Calculating Ventilation Area of Attic
Calculating Ventilation Area
OK, to determine how much ventilation area you need for your house, all you need to know is the square foot area of the attic to be ventilated. Let's use the following simple ranch house for an example.
The house measures 30 feet by 50 feet. There is a 22 foot by 26 foot attached non-heated garage. In reality, the garage doesn't figure into the calculations. That doesn't mean you shouldn't ventilate it, it just means that I will not include it in my calculations:
30 feet X 50 feet = 1,500 square feet
OK so now we know the square footage of the attic area to be ventilated. Now, let's figure out how much ventilation space we need. Remember, the minimum area requirement when using a continuous system is a ratio of 1:300. So, in our example, we will divide the 1,500 square feet by 300.
1,500 square feet / 300 = 5 square feet
See how simple that was! We know we need 5 square feet total of ventilation area. Based upon just about everyone's recommendations, 60 percent (or 3 square feet) of this should be in the soffit area and the remaining 40 percent (2 square feet) should be located at the ridge of the roof. Let's see how a standard ridge vent product will calculate out.
Standard ridge vent produces 18 square inches of ventilating area per lineal foot. Our roof is 50 feet long, so using a standard product we will get 900 square inches of ventilating space. Nine hundred square inches equals 6.25 square feet. So the ridge ventilation has exceeded the minimum requirement. That's great! But what happens if we choose to use those individual metal pot vents instead? The calculations before indicated that we needed 2 square feet of ridge or upper roof ventilation. Two square feet equals 288 square inches. A normal metal pot vent produces only 50 squares inches of free net area. So, we would need at least six metal vents in addition to the lower soffit ventilation to meet minimum ventilation requirements.
If you choose not to use continuous ventilation, guess what? The required ventilation area doubles! This means that you would not have any soffit ventilation. It then means that you would need a total of 10 square feet of ventilating area. How many metal pot vents you may ask? Believe it or not, you would need 29 of those individual metal pot vents to meet minimum requirements. Have you ever seen that many on a roof before? I know I haven't!
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Martha Hooper 11 Apr 2008, 10:55
Does having the heating system located in the attic contribute to the heat
build up and require additional ventilation. Also, my home has a mansard
roof line and I don't believe the builder installed adequate ventilation.
The home is located in South Carolina where it does get extremely hot!
Martha, 17 Apr 2008, 07:22
The furnace in the attic, in my opinion, would not add to the heat load. My
guess is that you are talking about summer months when the furnace is not
on. You should read my columns about Turbine Vents, and install several.
Mike 19 Apr 2008, 11:58
Hi Tim,
I have a question about my ridge vent. My upstairs rooms get very hot in summer. The attic is 40x24ft and I have a ridge vent that goes along the 40ft ridge. However, the slit in the attic for the vent is only 1 inch wide and not centered, but is one side of the ridge. According to the suggested minimum ventilation area - it is OK. My question is: is this slit too small to allow the vent to work properly? I live in East Tn - it is already getting warm upstairs and it is only spring. Thanks and great site.
joseph1 01 Jun 2008, 16:19
Tim,
Can you have too much ridge venting compared to soffit venting area? In your example above, 40% of 5 square ft equals only 2 square ft. and you would need only 5 of the 9 inch dia metal pots? This assumes soffit vents provide 60%. What is the open area of one of the standard soffit vents? Sorry I don't have exact dimensions, but look like about 3 inches x 12 inches. Thanks,
victor zammit 30 Jun 2008, 09:28
Hi Tim:
I live in East Tennessee. I have two ac units. The one upstairs is a ac?heatpump unit. My concern is the heat builup in the attic. the dimensions of the attic are as follows: 56 feet by 20. the house has a 12/12 pitch and the height at the center is approx 3 ft. i have 8 pot vents. I am thinking of replacing one pot vent with a 1500cfm exhaust fan. i also have soffit vent,BUT I have r30 insulation along knee wall and celing and this may impede flow of air to attic area. do you have any suggestion as to whether i need more intake or not. my soffit area runs the lenght of the house on both sides and is approx 18 inches wide. thanks vic zammit
Martin Pothier 30 Jun 2008, 22:24
I have a gable installed exhaust fan in the attic. The attic is about 1400
sq ft. How much vent area is needed for this attic.
Martin
Patrick Campbell 22 Jan 2009, 14:20
We have an attic with walk up stairs, which we would like to convert to
living space.
Right now the attic has poor insulation on the floors, ceiling and rafters. The attic also houses the furnace and A/C air handler. If we sprayed insulation as thick as possible on the attic rafters and attic walls, and conditioned the attic, would we still need ventilation in the attic? Why is attic ventliation necessary?
Joe Conroy 18 May 2009, 18:09
Tim,
My MasterFlow power vent dosen't turn on anymore. I suspect its burnt out. Given the following, would you Not replace it: Sq ft of attic = 4700 (plus 525 above attached garage); NFA of about 150 sq in in the soffits (continuous in only 3/4 of contiguous soffit); 6 RT 65 louvers for ~ 360 NFA. The Pro 1 Power Vent by GFA calls for 760 NFA for a 3000 sq ft house and I think that means in the soffits or facia. I'm guessing the non working Power vent burnt out because of insufficient air. What's your recommendation. Thanks.
Bill Hammer 02 Jun 2009, 12:27
How does having an A/C unit in the attic change the amount of air I must
exchange each hour?
Does it change the amount of intake vents I need?
farokh 11 Aug 2009, 19:15
My almost 55 years old house is in WestVancouver /BC/Canda.The house attic
needs insulation(R-50).There are 4 ventilation on soffits(E&W seides)and
three ventilation on the roof.Does the attic needs ridge
ventilation?Attic=1400sqft.lenghtof house38ftx18=684:144=4.75sqft.Pleaselet
me know,if the attic needs ridge ventilatin? or it would be enough that i
install two soffit vents on N&S and a vent on roof?Thank you
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