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?
I found a web page that talks about the relationship between the amount
of ridge vent and number of soffit vents.
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?
A ridge vent only goes on the highest ridge or ridges on your roofline. If
I understand your situation properly, it would go on the peak of the roof.
The guidelines are reasonable, but this is one case where more may be
Ridge vents are very common here in Atlanta. They seem to be a good solution
for allowing air to circulate through your attic and reduce attic
You will have to see if the roofer is willing to install the soffit vents
and factor that into the equation. I would do my own math, however. I had
about half of the required vents installed nearly 15 yrs ago and the time
has passed quickly meaning, I didn't add the difference. Now it's time for
new soffit and fascia so we'll be using perforated soffit this time. I
don't believe I can over-ventilate.
I already have soffit vents. My question is do the ridge vents work as
well as the little square vents every so many feet and on a hip roof are
the ridge vents on every ridge. I have I have six ridges. I'll talk to
the roofer in a day or two to see how many ridge vents he planned to
install and then as you say do the math to see how many soffit vents are
needed to take advantage of the ridge vents. From that web page I
posted, you need as much net free vent area in soffit vent and ridge
vent to ventilate properly?
The roofer is installing the ridge vents when he replaces the roof.
Since I have a hip roof, I have six ridges. Each ridge gets a vent?
If you look at that web page I posted, they imply you need an equal
amount of net free vent area for the soffit vents and the ridge vents or
the attic might not vent well? You can have excessive soffit vents but
if you don't have enough roof vents then the air got no place to get out?
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
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.
Yes, I know that. The OP had concerns about it, so I was
trying to explain why perfection is impossible and why it is
not a disaster if perfection is not achieved.
OK, first take the case of ridge vents with NO soffit venting.
The air in the attic heats up, hot air rises and passes out
of the ridge vent. Now there is negative pressure in the
attic. Air needs to come from somewhere. Generally, it will
come back in through the ridge vent, but the negative air
pressure in the attic tries to draw air from wherever it can.
So if you have ceiling light fixture with a bad cut around
the box and it has not been sealed with foam, then air will
pass through the openings drawing conditioned air from the
house. Same with any opening that can go to the attic space.
It is the same with less soffit vent sfootage than ridge vent
sfootage, only less so. So you want more soffit vents than
ridge vents to lessen this effect. It never entirely goes
away, but is negligible with more vents on the soffit side.
If you're in Florida, check with your local building department for their
advice and code requirements, and that the ridge vent installation meets
Miami-Dade requirements. Ridge vents may be the weak link in roof
construction when hurricanes are involved, and in the 2004-5 storms many
ridge vents were the initial point of roof damage, when they peeled off in
the wind like zippers. Regards --
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