29 PM   #2 (permalink)
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Attic fans are a solution to improperly designed ventilation. The ideal solution is to add adequate passive ventilation.

I don't like fans because they are power hungry and noisy, and represent a compromise in solutions rather than the ideal.

My house has inadequate intake venting, which I've pointed out multiple times to so-called experts, but none have validated my hunch. I actually put a contingency on the purchase of my house to add intake ventilation, but the morons added exhaust. I didn't want to explain the difference to so-called experts and delay the process, so I let it go.



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Old 06-21-2020, 03:39 PM   #3 (permalink)
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Certainly. My house has improperly designed ventilation, and passive ventilation seems to be the better way to go. I'm unsure if anyone would have known 60 years ago when the house was built.

The best way to get a bunch of effective net free exhaust area is a ridge vent. The roof however is relatively new, and has no ridge vent. The house is very long, and does not have a lot of gable area - I want to say it's 55-60ft long, and ~25' wide, with a not-very-steep roof (~20 degrees). In order to get the recommended exhaust area, I would need around 10 of these 18x12" vents, which would take up just about the entire front and rear house gable area:




Given that, is a fan the least bad of many bad solutions?
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Old 06-21-2020, 05:40 PM   #4 (permalink)
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Sounds like you need a ridge vent.
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Old 06-21-2020, 07:37 PM   #5 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
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Yeah, that intake situation looks questionable.
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Old 06-22-2020, 06:38 AM   #6 (permalink)
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I had a couple of those mushroom looking electric fans on the roof back in the day. When the attic got to a certain temp they would automatically come on. About 15 years ago a hurricane blew them both off my house (along with the chimney and a big portion of the shingles) and when they re-roofed the house they added about fifty feet of low-rise ridge vent and plywooded over the areas where the fans were.

The attic doesn't seem any hotter now than it used to be with the electric fans so the ridge vents seem to be working well.
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Old 06-22-2020, 08:10 AM   #7 (permalink)
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Let's assume a ridge vent isn't in the budget, at least not until the roof needs to be redone.

Unless I'm missing something and it's actually quite easy and relatively inexpensive to DIY a ridge vent.
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Old 06-22-2020, 11:48 AM   #8 (permalink)
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I expect a ridge vent to not be very complicated. My guess is remove ridge shingles, cut into the ridge decking to expose the attic, then install the ridge strip, and re-shingle the ridge.
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Old 06-23-2020, 07:46 AM   #9 (permalink)
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I had a mushroom fan and no ridge vent. You could see the dust accumulation around switchplates and sockets where air was being drawn between the interior walls up into the attic by the negative pressure.

We were having our roof redone, so we added a full length ridge vent. I also had them blank out where the fan was. Good riddance. Proper passive ventilation beats those stupid fans all the way up the hallway and back.

Ridge vent retrofitting is pretty easy, and I would've done it if our roof wasn't so near replacement:
https://homeguides.sfgate.com/add-ro...oof-43671.html

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Old 06-23-2020, 07:50 AM   #10 (permalink)
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