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BlueBSH 05-08-2012 02:42 PM

Solar Attic Fans
 
I am looking at putting solar attic fans in our house to help keep that space a tad bit cooler, with it getting in the 140+ degree temp range in the summer it needs air flow the soffit and top vent's aren't cutting it.

Almost all these I have looked at say cut a 19" Diameter hole in the roof, thats great and all, but what if your roof it 16" O.C. rafters?

Anyone put one of these in before? Any comments suggestions? thanks!

bill01 05-08-2012 03:23 PM

Pretty much the same solution used with electric powered ones. You can cut a 19 inch hole in the sheathing being careful not to cut the rafters and just have a rafter in your hole or just cut a 14.5 inch hole instead. Or you can try to find one designed for 16 centers.

BlueBSH 05-08-2012 03:35 PM

Quote:

Originally Posted by bill01 (Post 917269)
Pretty much the same solution used with electric powered ones. You can cut a 19 inch hole in the sheathing being careful not to cut the rafters and just have a rafter in your hole or just cut a 14.5 inch hole instead. Or you can try to find one designed for 16 centers.

any idea on if they help having them? I'm thinking about it because an energy auditor suggested maybe putting them in right now we just have gabel vents, the soffit vents and the top roof (ridge?) vent and in the summer very little air movement in there on calm days, and it gets HOT!

strategery 05-08-2012 05:00 PM

I have had one for over a year now. I also just have two gable vents.

You will probably hear tons of people telling you that they're a waste of money and that they will draw conditioned air up through the ceiling and into the attic.

I like mine. I had my attic floor air sealed so I'm not too concerned about losing conditioned air. For me it was about ventilation plus I'm not ready to put a new roof on yet. I measure my attic temp and humidity levels and it has lowered attic temps by several degrees depending on how sunny it is.

I had a roofer cut the hole and install it for me. I think I paid about 800 bucks.

JDubTheSolarGuy 05-14-2012 01:24 PM

Solar Attic Fans
 
Second what Bill said, just cut around the rafter. The fan won't care, and will not significantly affect flow.

As for depressurizing your home and pulling conditioned air out of your home, you should be ok even if your ceiling is not sealed as long as you do NOT over do it. Less is more - you are just trying to reduce the difference in temps between your attic and the inside of your home.

Make sure you solar panel is never shaded and is properly attached and rated for your wind speeds.

HTH,

JW

veesubotee 05-14-2012 02:47 PM

Quote:

Originally Posted by BlueBSH (Post 917242)
I am looking at putting solar attic fans in our house to help keep that space a tad bit cooler, with it getting in the 140+ degree temp range in the summer it needs air flow the soffit and top vent's aren't cutting it.

Like the original poster, I too have soffit vents (all open) front and back, as well as a ridge vent. While up on my roof (in my younger days), I have never been able to verify any airflow out of the ridge, nor air into the soffits and the attics are hot as hell..

I've thought about solar attic fans (lower flow) not causing negative pressure in the house, but wonder if the ridge vent would cause short circuiting of the air and not pull any air through the soffits. Or, would there in fact be divided airflow between the soffits and ridge?

JDubTheSolarGuy 05-15-2012 07:42 AM

veesub- you are right to consider the 'short circuit' effect. You can avoid it for the most part by placing the fan at least 6-8 feet away from any vents. The vents then become an intake as well.

If you can't move the fan far enough away, you have the option of closing up a section of the vent. I staple 30# felt to the back of the roof deck to close them up, that way its easily removed if necessary in the future.

HTH,

JW

veesubotee 05-15-2012 01:59 PM

Quote:

Originally Posted by JDubTheSolarGuy (Post 921744)
veesub- you are right to consider the 'short circuit' effect. You can avoid it for the most part by placing the fan at least 6-8 feet away from any vents. The vents then become an intake as well.

If you can't move the fan far enough away, you have the option of closing up a section of the vent. I staple 30# felt to the back of the roof deck to close them up, that way its easily removed if necessary in the future.

That's what I thought. As far as the vents are concerned, there is no way to escape them. The soffits are open the entire length of the house, front and back. This is where, ideally, the entrance air should come from. The ridge is another story as the fans are usually mounted relatively few feet below the ridge. It sounds like a real production to seal the ridge as there are no decks, just joists in my attics.

Oh well.

V

JDubTheSolarGuy 05-15-2012 02:24 PM

Quote:

Originally Posted by veesubotee (Post 921981)
That's what I thought. As far as the vents are concerned, there is no way to escape them. The soffits are open the entire length of the house, front and back. This is where, ideally, the entrance air should come from. The ridge is another story as the fans are usually mounted relatively few feet below the ridge. It sounds like a real production to seal the ridge as there are no decks, just joists in my attics.

Oh well.

V

You only need to seal up enough ridge near the fan to put free air flow at least 6 feet away. By 'roof deck' I mean the back of the sheeting (inside the attic) that the shingles are attached to. It is super simple to seal up. Buy a roll of #30 roofing felt, cut it to cover 6-8" on either side of the vent and staple with a staple gun. All you need to do is make it harder for the fan to draw air from those vents than the soffits. Works like a champ.

HTH

veesubotee 05-15-2012 05:06 PM

Quote:

Originally Posted by JDubTheSolarGuy (Post 921994)
You only need to seal up enough ridge near the fan to put free air flow at least 6 feet away. By 'roof deck' I mean the back of the sheeting (inside the attic) that the shingles are attached to. It is super simple to seal up. Buy a roll of #30 roofing felt, cut it to cover 6-8" on either side of the vent and staple with a staple gun. All you need to do is make it harder for the fan to draw air from those vents than the soffits. Works like a champ.

The 'deck' that I was referring to was actually what would be considered a floor (to stand on, or erect a ladder). I did some work in the attic and laid some sheets of sub-floor (scarfed during construction). Not sure if I'm directly under the ridge.

Now, just to clarify--hope this formats properly. Do you mean:

Ridge------------------=========------------ is that 8" or 8'
each side of fan?

Fan ( fan )

----- = ridge vent

===== is sealed area over fan

Is this what you mean?

O.K., it will not format correctly. What I drew was ( ) fan under sealed portion of ridge vent. Correct?

V

TarheelTerp 05-15-2012 05:43 PM

Quote:

Originally Posted by BlueBSH (Post 917242)
I am looking at putting solar attic fans in our house to help keep that space a tad bit cooler, with it getting in the 140+ degree temp range in the summer it needs air flow the soffit and top vent's aren't cutting it.

how large are the soffit openings?
they're almost never large enough to provide enough flow.

gregzoll 05-15-2012 06:15 PM

My attic has topped out at around 102 when I have been home to see the temp, at the peak of the temp while our ventilator is running, which is around 2:00, and by 5, the temp has dropped down to 87, and continues to drop overnight to be at the same as outside. Before we had the fan, I would see it get to 130 by the end of the day, and you could tell how much heat gain, due to the ac would cycle more, and it always seemed that it never got cool enough in our living room & kitchen, due to they take up that whole half of the house.

One other thing we did was put UV film on all our windows, which has also helped to keep the house cooler. Came home at lunch, and it was 72 inside, the AC had not kicked on, due to it is at 72 from 8am to 3:30 pm, then down to 71. I can say one thing, my bills so far have been a whole lot lower than they were last year at this same period, when we did not have the attic ventilator.

I went with the wired fan vs. the Solar, due to cost. It was cheaper to get the electric power fan then getting the solar powered. Plus we have tree cover over the roof, so the solar would not get enough sun to keep it running.

JDubTheSolarGuy 05-16-2012 09:12 AM

Quote:

Originally Posted by veesubotee (Post 922090)
The 'deck' that I was referring to was actually what would be considered a floor (to stand on, or erect a ladder). I did some work in the attic and laid some sheets of sub-floor (scarfed during construction). Not sure if I'm directly under the ridge.

Now, just to clarify--hope this formats properly. Do you mean:

Ridge------------------=========------------ is that 8" or 8'
each side of fan?

Fan ( fan )

----- = ridge vent

===== is sealed area over fan

Is this what you mean?

O.K., it will not format correctly. What I drew was ( ) fan under sealed portion of ridge vent. Correct?

V

Yes.

If you have to place the fan within 6' of a ridge vent, you will want to seal that vent from the inside of the attic to ensure you don't have free air flow from the vent within 6' of the fan.

8 inches not 8 feet, unless you really like stapling felt. :wink:

ksj9393 05-17-2012 09:55 AM

Ventilation mysteries
 
Unless you have a well-sealed attic, with sealed enclosures over can lights and all other ceiling penetrations, I would advise against any sort of powered attic vent.

You will see ventilation "rules-of-thumb" that range from 1sq ft of NFVA per 150 ft sq of attic space, to 1 ft^2 NFVA per 600 ft^2 attic. Further, the ratio of soffit to ridge (or mushroom) vent is often cited as 50:50. Since hot air rises, and decreases in pressure, inadequate soffit venting means make-up air will come from conditioned space, or exfiltration. Adding a powered fan will only worsen the situation.

Further, the data shows us that ventilation really does surprisingly little to lower the surface temp of roof decking - this is mostly dependent upon color of the cladding, etc. But if your attic space is hot, you've correctly identified the problem as inadequate ventilation. "Pulling" air from atic is not the solution, however, in my opinion. Rather, make certain you have at least 1 sq. ft. NFVA per 300 sq. ft. of attic area (minimum per IRC2009 with other conditions met), and divide it 60:40 in favor of soffit venting. Curiously, this runs contrary to IRC and others, but the science is good supporting greater soffit venting than ridge venting. (Again, my opinion.)

In other words, you may find that simply adding some soffit grates solves your attic heat issue, at lower cost, lower complexity, and lower risk of roof leak and other complications. AND, you just might discover your summer energy utilization decreases.


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