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Old 06-15-2008, 09:26 PM   #1
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Powered Attic Fan question


My house has trouble staying cool. I was considering adding a powered attic fan (activated by thermostat) to clear out the heat in the attic, I'm hoping that will help.

Attic is 1344 sq. ft (48x28). Currently I have 2 big louvered gable vents (18"x24") and 5 roof (round) vents (12" x 12"). My eaves are wood all the way so I guess I have no intake vents (i.e. no soffits).

I believe the CFM required based on a formula that Ed the Roofer posted was 940 CFM, divide by 300 and that determines the amount of intake sq. footage you need to support the fan. So I'd need about 3.5 sq. feet of intake? That doesn't seem like a lot at all...

What exactly constitutes intake? Do I need to cut out my eaves and install a couple of soffit vents on each side of the house? Or is the gable louver on the opposite side of the house able to do double duty as an intake as well?

Ed, you mentioned I'd need to add some soffits, how many do you recommend (like I said, roof is 48' long, so do what would you recommend as to the number of soffits to add & the placement?)

Am I better off with a gable vent powered fan or the kind that you stick in directly on the roof? I assume gable is better since I wouldn't have to cut any new holes in the roof so it's easier to install?

OR should I just add soffits since I already have the 2 gable vents + 5 round roof vents, so if I add enough soffits, it would vent naturally instead of a powered solution?

How do I know for sure that I in fact DO need more ventilation in the attic? My dad suggests taking the temperature of my ceiling below the attic and if it's not hot or warmer than the air in my top floor, then it means my attic insulation is preventing heat transfer into the house so I may not need to add any ventilation....(even though the attic is like 20-30 degrees hotter than my top floor on a hot day)....does that make sense?

Thanks a TON!


Last edited by spup345; 06-15-2008 at 09:41 PM.
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Old 06-15-2008, 10:59 PM   #2
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Powered Attic Fan question


Your best bet is to wait for ED to answer.If anyone can explain in detail,he can.Have a good night

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Old 06-16-2008, 05:32 AM   #3
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Powered Attic Fan question


I'll let Ed post all the indepth details, being he's such a stickler at that.

If you have proper intake, i suggest continuos soffit vents,
and proper air flow inside the attic, meaning the fresh air from the intake has a free opening to get inside the attic (baffles/open spaces) between the rafters/trusses,
and proper exhaust vent, in the roof,
than a power vent will not be needed to achieve your goal.

You need to use caution with mixing multiple types of roof exhaust ventilation.
I roofed a home in Febuary, it had burnt to the ground last summer and was a total lost except for the foundation/basement.
The fire marshal for Columbiana, Ohio stated he first suspected arson due to the speed of the fire spreading,
later he determined is was the (attic air flow).

Explanation:
The home was older, originaly had no ventilation but the Gable Vents.
Home owner had a new roof installed that included Pot Vents in the roof.
Later in the years the home owner had new siding installed, with continues soffit vents, plus added ridge vent to the roof (but they left the pot vents and gable vents).

Pot, Power and Ridge vents are all classified as exhaust vents,
Gable is classified as both exhaust and intake.

With the nort west Canadian winds hitting the home, the gable vent (on that side), and the intake at the eaves over hang, both were sucking so much air that it siphon'd the air through the ridge vents as well, resulting in a wild fire effect in the attic.
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Old 06-16-2008, 04:56 PM   #4
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Quote:
Originally Posted by spup345 View Post
My house has trouble staying cool. I was considering adding a powered attic fan (activated by thermostat) to clear out the heat in the attic, I'm hoping that will help.

Attic is 1344 sq. ft (48x28). Currently I have 2 big louvered gable vents (18"x24") and 5 roof (round) vents (12" x 12"). My eaves are wood all the way so I guess I have no intake vents (i.e. no soffits).

I believe the CFM required based on a formula that Ed the Roofer posted was 940 CFM, divide by 300 and that determines the amount of intake sq. footage you need to support the fan. So I'd need about 3.5 sq. feet of intake? That doesn't seem like a lot at all...

What exactly constitutes intake? Do I need to cut out my eaves and install a couple of soffit vents on each side of the house? Or is the gable louver on the opposite side of the house able to do double duty as an intake as well?

Ed, you mentioned I'd need to add some soffits, how many do you recommend (like I said, roof is 48' long, so do what would you recommend as to the number of soffits to add & the placement?)

Am I better off with a gable vent powered fan or the kind that you stick in directly on the roof? I assume gable is better since I wouldn't have to cut any new holes in the roof so it's easier to install?

OR should I just add soffits since I already have the 2 gable vents + 5 round roof vents, so if I add enough soffits, it would vent naturally instead of a powered solution?

How do I know for sure that I in fact DO need more ventilation in the attic? My dad suggests taking the temperature of my ceiling below the attic and if it's not hot or warmer than the air in my top floor, then it means my attic insulation is preventing heat transfer into the house so I may not need to add any ventilation....(even though the attic is like 20-30 degrees hotter than my top floor on a hot day)....does that make sense?

Thanks a TON!


First, the square footage formula is for determining the total amount of NFVA required for staic air vents.

Here are the basics for that, which would be your attic square footage, (for a walkable pitched roof), which would need to be multiplied by .7 to determine the cfm required.


Benefits:
  • One power attic ventilator is usually sufficient, unless it's a very large attic
  • Roof-mount power vents are ideal for hip roofs
  • Feature a preset adjustable thermostat for maximum homeowner convenience
  • Gable-mounted power vents are an excellent option for tile roof ventilation
Design Considerations:
  • Most power attic ventilators only have a thermostat, so they only monitor the heat in the attic. Moisture buildup is also a problem in the attic. A solution is a power attic ventilator featuring a combination thermostat/humidistat. (See "Plus" models.)
  • For proper attic ventilation, the Home Ventilation Institute recommends a system that provides at least 10 air exchanges an hour.
  • To determine the fan capacity needed to provide this minimum airflow, use the following formula: attic square feet x 0.7 = CFM (cubic feet of air moved per minute).
  • Power vents are rated by CFM. Look for a power vent that provides at least the minimum CFM requirement. Remember, the higher the CFM the more air exchanges per hour.
  • Note: for roofs 7/12 or higher, you may want to add 20% more CFM capacity to handle the larger volume of attic space; 30% more for roofs 11/12 and higher
  • Keep in mind that there are energy costs associated with operating a power attic ventilator
  • Adequate soffit venting must be installed for best performance
  • If using gable vents, gable vents must be installed on the down-wind end of the home
Now, for your question regarding how much soffit intake ventilation is required, per the HVI, (Home Ventilation Institute), lets see what they have to say from the following link:

http://www.hvi.org/resourcelibrary/HowMuchVent.html

Powered attic ventilators – PAVs
Powered attic ventilators should provide at least 10 air changes per hour. Multiplying the total square footage of the attic by 0.7 will provide the rate required. For particularly dark or steep roofs, we recommend a slightly higher rating.
Attic area in square feetCFM required+15% for dark/steep roofs1,000 square feet700 CFM805 CFM1,000 square feet1,400 CFM1,610 CFM1,000 square feet2,100 CFM2,415 CFM
The air being exhausted must be replaced by outside air drawn through vents under the eaves in the soffit. To calculate the total minimum soffit vent intake area in square inches, divide the CFM of the PAV by 300 and multiply the result by 144.
CFM of PAV Soffit vent in net square inches805 CFM386 net square inches1,610 CFM773 net square inches2,415 CFM 1,160 net square inches

A minimum of one square foot of inlet area for every 300 CFM of HVI-Certified fan capacity is required for proper fan operation.
  • Use only soffit vents as fresh-air intakes for powered attic ventilators.
  • Do not use gable vents because it's possible to draw rain and snow into the attic.
Now, lets do the math:

1,344 square feet of attic floor space x .7 = 940.80 CFM Required

941 divided by 300 = 3.137 square feet of Intake Ventilation required.

Ventilation products are rated by NFVA in square inches, so the 3.137 needs to be converted into square inches.

3,137 sq ft x 144 square inches per square foot = 451.73 square inches required.

If you install a continuous 3" wide screened in soffit intake vent, which provides an NFVA of 9 square inches per foot, then you would need 50.19 lineal feet, so call it 52 feet as the absolute "Minimum" amount required.

I would run continuous soffit venting for intake the entire length of both sides, which would be 96 feet. More is better, otherwise you may suffocate the PAV from performing properly, as you have already read in the other topic you posted in.

Remember, you will not be buying a PAV with the exact calculations provided and it will be providing a larger amount of CFM, so that is another reason to go with the additional soffit fresh air intake ventilation.

Ed

Last edited by Ed the Roofer; 06-16-2008 at 04:58 PM.
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Old 06-22-2008, 06:36 AM   #5
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Ed- You are the best, thanks!
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Old 06-30-2008, 01:46 PM   #6
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how cheap can i go on a fan ????
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Old 06-30-2008, 06:44 PM   #7
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Powered Attic Fan question


Check out e-bay.

Or, if you want a really cheap one, wait until a roofing crew is removing one to put in a ventilation system that works better and see if you can get it before they throw it away in the dumpster.

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Old 06-30-2008, 07:50 PM   #8
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ED ,,,what about a better quality window fan ????
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Old 06-30-2008, 08:04 PM   #9
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I think you will have to ask that in the Window Fan forum. This forum is for roofing related questions.

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Old 06-30-2008, 08:10 PM   #10
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Quote:
Originally Posted by Ed the Roofer View Post
I think you will have to ask that in the Window Fan forum. This forum is for roofing related questions.

Ed
and where is that ,,,,,what do attic temp's average in say 92 deg sunny days with just gable vents at each end ??

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