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Are Powered Attic Fans A Waste of Money?


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I think so but for years have had a hard time convincing people - especially those in the southern states - of that fact.

Here is what a couple of contributing editors at the Journal of Light Construction Paul Fisette is a well-known building scientist) have to say about the subject.

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Click here

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Mike

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  • 1 year later...

Hi,

Thanks for letting me know about the link. I've removed it and replaced it with three new links. I don't know if the one out of those three that Fisette wrote is the one I'd previously linked or not but I'm sure his answer is probably about the same.

ONE TEAM - ONE FIGHT!!!

Mike

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  • 4 weeks later...

My house does not have any soffit vents because the overhang is too small. There are no ridge vents. I installed multiple gable vents and a power vent. I experimented leaving the fan on during hot days and shutting it down.

There is a huge difference in attic temperature when the fan is running.

I have concluded that in my own house, the power vent is helpful.

I recommend soffit vents and a ridge vent where feasible.

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Power vents have distinct applications that are useful; Hocksteins is one, and I've done a couple others that are similar, where I set up standard roof vents in different locations designed to pull air from the areas I needed it pulled from. The power fan was located so it would pull air down the length of a long attic "plenum" and exhaust it. Worked great.

Power vents that are just stuck on the roof, with no regard for design engineering, are worse than worthless; they create problems.

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"Power vents that are just stuck on the roof, with no regard for design engineering, are worse than worthless; they create problems."

I think the same could be said for any roof vent. Attic ventilation needs to be designed as carefully as any other system in a building, leaving half of it up to the siding crew and the rest up to the roofer is a recipe for disaster. Other than that, I couldn't agree more.

Tom

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My house does not have any soffit vents because the overhang is too small.

Easily remedied without hacking into the roof at a half dozen places.

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tn_20098281164_DripEdgeFlashingVented.jpg

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http://www.airvent.com/pdf/installation ... nstall.pdf

http://www.airvent.com/pdf/installation ... nstSht.pdf

ONE TEAM - ONE FIGHT!!!

Mike

Wouldn't these be a problem in areas that experience ice damming? With that opening hanging right above the gutter it would provide a nice highway for ice.

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There's a few problems with those, but they're surmountable, although it means new gutters and more work.

Ice damming can be controlled by wrapping the IWS down over the fascia, installing the gutter, then the AirVent (wrapping into the gutter), then the shingles. Felt details get kinda lost, which means tearing up some roofing to get the felts right.

The Airvent extends the drip edge out further than "normal", so you have to install big (5") gutters on the house to catch high volume runoff.

So, in some ways, hacking holes in the roof is about 6 of one, and the other way is a half dozen.

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Ice damming can be controlled by wrapping the IWS down over the fascia, installing the gutter, then the AirVent (wrapping into the gutter), then the shingles. Felt details get kinda lost, which means tearing up some roofing to get the felts right.

Ummmmm...

If you install IWS first and wrap it down the fascia, and then install the air vent on top of it, just how does the air get through the IWS?

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Dammit, I hate it when people show me I'm stupid.

You're right, it won't work.

That's what happens when you answer questions in the early AM while amped up on too strong coffee.

We're back to tearing up the roof for more vents.

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My house does not have any soffit vents because the overhang is too small.

Easily remedied without hacking into the roof at a half dozen places.

Click to Enlarge
tn_20098281164_DripEdgeFlashingVented.jpg

3.33 KB

http://www.airvent.com/pdf/installation ... nstall.pdf

http://www.airvent.com/pdf/installation ... nstSht.pdf

ONE TEAM - ONE FIGHT!!!

Mike

Mike-

That is a great option that I was not aware of when I replaced my roof 9 years ago. I am not even sure if it was available. For now, the current conditions seem to be adequate. There is no moisture in my attic and the temperature is reduced by the fan during hot weather. I should also add that my roof sheathing is tongue and groove boards that hold up much better than plywood and OSB.

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Which brings up a point I've found most critical with attic venting, and that is........

Is the roof sheathing old growth lumber, or a plywood or OSB sheathing?

I see a few hundred (sheathed with old growth roof board) roofs a year that are set up completely wrong, and by modern code standards, should be mold farms and piles of rot.

But they're not, because the lumber breathes and equalizes moisture out of the structure, unlike the engineered lumber and sheathing modern buildings use.

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I like the part about create enough negative indoor pressure to make gas-fired appliances back-drafting and then we can change the temperature of in attic space and creating dew point temperature on surfaces, leading to mold issue. I think these are worst case scenario, but two things that I would not want to do to any home.

Test in and test out.

John

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  • 1 year later...

My house does not have any soffit vents because the overhang is too small. There are no ridge vents. I installed multiple gable vents and a power vent. I experimented leaving the fan on during hot days and shutting it down.

There is a huge difference in attic temperature when the fan is running.

I have concluded that in my own house, the power vent is helpful.

I recommend soffit vents and a ridge vent where feasible.

The question is, have you saved any money? I doubt it, as the majority of the heat gain through the ceiling is from radiant heat. Lowering the air temperature in the attic saves very little energy. The only way it makes financial sense is if the vents are solar powered and don't cost you anything to run. You can read the details in the attached file.

Of course, there may be other good reasons to increase attic ventilation in a heating climate, especially if you can't control the humidity indoors.

Download Attachment: icon_adobe.gif Attic Vent Report Final.pdf

1990.69 KB

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Hi Rob,

Yeah, I guess that would be a problem up in your neck of the woods where they have so many Cape Cods, salt boxes and other styles of home without any real overhang at the eaves and ice and snow is an issue. I think if I had to deal with that on a daily basis, I'd recommend this kind of vent if I were in snow country.

ONE TEAM - ONE FIGHT!!!

Mike

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Hi Rob,

Yeah, I guess that would be a problem up in your neck of the woods where they have so many Cape Cods, salt boxes and other styles of home without any real overhang at the eaves and ice and snow is an issue. I think if I had to deal with that on a daily basis, I'd recommend this kind of vent if I were in snow country.

ONE TEAM - ONE FIGHT!!!

Mike

Seems like a good choice for hurricane country too, especially if the overhang is kept short to avoid catching the wind. The 1 1/2" height of the Coravent might reduce the entry of wind driven rain better than those thin soffit vents.

Just need to check on code acceptance.

Marc

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Passive ventilation schemes make very little sense, even less so in hurricane country. It's my understanding that roofs fly when they generate lift, and buildings implode/explode due to pressure extremes. Both of those are significantly impacted by ventilation, no?

Impacted? I dunno. Influenced? Sure. It's in the report that Mark C linked us to earlier on this thread.

Marc

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Passive ventilation schemes make very little sense, even less so in hurricane country. It's my understanding that roofs fly when they generate lift, and buildings implode/explode due to pressure extremes. Both of those are significantly impacted by ventilation, no?

I dunno; I think it depends a lot on climate. Around here, I seem to find more problems with roof decking fungi and such in houses where the homeowners thought adding a powered ventilation system would be an improvement. It wasn't.

On the other hand, I grew up downstate from you in Dutchess County and I remember well the first time my father used gable end vents and eave vents to rid an attic of condensation. It was a schoolmate's home. It had been done for less than a month and my friend's father was complaining about condensation in the attic. I was over there to see my friend when my father and uncle rolled up. At first I thought I'd screwed up and was about to get my ass whupped, but then the architect pulled up and I breathed a sigh of relief.

My son's father wanted a powered attic fan but my father thought that was just goofy given the fact that we would routinely have two or three storms where at least a foot of snow would end up on the roof each winter. Then the architect suggested cutting vents in the gable end walls of the attic. Jeez, you would have thought he'd suggested setting the house on fire. I remember my father and uncle standing there for most of an hour arguing with the architect about the architect's dumb idea of cutting vents in the perfectly good gable end walls of the attic. The old man was dead set against it. The architect was going on and on about the science of ventilation and how air moved and I could see that the old man was pretty skeptical. He listened though and he put the vents in. The attic dried out and my old man became a believer in passive ventilation after that.

Today we know that gable end vents are damned near useless unless oriented on the same axis as the prevailing winds, but we didn't learn that overnight and some still haven't learned it - as the plethora of new homes going up with gable end vents will attest to.

There are a lot of houses that lose their roofs in hurricanes but there are also a lot that don't lose their roofs that have ridge vents and soffit or eave vents. I think the building scientists and engineers spend a lot of time studying those houses to see what works and what failed and then they make their recommendations based on those best practices that work.

If passive ventilation weren't working, I think we wouldn't see it advocated so much by the building scientists and engineers.

ONE TEAM - ONE FIGHT!!!

Mike

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I think you missed my point.

There are a thousand 'one size fits all' passive ventilation products out there, and little if any actual building science or engineering. We talk all the time about the importance of properly sizing air handling equipment, there are formulae for all of the critical components, yet proper ventilation is left to a stupid 'rule of thumb' and a monkey with a hammer to figure it out. I say it every time this topic comes up: passive systems should be engineered to work, or engineered out. It's simply amazing that the current scheme works as often as it does.

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A while back there was a post on another thread by Mike B about someone from a few centuries ago who wrote a fine book about fireplace design. The width, depth, height of opening, profile, size and height of the flue all had to be right to accomplish a fireplace that delivered the most heat into the room. I couldn't find the post but I just thought that that idea rhymes with this thread...that all dimensions and circumstances have to be considered and coordinated in order to produce an optimum attic ventilation plan (if you're depending on a natural draft arrangement).

Marc

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