Attic Ventilation..

[JD]

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?

[Kyle Boatright]

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 better.

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 temperatures.

[C & E]

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.

[HeyBub]

Ridge vents are cheap. Put 'em everywhere there's a peak.

You can't have too much soffit venting. Honest.

[JD]

A roof that is shaped like an A would only have one peak, generally so one ridge vent. My roof slopes up from all sides so it has more than one peak. Does that make sense?

From what I can tell, it's a matter of ridge vent to soffit vent ratio. You need an equal amount of both to make ventilation work?

I'm in west Texas and most roofs here have the little square vents every so many feet. I've never seen a ridge vent.

[Robert Allison]

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

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

[JD]

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?

[JD]

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?

[bigjim]

[JD]

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.

[JD]

No.

Choose > Get a fan too!

[Robert Allison]

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.

[CJT]

I don't see the issue. Having unequal areas just means the velocities will differ. You will never have soffits distributed over the entire relevant area, nor is that necessary.

I don't see how that follows. How about some physics?

(This happens around ceiling lights,

[Robert Allison]

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.

[JimR]

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 --