Attic vents

[Sidney Schwartz]

I read with interest this exchange from an earlier post....

"Ridge vents work with soffit vents, and suck air in the bottom and out the top. Gable vents work with other gable vents, and air blows in one, and out the other. Mixing the two makes the situation complicated. A single gable vent and a ridge vent is certainly better than just the ridge vent, but probably not as good as either a pair of gables at opposite ends of the house, or a matched set of ridge and soffit vents. Do you only have one open gable? If you have two, and they're working, why are you installing a ridge vent? "

Our roof was replaced 12 years ago. The original roof had gable vents and soffit vents, no can vents. The outfit that replaced the roof installed can vents. One section of the roof already needs to be replaced. I had a different roofer come out to assess why. He said cause was condensation due to improper venting, and that the cause of the improper venting was that the gable vents were not blocked off when the can vents were installed. According to him, having both gable and can vents made both less effective. Being ignorant about all this, I'm trying to get some confirmation that what he told me is in the realm of possibility. The roof obviously need to be replaced in any case, but I'd like to be able to make sure that we don't just have the same problem. There's also the question of whether or not the people who installed the roof failed to do it properly.

Thanks in advance for your assistance.

[tbasc]

Look at the Building Science Corporation web site for climate specific recommendations based on research.

Your problem may be the result of moisture from plumbing vents ending in the attic or general moisture moving into the attic from the rest of the house.

TB

[PipeDown]

That does seem counterintuitive.

I imagine the point was that the momentum of moving air created by convection is more effective than just passive diffusion through a larger ventlating area.

In an actual roof, convection, conduction and diffusion of heat would all play a roll so its hard to say if he was actually right or just mis-using some wisdom incompletely passed on to him.

Depending on where the condensation occured, any number of causes might apply. To guess would require writing a book.

My roof has soffit and gable vents with one having a fan on a thermostat and several can vents. I live in CA so it never freezes and condensation is a lesser concern. Just about anything that keeps the water out works here.

[Ronald']

The gable vents short circuit outside air to the roof vents. Therefore the air flow from the soffits is reduced. Block the gable vents and air flow from the soffits will increase and better displace the total attic air volume.

[CJT]

Interesting theory. Has anybody done any scientific research to determine whether it's true?

[Sidney Schwartz]

The roofer who told me this has been in business for more than 20 years and is a "master installer", whatever that means. That isn't scientific proof but at least lends some credibility to the theory. I'll probably have another roofer look at it and see if he confirms what the first roofer said.

Sid

[Ronald']

I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.

[Ronald']

I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.

[Ronald']

I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.

[PipeDown]

Makes complete sense to me. Hot air will rise and exit from the highest point (ridge or can vents) and cooler ari will be brought in to replace it. Since air like water and electricity will always flow along the easiest route, it would come from the gable vents as they would be closest.

If you want a scientific experiment, try a few well positioned smoke bombs on a hot but otherwise still day.

[jdk]

when they did my new roof they closed the gable vents and put in ridge vents along with the soffits already there.

[CJT]

The devil is in the details, methinks. Air could as easily flow from both the gables and soffits to the ridge and result in a cooler attic temperature.

[Sidney Schwartz]

I would think that air flowing from the gables to the ridge vent would be almost useless because it is hardly traveling any vertical distance and therefore leaving most of the attic air untouched. The gable vents are supposed to be where the air exits, not where it enters. Everything I've read says that the air should be entering through the soffits, rising along the inside of the roof, and exiting the vents. If the air coming in the gable vents is reducing the airflow from the soffits, I can see how this would cause the overall ventilation to be inadequate.

Sidney

[Sidney Schwartz]

That's exactly what I was told should have been done with our house but wasn't. :(

Sidney

[Sidney Schwartz]

Many thanks to everyone who has answered so far. My next problem will be going to the roofer who did the job 12 years ago and trying to get him to make good on the work that was done incorrectly. Oh joy.

Sidney

[mm]

I am not a pro, just a shlepper here.

Are can vents like roof fans, but without the fan?

It seems to me, that if you have a room with 40 people and one door, then all 40 people will leave through that door, but if you put in a second door, then maybe 22 people will leave through the first door and 18 will leave thorugh the other door. So each will be less effective than the first door was. But for the life of me, I can't see how two doors or more vents could, in total, be less effective than one door or one set of vents.

Of course, I'm no pro or anything. I don't even know what a can vent is. (Is it anything like a convent?)

This also makes me wonder how you could have too much condensation

*because* of too many vents. Is condensation caused by other things than taking showers and maybe cooking in the house below? The instructuions for my roof fan said that I should provide a manual switch to turn it on when it is humid in the attic, even if it isn't hot in the attic. That was 23 years ago. Maybe by now there would be a humidity switch.

[Sidney Schwartz]

I think the more common term is roof line vents. They're the little square vents that are placed parallel to the ridgeline and a couple feet below. They are not powered.

Apparently it's the placement of the vents and not the number of vents that's causing a problem. Ideally the cooler air should be entering at the lowest point, which woudl be the soffits, and exiting at the highest point which would be the ridge or roof line vents. The gable vents are interfering with this flow of air. I guess it's like having a pipe with a leak in the middle...not everything that goes in one end is going to exit the other end.

We do have a powered attic fan that runs off a thermostat. The fellow who diagnosed the vent problem commented that a powered fan should not be necessary for the small area we're talking about. It was installed by the previous owners who tended to overdo things.

Sidney

[mm]

OK. Closer to square sardine cans. I was thinking of spinach cans.

Well, I see what you mean, but I would want to hear this from a second person who does this or designs this. I can imagine a house with cross ventilation, especially on the second floor there will be a breeze. Say a third window or fourth gets opened in a room in the middle. Is that really going to lead to less fresh air in the second floor? I can hardly believe that.

BTW, do you also have soffit vents?

Baloney. This seems like a not-unusual belief, and roof fans themselves are not that common, IME, but my experience runs counter to the notion that powered roof fans are not tremendously valuable. My townhouse is only 700 sq. feet per floor. I have a ridge vent that runs the whole width of the house, an inverted V with outlets towards the front and towards the back. I have soffitt vents that run the whole width of the house, both in the front and in the back of the house.

I live in Baltimore, and whenever the high on a given day was 80 or more outside, it was too hot when I got home from work to go upstairs, and too hot to sleep there, without using AC, which I normally don't use. I would stay downstairs, sleep in the basement, and go up in the morning to wash and change clothes. After about three months of that, I got the fan in in early September and the temp of the second floor went down from about 95 to 85. The temp of the attic went down from 140 or 150 to about 95 or 100. It wasn't too hot up there to work at my desk, and later, by not covering myself up with a blanket and sometimes not wearing clothes, I was able to be quite comfortable sleeping. Without AC. Even when the high was 95 or 100 on a given day.

BTW, after about 15 years with the fan, I was up there looking at something and saw that my soffitt screens were covered the width of the house with a layer, something like the layer on a drier lint screen. It peeled or brushed off. Made from floating plant seeds and I don't know what else. ISTM that shows how much more circulation there was to my attic with the fan than anyone without a fan gets.

There is one new townhouse development near me. I think that each "house" is two houses, one facing the front and the other facing the back. And each roof of these two houses has 4, well now I have to go look again. I thought they were fans, but maybe they are just vents. When I went to look the first time, I talked to two owners and neither knew what he had on his roof!!

[mm]

On Fri, 7 Apr 2006 00:14:24 -0700, "Sidney Schwartz" wrote: geline and a couple feet below.

I read your answer to me first, but now I've read some other answers.

I might be convinced eventually that the gables should be closed, but no way is the roof fan not the best part of ventilation that you have. The temp of my upstairs and the lint on my soffit vents proves that to me.

What I would do, starting now, would be to keep a chart of the outside day time temp, maybe just the high, and whether it is sunny, partly cloudy, etc. (because solar radiation is at least as important as air temp), and what the temp in the attic is. If it is too hard to go up there you could measure the temp say, one foot from the ceiling (but that only works if you don't have the AC on. If you have the AC on, that will control the temp in the second floor.)

Then, when the gables are shut (in a manner that can be undone, because there is no need under any theory I can think of for anything more permanent.) you can continue to keep this chart, and I think 5 or

10 days of data before and 5 or 10 days after the gables are shut will be enough to show a clear difference, if there is one.

BTW, what problem are you trying to solve? Why do you have men looking at the attic at all? Was this comment offhand, or in response to a particular problem? Often times we find that deep background (well, not as deep as where you spent your honeymoon) can be a big factor in answers.

[mm]

I would be interested in other opinions about whether this** is making good. He gave a price based on what he did, or planned to do. Did soethign happen that made him not close the gables when he had intended to. Or did he just think they didn't need closing?

If the latter, why should he have to do that now for free, or for whatever compromise price you have in mind?

**I'll admit I'm not positive about what you have in mind.

It's not like he did something but did it wrong. He didnt' do anything to your gables, right? So he didn't charge you for doing anything to them, I would hope.

To close a gable, doesn't one just nail a piece of plywood to the inside. But I think the big problem might be challenging his opinion of what you need.

Like I say, what is the underlying problem? Is it hotter in the attic than it should be? It's not hot yet in Baltimore, and I don't remember what the fan thermostat is set at, but I think my attic gets to be about 10 degrees hotter in the middle of the day than the fan is set for, or than the outside temp, whichever is higher. But I've never measured. I will if you want me too.