# Solar Attic Fans



## CLConstruction (Feb 25, 2011)

Hey everyone,

My client wants to use solar powered vent fans on his roof. Anyone have an experience with these? Are they any good? Thanks


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## Dirtywhiteboy (Oct 15, 2010)

CLConstruction said:


> Hey everyone,
> 
> My client wants to use solar powered vent fans on his roof. Anyone have an experience with these? Are they any good? Thanks


They're very easy to put in:whistling but you make it sound very difficult and charge him for it:thumbsup: who cares if they're any good


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## seeyou (Dec 12, 2008)

CLConstruction said:


> Hey everyone,
> 
> My client wants to use solar powered vent fans on his roof. Anyone have an experience with these? *Are they any good*? Thanks


Upside - no wiring necessary. No power usage.

Downside - more expensive initially (but that is offset by not having to hire an electrician or run wire). They are not as powerful as hardwired units. They stop running when the sun goes down. 

There are some applications where they are the best solution and some where they are not. I don't have enough experience with them to discuss longevity.


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## SLSTech (Sep 13, 2008)

It depends, but generally NO

http://blog.sls-construction.com/2011/rip-solar-powered-attic-ventilators


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## Grumpy (Oct 8, 2003)

I've been installing solar attic fans for about 4 years now and no complaints. I have 3 on my own personal residence. So I think I can say I've got a little experience with solar fans. 

I do not have an electricians license nor do I have electrical insurance work so I can not legally make the connection of power to fan. Most of the roofers in my area including myself will write in our proposal "Replace attic fan blah blah blah (no electric)." Now the customer either has a non operational fan or has to pay an electrician to come wire the fan. I don't know any electrician who would come out for less than $200.

I bring up the electricians fee for a reason. A solar powered fan costs about $200 more than a traditional fan, so when you look at it that way; it's break even.


Here is something to consider with a solar powered attic fan, they are not as powerful as traditional hard wired attic fans. So if the roof has one fan now, it may not be enough so be sure to run a NFA calculation. Here is another thing to consider, most solar powered attic fans are actually only 500-550 CFM, literally half a standard wired attic fan. I prefer the Air Vent SOlar Cool which is 880 CFM. It's slightly more than the GAF 550 CFM, but you get what you pay for.



FWIW I know they work because I always have to clean my intake vents of dust that they are sucking inside. Also when I clean my gutters I check the fans out. I was concerned they wouldn't work well in an overcast day but they turn just fine in just about any light. In the past when I mentioned I check them periodicially someone arguing against the fans latched onto that comment and said he'd prefer to install something he never had to check. Well, I don't "have" to check them I just do it for my own peace of mind. It is still a newer technology afterall.


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## seeyou (Dec 12, 2008)

Grumpy said:


> I've been installing solar attic fans for about 4 years now and no complaints. I have 3 on my own personal residence. So I think I can say I've got a little experience with solar fans.
> 
> I do not have an electricians license nor do I have electrical insurance work so I can not legally make the connection of power to fan. Most of the roofers in my area including myself will write in our proposal "Replace attic fan blah blah blah (no electric)." Now the customer either has a non operational fan or has to pay an electrician to come wire the fan. I don't know any electrician who would come out for less than $200.
> 
> ...


We often find hardwired units that have failed long ago and the HO just assumed they were still working. If they have moving parts, they need to be checked periodically.


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## [email protected] (Oct 3, 2011)

*It depends, of course*

Grumpy's experience notwithstanding, I think it's good to ask just what you're putting one on for.

If you have done the full ventilation analysis and you're replacing an existing fan and the CFM works, then yes, go ahead, if it's the only way to vent the attic.

And, if you're replacing you can't entirely ditch the electrical work, since we're not supposed to abandon electrical endings. I usually install a receptacle in place of the fan's electrical box.

Just because they're cool doesn't mean they're right for your application. If you can double the 1/300 attic ventilation rule with standard intake/exhaust vents, why bother with any power fan?

I have gotten rid of more of these than have installed. Usually, only in situations with very challenging hips have I thought one was necessary.

What think you of that, Grumpy?

Aaron

www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

The roof currently has no vent fans. There is a lot of standing heat in the summer because he has a dark roof. There is no AC in the house at all. He asked me to install vent fans to help pull the heat out of the attic so it is not so unbearable inside.


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## [email protected] (Oct 3, 2011)

*Soffit venting*

So, what is the square footage of the attic, and what soffit and exhaust venting is installed currently? Do you have 8x16 louvered soffit intakes, 4x16, little round guys, cheese-grater looking full-lanced soffit panels ... anything? And on top, standard mushroom vents? and how many?

Aaron

www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

The roof is a 6/12 and is about 1200sq ft. there are the standard blocks with three round holes installed in every other rafter bay. and a mushroom vent at the top of the bay, i believe there are 5 total. it has one 12x16 gable vent on the north side.


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## [email protected] (Oct 3, 2011)

So, a pretty standard gable roof, about 30x 40, with a bump out / bay window in one place?

Aaron


www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

It's a straight gable roof on the second floor. No bump outs or bay windows.


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## [email protected] (Oct 3, 2011)

I'm not familiar with the soffit venting you mentioned (maybe it's different here in Packerland) ... how big are the circles, would you say? 2"? 3"? 4"?

And there are three circles per block, and one block every other bay? So maybe 12 or 13 blocks per side of the house?

Aaron

www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

There are 8 2x6 blocks that are mounted between the trusses with 4 2" holes in each block.


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## [email protected] (Oct 3, 2011)

OK. Are you ready for a long and technical answer? 'Cause here it comes.

The best and easiest way (in my opinion) to vent a standard gable attic is to use standard static soffit intake and ridge exhaust, no power fans.

First thing to do: if the gable vent is more than a decoration (if it is actually a vent and not just a nice siding accessory) then BLOCK IT UP. It's messing with your airflow.

Second: Check that the air flow can actually get from the soffit to the exhaust vents. If the insulator went crazy and blocked the rafter bay at the top plate and no air can get past, that leads to a very hot home. Pop up the attic hatch and see if you can observe daylight coming up from where you expect the soffit vents to be at the edge of the roof. If you see daylight, you're probably good. If not, keep looking and determine if air can get past. If air can't get past, then unblock the rafter bays and see how much the experience of the home improves. Just because there are soffit *holes* and exhaust *holes* doesn't mean air can actually flow between them.

Third: Do the numbers. FHA/HUD has a calculation to determine what constitutes "enough" ventilation. It has seven steps, and I won't go through them all, so here's a shortcut:

a 1200 sf attic needs 288 "square inches" of ventilation at the top, and at the eaves. If you have 5 mushroom vents (which typically give you 50 square inches of ventilation each) then you have 250 square inches of exhuast ventilation...so you're almost right on, but a little short of target. If you can double the FHA/HUD number, you'll have a happier homeowner.

For intake, one 2" intake is worth about 1.5 square inches of air flow. 4 of these per 2x6 block is 6 square inches of ventilation per block. If there are 8 blocks for the whole house, you are severely short on intake, only 48 square inches for the whole house. If there are eight blocks per every other rafter bay, you're better off, but I'll need more info to know if you are sufficiently vented at that point.

If it is only 8 blocks in the soffits for the whole house, here's what I would do (after blocking the gable vent and verifying airflow past the top plate): cut in 4 new mushroom vents, and come up with a new soffit venting system that will get you to 500 inches of intake at the soffit.

That would do a ton to address the issue. 

If you simply add a fan to a system with inadequate soffit intake, you get nowhere, really. You did work, you billed the client, and the place is still too hot to enjoy. Additionally, the technical types don't want you to mix power fans with mushroom vents, so they would have you not only block the gable vents, but also all the mushroom vents, if you went with a power vent. And you'd still have to have enough soffit intake venting if you wanted that system to work.

The Feb '11 issue of Roofing Contractor from the NRCA has a nice article on the venting changes you would be better off not to make as a contractor.

So, is it 8 blocks per soffit bay, or 8 blocks for the whole house?

Aaron

www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

there are 8 blocks per side. 16 for the whole roof.


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## [email protected] (Oct 3, 2011)

Well, there you have it then.

In the best case, a gable roof will have:
1) soffit intake that gets past the top plate and insulation, and goes to the exhaust vents.
2) Slightly MORE soffit intake than exhaust.
3) No other air flow dynamic disruptions (power fans, gable vents, all that).

It looks to me like the house is severely under for intake ventilation, at about 96 square inches total. That will keep a house hotter than inhabitants would prefer.

Can you add (not that it's the right look, not that it's not) 16x8 louvered soffit vents? Each one of those will get you 65 inches of intake. You would want to add two of those per side, and one mushroom vent, just to get to FHA minimum. Add three more mushroom vents and two more 16x8 per side to have happier homeowners yet.

Do y'all do 16x8 louvered soffit vents out in those parts?

Did all that make sense?

Aaron

www.american-roofing.biz


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## CLConstruction (Feb 25, 2011)

The house has an open soffit. so adding the louvered vents is not an option. The blocks i was talking about are the ones that sit on top of the wall plates, and space the trusses apart. Can i just add more holes along the eave? I have 16 more blocks that i can drill in.


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## [email protected] (Oct 3, 2011)

Yes, you can drill more holes, but I don't know that you can drill enough 2" holes to provide as much intake as the formula wants you to.

Can you make the holes 2-1/2" inches? Can you get to 3"? You've only got 14" total across the rafter space, I know. 

And, as they are holes in a 2x6 as I now understand, you would get more like 10 inches per block of intake.

Is there a balance between bigger holes for air flow and critter control through smaller holes?

Can you make them 2" tall and 10" wide? There's almost 20 inches of airflow (factoring in the screen) per rafter bay, rather than 10-ish. How about two of them per block 2" tall and 5" wide? 4" x 5"?

Basically, you want to put enough holes in the blocks to total up to the 288 square inches I mentioned earlier for soffit intake.

And keep the raccoons / wasps out.

Additionally, the system works better with more intake than exhaust, so you won't get anywhere adding more exhaust (or a power fan) with constricted intake. 

So, I think the best you can hope for is to make as many holes as large as you dare in the blocks, and understand that the design limits the airflow.

You can make a definite improvement. You can get right up close to FHA minimum recommendations. But I don't think your client will get their money's worth from a power fan in this situation, and I do think that if you give them one anyway because they asked you for one and they're cool that you run the risk of spending a bunch of time and money and not actually accomplishing the goal of a more comfortable structure.

We've got a lot of info on the table. Anybody else out there have an opinion?


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## CLConstruction (Feb 25, 2011)

Thanks Aaron for all your help. The trusses are 24 O.C. so i have more room to drill. I will fix the flow problem and see where it goes from there. It is my neighbors house so i can check back in the summer. Thanks again!


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## SLSTech (Sep 13, 2008)

CLConstruction said:


> The roof currently has no vent fans. There is a lot of standing heat in the summer because he has a dark roof. There is no AC in the house at all. He asked me to install vent fans to help pull the heat out of the attic so it is not so unbearable inside.


I am going to leave the venting alone (esp. seeing it appears Aaron is doing great in that area) & simply ask how much insulation does he have in said attic? Is the attic door insulated? A hot attic isn't the real issue, so much as how much heat makes it from the attic inside. http://blog.sls-construction.com/tag/air-sealing-insulation (last 5 posts have been dedicated to the attic)


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## CLConstruction (Feb 25, 2011)

In regards to the insulation, i have no idea. I have not yet been in the attic. The house was built about 15 years ago by a builder with a reputation for being cheap. So my best guess would be just enough to pass the inspection. I believe that R38 is required in the attic up here. Don't quote me on that though.


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## jessegee (May 14, 2013)

Its generally difficult to justify their costs


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## Dirtywhiteboy (Oct 15, 2010)

jessegee said:


> Its generally difficult to justify their costs


Why:blink: it's the best way to move air and cool the house:blink:


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## SLSTech (Sep 13, 2008)

Dirtywhiteboy said:


> Why:blink: it's the best way to move air and cool the house


  :no: - they only make sense in very limited situations


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## Dirtywhiteboy (Oct 15, 2010)

They work very well over here.


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