where can I find the sq in. vent to cu ft. area requirements for venting an attic…please and thankyou…?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Fine Homebuilding is excited to be the official media partner of the 2024 Building Science Symposium series! This event offers builders, tradesmen, architects, designers and suppliers to discuss topics ranging…
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
There are several "schools of thought" on this subject. Depending on where your house is located and how it was built, the rule of thumb is one (1) ft2 of net free vented area for every 100 to 300 ft2.
In Upper Michigan, I prefer to use the 1 ft2 of net free vented area to every 100 ft2. This should be distributed between the soffit system and the ridge vent.
thanks for the reply - but is that area number actually ft3 or ft2?...that is, is it for an attic vent, or just an insulated roof with an air gap for venting ?
Not cubic feet. You're trying to cool the roof, not the air in the attic. That's just a bonus.
Edited 2/27/2005 7:39 pm ET by Uncle Dunc
so the vents are to cool the roof...and if you want to cool the attic , then you go with an active system, with appropriate cfm rating....gotcha, I think...thanks.
R806.2. The ratio is 1 to 300 if at least 50% and not more than 80% of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet above the eave or cornice vents.
This is from the 2003 IRC One and Two Family Dwelling code
First: the vent "rules" apply to "free vent" area - not the rough opening but what's left after covering with a grill. Rule of thumb - 50% of the rough opening. It should be stamped on the grill/register
Are you looking to make a building inspector happy or actually adequately vent your roof?
In having looked at a lot of attics over the years, I have found little relationship between actual venting and the "rules" and needed venting.
If you're more concerned with mildew/mold growth on the sheathing, get your basement/crawlspace/slab dry first.
View Image
Sojourners: Christians for Justice and Peace
I'm actually asking this on behalf of a friend, so I'm not sure on all the specifics ( actual cu ft) but it is an odd "art" house - about 1000 sq ft, 2/12 pitch disguised by facades, no overhang, and he was hoping to put a slot type vent under the trim at top and bottom of the 2/12 - he wants to avoid any vent on top, if possible, and do the minimum venting necessary....
>>he wants to avoid any vent on top, There are vents specifically designed for that sort of application, although in an "art house" (50's modern?) the detailing will probably vary.He should talk to some commercial roofers who do big flat stuff like factories.
View Image
Sojourners: Christians for Justice and Peace