How many soffit vents are generally used to ventilate an attic? We have an attic that is about 2400 sq ft total with an 8/12 pitch. The electricians installed 2 fans, 1 on each end of the house. How many vents should be installed for good ventilation? A local roofer recomended 20 all around the house. Does this sound about right to you guys?
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Remove the fans, install ridge vent, install about 1000 sq in of soffit vent.
happy?
Where does the supply air come from for those fans? A few soffit vents won't do. The fans I've seen need something like 4 sq. feet of free open space per fan. What I've seen is one fan on one end and a gable end vent on the other side of the house. Or am I not understanding what's going on here?
Install continuous soffet venting for now.
When the time comes for a new roof, kill the fans and install a ridge vent.
J. D. Reynolds
Home Improvements
We don't normally use fans but we don't live in the desert either... Anyway,
From International Residential code:
R806.2 Minimum area. The total net free ventilating area shall not be less than 1 to 150 of the space ventilated except that the total area is permitted to be reduced to 1 to 300, provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1 to 300 when a vapor barrier having a transmission rate not exceeding 1 perm (57.4 mg/s•m²•Pa) is installed on the warm side of the ceiling.
I cut and pasted the above paragraph from this web page which, at a glance, looks pretty good: http://headrick.net/buildcode.html but has a bunch of commentary toward the end that seems unnecessary to me.
Bottom line: I'd go with continuous soffit vents at all horizontal eaves. Insulation baffles are necessary to keep loose fill attic insulation from interfering with air flow too. BTW - we don't use ceiling vapor barriers in my state - I think they use them up north.
Not bad free info from a crook builder who makes 233% profit on houses (or whatever it was that you insinuated) - Hu?
Well I still think there are GCs making a fortune, but I am starting to think its mostly these large companys that are doing it. Example, my aunt is a broker in N. California with a very large company. They are building houses by the nieghborhoods there and she says the GCs are making somewhere in the ballpark of 100G per house. But anyway, thanks for the free advise. That is what I like about this place and why I am still posting questions here. Why do you think ridge vents are better? I can understand the heat will naturally go up and stay cool in the summber but I like the idea of a temperature controlled fan to keep the temp of the attic at desired levels. For instance, in the winter do you really want all the heat escaping the attic? Seems to me you would want it warmer up there which fans could do as they would be shut off. No?The roof is a hip roof with a fan mounted at the peak of the roof, one on each side. So it will get its air supply from the soffits. I'm just trying to figure out how many soffit vents would be needed. As stated earlier a local roofer suggested 20, so I think I'll just go that route.Thanks again,Johnny
Edited 12/14/2005 9:15 am ET by Johnny1985489
I am not a roofer ,But I think that the fans are fine, as long as ( and very important here) you properly calculate the proper free air ratio ( faa ) I also think you want the opposite, to keep the warm air out of the attic in the winter.ect This leads to frost, which leads to moisture problems. which can lead to mold, and drywall warping.
I can understand the heat will naturally go up and stay cool in the summber but I like the idea of a temperature controlled fan to keep the temp of the attic at desired levels. For instance, in the winter do you really want all the heat escaping the attic? Seems to me you would want it warmer up there which fans could do as they would be shut off. No?
That's precisely the question which Building Sciences tries to answer. They have a rather informative web site with some interesting info on this topic.
Now, here's the trick question for where you are in NM--does leaving the windows on your car cracked open really make a difference in the temp in summer? We are talking about the same sorts of air volume change.
Ok, in our cars, we get a 4-5º temp change in summer. That's down to 105-110º in our cars; about 130-135º in our attics.
Now, in NM, the real question becomes, does the combination of altitude and snowy weather make the winters wet enough that there is an actual risk of ice dams. (But that, then starts up the argument about whether vented roofs have no ice dams, a subject with much emotional content, and not nearly enough science.)
Now, my rule-of-thumb for venting, is that the areas ought to match. So, cfm "out" needs to match cfm "in." A lot of the time, a continuous soffit vent is much better than almost any quantity of round (or, heaven help us, rectangular) vents--which probably ought to be in every other or every third rafter bay, closer to 20-30 the side, not total.
But, I think you are on a good track, asking these questions.
Occupational hazard of my occupation not being around (sorry Bubba)
For instance, in the winter do you really want all the heat escaping the attic? Seems to me you would want it warmer up there which fans could do as they would be shut off. No?
Simple answer....no.
Once your heat has escaped the living space and reached the attic, it`s doing you no good. Preventing it from getting that far is where the concern ought to be.
Ideally, you want the interior surface of the roof decking the same temperature as the exterior surface. So, that heat you are looking to trap inside your attic is actually working against you.
J. D. Reynolds
Home Improvements
>> Why do you think ridge vents are better? <<
My statement about powered ventilators was rather non-committal - or so I thought. I just don't have much experience with them, other than a house I owned in the past.
I'd just prefer to have a passive device like ridge vents. No mechanics to break down, no directly associated power bills - etc.
Now that you said it is a hip roof, I'd say that matters. Ridge vents don't work so good with hip roofs for one simple reason - there isn't much ridge. There is some kind of 'hip vent' but my roofer told me he would not install them. That was all I needed to know. A house I'm building right now is a hip, and the roofer tried to talk me into a power vent. I ended up going with 6 "pot vents" instead - or at least that's what I call 'em.
Anyway, I don't necessarily feel powered ventilators are inferior - just not my choice.
In my experience the code rules on attic venting have little basis in reality.
Talk to an local experienced home inspector to find out what works in your area.
>> In my experience the code rules on attic venting have little basis in reality.<<
Wanna expand on that?
>>>> In my experience the code rules on attic venting have little basis in reality.<<>>Wanna expand on that?Sure: I look at about 400-500 attics a year, looking for, among other things, signs of inadequate venting. (Tough to see signs of over=heating <G>)I see plenty of attics with "inadequate" venting but no signs of excessive moisture, and a fair number of attics with code compliant venting which still have moisture problems.In my opinion and experience, preventing moisture accumulations in the basement/cellar/crawlspace which lead to excessive moisture in the attic is much more important.
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Sojourners: Christians for Justice and Peace
Seems like this one could end up being a venting debate . <G> ZRemember that one?
Ive reworked a lot of houses and looked at still yet a bunch .
Ive found shingles to fail prematurely blamming a defective power ventilator . No telling when the last day it worked .
Ive found upgrade PVs installed with the last new roof and even found cardboard taped over the gable vents with an improper number of soffit vents to NONE. One such house I bought had whistleing outlets. That was cool to observe.
Just when you think people cant screw somthing up they invent a way unfounded.
Lady told me she had lost her lights in all bedrooms and sparks were flying in one of my houses . I took the lights apart which were enclosed fixtures and found 2 100 watt light bulbs and the light wiring fried. I bet Ive got a hundred of those stories.
Back to the PVs , negative pressure has been the worst charge by me finding fault , but there are several more .
Tim
Ditto ph882 and JD's posts.
The source of moisture in the winter is from the people in the house breathing and the more obvious evaporative sources. Needs to be vented out of the attic to avoid winter frost and its problems.
I think continuous soffit venting is the best, however, it may not be practical due to the existing method of soffit construction. In that case you will have to add individual vents - calculation required. The roofer said 20 - but what size?, even at the local big box, I can buy 3 different sizes in a round style ranging from about 1" to 3-1/2" - obviously they do not all supply the same FVA (Free Vent Area). The fans will only expel as much air as they can draw through the vents.
Jim
Never underestimate the value of a sharp pencil or good light.
Whatever the soffit vent sizes/quantities are is a matter of code calculations. Follow them and you've done your best. BUT....there are many things that make for proper ventilation.
If your roof decking is installed "high" off the facia (exposing a gap) and you have no drip edge, you have a continuous soffit vent and red wasp "highway" to your attic. I just cured this on our 20 year old home.
If your soffit vents are 8"x16" and are properly installed, you will have approx 6"x14" of open venting. HOWEVER, if the installer decides to cut 4"x8" holes in the soffits, the venting is reduced by more than half. I just fixed that on the same house.
You can have all venting installed correctly and STILL have poor venting when the insulation installer pushed the batts far enough to close the gap between the top plate and roof decking. I just fixed that as well.
I hate roof penetrations (leaks), so I prefer a gable fan with humidity and temperature relays combined with plenty of free soffit venting.
Troy Sprout
Square, Level & Plumb Renovations
We use James Hardie perforated soffit. It is continuious around the whole, and the install kills two birds with one stone: soffit and venting.
There is a formula for you question. You can probably find it online doing a search for attic venting.
We do not use fans, we use continious ridge vent. We also use Tech Shield decking. It has a radiant heat barrier applied to the underside of the decking from the factory, good for hot climates. Our cost is $2.00 higher per sheet for the Tech Shield.
How is your home coming? Are you stll building for $42.00 per SF?
My home has barely begun. It was my fathers house that we just finished with. We are working on some final touches still like soffits ect. My house had the footings dug about a month ago but after many weather delays and plumber mistakes we are still waiting for the slab. The plumber is fixing some things as I write this so if all goes well the slab will be poured tomorrow or monday.
The most important thing to start with is your climate. Do you have freezing winters? Snow? High humidity? How much rain per year? How much time with the heat on vs how much with the A/C on? Things that are critically important in one place are irrelevant in others.
-- J.S.
Well we just moved here less than a year ago but what I know of the climate so far is this. The summers are ungodly hot! 100+ for 2-3 months straight. Occasionally humid as we get localized electrical storms during the summer. There is more rain in the summer than winter. But not much rain all year, couple inches throughout the summer, maybe 1 in winter. It hasn't rained in a couple months and this is december. So the AC will run most of the year. During the winter however it does get below freezing. We had a couple weeks that were low 20s every day. One day hit 9. So its generally a hot dry climate with occasional humidity and occasional rain. Even some snow once every couple years. But probably not enough to stick. We are 3500ft up. Maybe that helps I dont know.
Edited 12/15/2005 8:13 pm ET by Johnny1985489
>> We also use Tech Shield decking. <<
I saw a sample of that stuff at one of the building supplies and was intrigued. Seems like it would be a great product for largely cooling climates. At $2 a sheet the upcharge seems somewhat negligible - in most cases, under $200 a house. I wondered though if it might increase the temperature at the exterior roof surface.
Thoughts?
Overall, it cools the Attic and is suppose to prolong the life of the roof, according to the manufacturer.
Did you see a light bulb test, set up with two floods and a thermometer in each cavity. One cavity has regular decking and the other Tech Shield. It can produce temprature variables of 30+ degrees.
$2.00 per sheet is our current price, I have seen it as much as $4.00 per sheet.
Most of the homes we build have 220+ sheets of decking. We offer the product as an upgrade. So, the upgrade cost can be $800.00 + profit and overhead.
220+ sheets - that some pretty sizable roofs. I'm guessing most of them are 12:12... or so. Do you build a lot of 1 story houses? BTW - you need to put some more of these houses on your web site... I read your description of one in the business/FHB authors wanted thread... not too shabby ;-)
Actually, most are 8/12. Some of the homes I design, and I try to be trendy (steep roof) and be carpenter friendly (8/12).
About 1/2 of the homes we have built in the last two years have been 1 story, many have large covered patios, guest house connected with covered porch / breezways and 3 + car Garages.
One of our homes, currently under construction has a porch and gazebo in front, large rear porch, 24 x 26 porte cochere and a 3 car garage / shop area with guest quarters above.
I have submitted request to our webmaster that I needed to update, add pics and more content to the site. No response for several weeks, he is about to be fired.