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Question DetailsAsked on 2/16/2014

I have 70' of roof on a single story ranch. How many box vents and soffit vents do I need for proper ventilation?

I have 70' of roof. 20' of the roof is attached garage with unfinished interior in the garage. There is some old ridge vents installed in 10' sections by previous homeowner. Instead of continuous ridge vent, they have 5 to 6 feet separating each. I can't imagine a licensed roofer would have done this. One is leaking. So due to funds for a new roof (2 layers on it now), I plan on tearing off the ridge vents, laying down felt, and just shingling over the ridge to stop the potential leaks. I currently have 6 or 7 box vents on the back side of the house. I have 8 soffit vents on the front of the house and 2 on the back side due to additions the previous home owner added. Do I need more soffit vents on the front? I have no place to put additional soffit vents in the back without getting into major remodeling (that is planned for the future). I also need to replace some rotted facia and put up new gutters. Any suggestions or information would be appreciated.

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6 Answers

0
Votes

My common answer for the box/turtle/750 vents would be 1 per/375sq ft

Answered 5 years ago by cewoodford

1
Vote

AirVent has some great information on Roof Venting. I would suggest that you start there and a quick read will give you all the technical information that you need.


The rule of thumb is 1 square foot of ventilation (split equally between intake and exhaust) for every 150 square feet of foot print of the home.


If you are going to error on the split, be sure to have more intake than exhaust so that you don't exacerbate the stack effect of the home.


Ridge vents are not the issue as much as the type of ridge vent used. The new ridge vents are baffled and much more effective at keeping wind driving rain and snow out of them. From what I have seen, if the rain is hard or sideways enough to make a good quality ridge vent leak, it will probably leak at a box vent as well.


http://airvent.com/homeowner/whyVent/evaluate.shtml

Source: http://www.windowsonwashington.net/bl...

Answered 5 years ago by WoWHomeSolutions

0
Votes

I prefer the look of a continuous ridge vent over box vents.

Answered 5 years ago by Davidhughes

0
Votes

Your requirements will depend on the region in the country. In far southern climates (like here in Texas) we require 1 sq. foot net ventilation per 150 sq. ft. of attic space. You should have the intake vents (at the soffit) evenly distributed for the best performance. Otherwise, you could wind up with hot pockets in the attic in which humidity/moisture will accumulate due to the lack of air flow. The box vents you are referring to should have been removed when the ridge vents were installed unless they are installed on a part of the roof that ridge vents could not service. When left in place the cooler air from outside is drafted in through the box vents and exits the ridge vents, rather than drafting at the bottom of the attic at the soffits. Therefore, you are only moving air through part of the the attic and leaving stagnant air down by your insulation and ceiling framing. Remove the box vents and install the ridge vents properly. They are much more efficient than the box vents and calculate at a higher rate of air exchange than the box vents.

Answered 5 years ago by Todd's Home Services

0
Votes

1) I would not ask how much is necessary - I would go with the more is better philosophy There is no such thing as excess eave ventilation, too much does not hurt anything (as long as you arer not taking away structural support), and every bit of increase reduces the resistance to airflow through the rafter spaces. Also, not providing eave vents at every rafter bay defeats the purpose because the really hot zone is between the rafters, and especially if they are deep (8 or 10") the temperature up against the underside of the roof in an unventilated bay can be 30-40 degrees hotter than an adjacent ventilated bay. At the low airflow velocities from eave to ridge vents on windless days, most of the airflow hugs the underside of the roof and occurs in the rafter bay area. You would be amazed if you saw a smoke gun test of the ariflow. I have seen houses with both ridge and ridge venting where a smoke test showed the smoke moving horizontally through the attic spread out from gable vent to gable vent, with probably 80-90% of the flow undisturbed by the airflow moving from eave to ridge between deep rafters or trusses.

2) I would go with full width eave vents everywhere you can, get rid of the box vents except where you cannot do eave venting and place them as close to the eave as you can in that area, and use full length (except gable overhang area) ridge vent over the house. If the garage is not air conditioned and you do not have real winters but do have hot summers, or if it is totally unheated, I would continue the eave and ridge venting through the garage area too.

3) Several full-scale studies have shown that the 1 SF per 350 or 400 SF of attic space is way undersized - get it down to 1:250 or so and attic temp commonly drops about 20-30 degrees in summertime, and at 1:150 or so can total a 50 degree reduction. I know my house, with full-width (excluding rafters, of course) eave vents 5-1/2 inches clear in height runs about 110 degrees in the peak summer solar heat, whereas a neighbor's otherwise identical house with exact same roof with 4" clear eave openings every other rafter (with full-height blocking in alternate rafter bays) ran 145 degrees within a half hour of my measurement, and dropped to 110-120 range after removing the blocking and replacing it with metal plate rafter bracing.

Answered 5 years ago by LCD

0
Votes

A vast majority of homes don't have enough to put it simply. I would slightly disagree with LCD in that you can have too much in areas where you don't need it. Example: There is building science observations to show that venting in some hot and humid climates can create more issues than solve. That being said, I haven't seen any homes with too much.

Answered 5 years ago by Davidhughes




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