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rafter vents for the attic how many to install

D

deene

Member
so i am redoing the insulation in the attic and want to install new rafter vents as well these will draw air from the soffit vents to the roof vents, how do i know how many to install? thanks for any insight.
 
The amount of ventilation varies by climate region, but section 806 of the IRC says the minimum vent area is 1 sq/ft per 150 sg/ft of attic. You want the amount of eave ventilation to be equal to the amount of roof ventilation. Look on the Web for a document called Principles of Attic Ventilation by a company called Air Vent and then look at section 3. That would be where I would start (as I am in the process of doing the same thing you are).
 
I agree !! The temperature in your attic should be the same as the outside temperature. The more vents the better. I finally put a [ower vent in and it does a great job< the whole house is cooler on a hot day
 
Bad part about a power vent is if it is run by a temp. thermostat, when or if a fire should get in the attic it will continue to run causing a blower type effect. You should have one that is humidity controlled to afford this which in a dryer region defeats it's purpose.
 
When I was building. We would size the gable end outlets or roof vents according to local regs. The Insulator would decide eve venting. Normal was every other joist/rafter space. HTH
 
X2 on ridge vent, and soffit vent all the way around. I should of put a roof vent on the S end of our house, because we have a hip roof, so the ridge is much shorter than the house. But when there is a breeze I'm sure we get some flow through ventilation. We added an extra foot of insulation when we re-roofed.
 
A lot depends on where you are. If you are in the south where you have long hot summers use as many vents as you can. If you are in the north where keeping heat in the attic is important then use very few.
 
Do not install ridge venting along with any other roof vents present. The air will short cycle from the roof vents to the ridge instead of drawing air from the soffits.
 
The square footage of the inlet vents needs to be about the same area as the outlets. There's no advantage in having more inlet area than outlet, but it's OK to have a bit more outlet area, since air expands as it warms. If you have a ridge vent, you should be able to find the manufacturer's instructions for your particular system on-line; they should tell you how much inlet area you need. Here are the instructions for the popular GAF Snow Country ridge vents.
GAF Snow Country installation instructions PDF
 


Deene, I think that you may be asking about baffles such as "Propa- Vent", which are installed between every two rafters to keep your soffit vents from getting blocked by new insulation. You want them between EVERY pair of rafters.
 
> Deene, I think that you may be asking about baffles such as Propa- Vent, which are installed between every two rafters to keep your soffit vents from getting blocked by new insulation. You want them between EVERY pair of rafters.

Our house, built in the early seventies, has blocking installed between the roof trusses which completely defeats the soffit vents. Can you say 'ridiculously huge ice dams', boys and girls?
 
Got rid of the ridge vent on our place. Snow would sit on it melt and put water into the attic. Put in gable vents to replace it.
 
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