JD wrote:
Robert Allison wrote:

JD wrote:

I'm in the process of getting a new roof and the first roofer wants
to install ridge vents, which I wasn't familiar with. I've done some
research and ridge vents seem to be a good idea? I have some questions:

I have a hip roof so do the ridge vents go on every ridge or just the
top or highest ridge?


It goes on every ridge that is over attic space. The "ridges" on the
side of your hips are not ridges, but hips.

I found a web page that talks about the relationship between the
amount of ridge vent and number of soffit vents.

http://www.rd.com/familyhandyman/content/18231/0

If you scroll about half way down the page they talk about Minimum
Venting Requirements. Is the information they provide correct? It
says a 1,500 sq ft attic will need 10 sq ft of venting, half on the
roof and half on the soffits. They say the factor is 1 sq ft of
venting for each 150 sq ft of attic. Is that correct?


It is a good rule of thumb. As much as you can get is the better answer.

The key being one needs the same amount of soffit and roof vent Net Free
Vent Area or you have bad ventilation. The upper vent can't remove more
than the lower vent can provide. One needs an equal amount of in and out
vents. No? It's a flow problem.


In the best case scenario, you want equal intake and exhaust.
In the real world, you want as much as you can get. If you
have a short ridge due to your hip roof, then you may need to
add a couple of passive vents (the ones you refer to as square
vents). You need to add these as high on the roof as
possible. This will increase your exhaust sfootage. If you
didn't have ridge vents before, then you probably already have
some.

Let me rephrase the equality issue. If you don't have an
equal amount of ridge and soffit vents, then your ventilation
will not be perfect. If you have a difference, it is best to
have more ridge or passive venting. But if you don't have an
equal amount, it will not be worse, it will just not be as
good as it could be.

The one drawback to having too few soffit vents (less sf than
ridge vents) is that the attic may tend to draw air out of the
house itself, thus sucking out conditioned air. (This happens
around ceiling lights, ac vents or other ceiling penetrations
and even through outlets if the top plates is not foamed.)

So, if you have to have an imbalance, it is best to have more
soffit venting than ridge. The air will still move out
through the ridge vents, causing negative pressure in the
attic, thus drawing fresh air in through the soffit.

--
Robert Allison
Rimshot, Inc.
Georgetown, TX