Just curious, is there a formula that will tell you how many sq/in of vent ratio to sq/ft of attic space?? A lady called me the other day and said her attic was damp. She says she has vents and an attic fan w/thermostat. She says she needs more vents. I think the ceiling is insulated but not the roof so I assume this is a cold space. Gonna have a look this Sat. and see what’s up.
Thanks,
Bill D.
Replies
I think Cor A Vent's website has some calculators for figuring this sort of thing if you're looking to install a ridge vent. If she needs more ventilation that is.
http://www.cor-a-vent.com/v600.cfm
Scroll to the bottom.
IRC says 1/150 of the area of the attic space. Reading the code further there is an opportunity to have less depending where the vents are located. But I read that part of the code three times and I could not make heads or tails out of what it meant.
Best is to provide soffit venting to allow cool air in (continuous would be best). Then I do like to use a continuous ridge vent like Cor-A-Vent to let the heated air out. This way the venting is natural. However, there's a problem if your roof is a hip with a short or no ridge. Then you have to use roof vent caps, or a whole house exhaust fan set on a thermostat.
I've seen small dormers with louver vents added to roofs to provide attic venting.
Uke,
I see cor a vent has a vent for hip ridge.
I've wondered about that here in TX. I have lots of soffit venting but only have 3 whirlybirds on the roof.Anyone ever use the hip ridge venting??Pete
I never saw the hip vent before. Interesting.
Has anyone out there in the Midwest, or more specifically, in the Chicago and surrounding areas ever used a continuous hip vent such as available from Cor-A-Vent. I'd like to know the feed back on that from he trades.
Anyone ever use the hip ridge venting??
I try not to. Put more into commercial work than residential, though.
But, that's more due to the crews installing the material create more problems than the vents solve.
You have to be super careful in dry areas with intense rainfal (like around Austin-Leander) as they will leak in a texas thunderstorm.
Some of which goes back to iffy installation, too.Occupational hazard of my occupation not being around (sorry Bubba)
A wet attic? Meaning wetness under the roof deck? Has she been up there? I guess you'll see for yourself.
If it's dampness under the roof deck, then it could be that there lack of soffit venting results in the fans drawing up air from the house. The moisture also could be due to improperly connect bath fans. Clearly, they ought not to dump into the attic. Nor should they exit the house right under soffit venting, or the moisture gets sucked right up into the attic.
If there is no ridge vent or enough vent area high up, even just a lot of water use in the house (bathing, cooking), with a poorly sealed boundary between living space and attic, can dump water vapor up there to condense.
Then there is crawl space moisture running up through wall cavities and into the attic.
You'll have to report back on the overall configuration of the house and the lady's living habits. Her problem may not be a matter of vent area at all, but more a matter of excessive water vapor from below.
<lack of soffit venting results in the fans drawing up air from the house>i'd say you got it. OP should check that out before anything else. put in a bit of soffit venting and seal a few holes to the warmth below and see if it improves. Make sure the soffit vents can actually draw into the attic - rafter heels not framed closed or blocked by batts. j
Yeah, generally something on the order of one square foot vent for every 150 square feet of roof (which works out very close to one square inch of vent for each square foot of roof).
But more importantly, the venting should be split about 50/50 between rooftop and eave, and the eaves should have the venting fairly evenly distributed around the perimeter. The total square footage of vent space isn't as important as the distribution, such that there are no "dead" areas, especially near the eaves.
Gable end vents are often said to be counter-productive because they "short-circuit" airflow between eaves and rooftop.
And of course there should be no bathroom vent fans dumping into the attic, and at least nominal efforts should be taken to prevent other air leaks from the house into the attic.
Well... the guys above have it almost right. Without looking it up - IRC says that there must be 1 sq foot of ventilation for every 150 sq feet of "floor" space in the attic but that the ratio can be reduced to 1:300 provided that at least 50% but not more than 80% of that ventilation is in the ridge area of the attic space and the balance in the horizontal eaves. I think there is an allowance for a reduction from the 1:150 requirement if you use a ceiling plastic vapor barrier, but I don't know the details of that one since we don't use ceiling vapor barriers in the area where I live.
When analyzing the woman's situation, the first thing I'd be doing is looking for opportunities for more ridge vent. It's difficult to get too much ventilation there. Secondly I'd look at the soffits for more ventilation opportunities. Third I'd go up in the attic to actually see if it seems damp and to look for existing moisture damage but to also see if the ceiling insulation (attic floor) is blocking air flow from the soffit areas to the open space in the attic. And while you are up there, look at any existing ridge vent to be sure it isn't blocked.
Further, if there are gable end vents, that can complicate things, so, if so, report back and we will tell you how to handle that.
Or, better still go to this web site for AirVent Inc and spend some time reading. You will have all the info you need...
PS - what Dan said: >>And of course there should be no bathroom vent fans dumping into the attic, and at least nominal efforts should be taken to prevent other air leaks from the house into the attic. << The first part being extremely important.
And, of course, while up there one should check for evidence of roof leaks.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
Thanks for the links and all the advice.
From what she has said I think the attic is drawing air from the living space and not outside (I think).
Also she says that the fan in the attic is on a thermostat for temp in the attic. Would there be a switch that would monitor humidity?
And last, I know she just had the roof reshingled about 1-2 years ago. I know the guys and recommend them often, they do good work AND (the bonus !!) they come back and fix (for free,if it's their fault) any problems until it's right. I remember being there as they were starting the roof ( as I recommended them for the job) and the sheathing is older 1x6 with gaps of 1/2".
Anywho I'll post back after Saturday (maybe with pics) and give a full report. But, right now I've got to go in yet another attic(what's with all the attic work these days) and install the wood shashes that I restored. Maybe post some of those pics too (if I can figure out that dang dig-gi-tal carmera thingy).
If anyone else has more input I'll welcome it.
Again thanks to all.
Bill D.
As noted, 1 sq ft vent per 1,000 sq ft of attic floor IF there isn't a moisture barrier under the insulation -- 1:300 if there is.
Most times though, in my area, NW Ohio, attic moisture problems reflect basement/crawlspace moisture problems
Other possible causes: plumbing vent terminating in the attic (use your nose to diagnose) bath/kitchen fans terminating in the attic; furnace humidifier set way too high (but that also shows as condenstaion in the living area.
I have inspecterd plenty of houses with "inadequate" venting but no visible evidence of attic moisture problems.
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