My attic has 6 eave vents and 6 roof vents, all on the same side of the roof. I’m not getting any air circulation elsewhere in my attic. I want to add more vents, so my question is two fold.
Is there some standard calculation to determine the number of vents needed?
Is there such a thing as too much ventilation?
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Replies
ridge vent
I was thinking of gable vents.
Gable end vents are quite out of vogue and now considered marginally effective.
Go with ridge vents and add some soffit vents.
Matt
>>Is there some standard calculation to determine the number of vents needed?
1 sg ft free vent space for every 300 sg ft attic floor space (or 150 if no VB in ceiling) divided 50/50 base and peak.
OTOH, that formula bears no relation to the real world I see doing home inspections.
How do you know you aren't getting any air circulation?
The roof is roughly shaped like a C from a birds eye view. The roof vents and soffit vents are both on the same plane of the roof (the "back" of the C), so I don't think that there would be good circulation in the "open" area of the C.
Does that make any sense?? I attached a quick sketch, but don't know if it will go through.
In this case gable vents might make some sense. The problem with them, though, is that they let the air in (or out) fairly high up, with no provision for air to enter lower. If possible, adding eave vents along the gable sides would be recommended.
This is going to be a retrofit operation, hopefully without any re-roofing involved. I think I can cut in some eave vents also.
Thanks so much to everyone for the info. It helps allot.
I guess you guys use a different building code - without looking it up, 1:150 unless at least 50% and not more than 80% is in the upper 1/3 of the attic roof space then it can be reduced ot 1:300. That could be just a NC thing though... OTOH I know that home inspectors generally don't deal with code - or at least not for existing homes (non new construction). Matt
http://www.airvent.com/homeowner/products/staticVents-roofLouvers.shtml
Correct me if I am wrong, when the code says 1 sq ft per 150 or 300 sq ft. That means "net free area", which is not the size of the hole but is actually generally 1/3 to 1/2 of the hole size. The above link is for some standard static (read non-powered) roof vents. These vents generally provide about 50 sq inches (about 1/3 of one sq ft) of net venting.
Keep that in mind when calculating the number of vents!
FF
A builder told me something that made a lot of sense--don't mix the ridge/eve vent system with roof/gable system. He says the ridge/eve system acts like a syphon and adding vents anywhere else is like poking holes in the syphon hose. Poke holes and you lose your syphon. Made sense.
Agreed - another way to put it is that if you have a eve/redge vent system and introduce gable end vents, for example, the gable end vents short circuit the even flow from eves to ridge, and you end up with dead air pockets. Matt
Generally the best configuration is a combination of eave vents and ridge vent.
Depends somewhat on where you live, but if you figure about half a square inch (some say an inch) of ventillation for each square foot of attic space you'll come close to recommendations. The recommended number of square inches should be divided about equally between eaves and rooftop, though with a ridge vent you'll usually end up with more than the recommended amount on rooftop (which is not a problem, provided eave vents are the recommended amount).
The square inch rating of a vent is generally stamped on the vent somewhere. For ridge vents it's specified in square inches per foot.
For standard roof construction the only time you can have "too much" ventillation is when a tornado takes the roof off.
" I’m not getting any air circulation elsewhere in my attic. I want to add more vents,"
Why do you want air circulation? What problem do you have now that you are hoping more air circulation will solve?
Rich Beckman
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