What Size Fresh Air Intakes Do I Need?
Reader BOB in GRAHAM writes: “I have a 40 x 60 pole building. There is a ridge vent along the 60′. I recently installed Sheetrock on the ceiling and blew in insulation. My question is what size fresh air intake vents do they need to keep the attic cooler. I was thinking I need Air vents on each side of the end wall at the lowest points. The other thing is I can only pull fresh air in at one end of the building. As the other end is open to the ridge vent, separate from the other 60′. Should I put an electric intake fan rather than just an air vent?”
Here are ventilation requirements:
Provision of 1 square foot of NFVA (Net Free Ventilation Area) for each 150 square feet of attic floor. One important note – attic floor area is just as it reads – area – not volume. This is a minimum requirement and does not stipulate required ventilation openings provide intake (low), exhaust (high), or both.
Provision of 1 square foot of NFVA for each 300 square feet of attic floor if both following conditions are applicable:
At least 40%, but not more than 50% of NFVA is provided by vents located not more than 3 feet below roof’s highest point.
Provision for a minimum 1 inch air space between roof sheathing and insulation in attic at vent location.
Your 40 x 60 building would therefore require at least eight square feet ( 40 x 60 = 2400 / 300 ) or 1152 square inches of NFVA, with at least 50% of this been at eaves (vented sidewall soffits) or gable vents located in lower half of attic space.
While an electric powered fan would move air within your attic area, you do need to provide adequate intake area. 576 square inches of NFVA is a lot of gable vent (assuming you have no soffit intake vents).
Last questions. I found a 36×36 square gable vent that has a NFVA of 585. I need 576 on both sides of my building so this vent should be sufficient, correct? Also, with me using the 1/300 ventilation rule the gable vent company who has this vent recommended using a vapor barrier in the attic too. I believe he said that it needs to be no more than 1 perm and to install it on the warm side of the attic. He said this is needed because I’m not using the 1/150 rule. He said if I have 16 sq ft on ventilation instead of 8 I wouldn’t need a vapor barrier. That’s not really an option though. That many gable vents wouldn’t look good. So what do you think? Do I really need a vapor barrier using the 1/300 rule? My roof panels have a felt-like material on the underside of them that I believe is called “drip x”. I was under the impression that this was a vapor barrier and when warm air rises and hits those cold roof panels the “drip x” keeps it from condensing. Do I really need a vapor barrier along the warm side of the attic too? That sounds like overkill to me but I want to do it right too. The plan is to fully finish the inside of my outbuilding eventually with a ceiling that will be white ribbed panels with a r-38 blown in fiberglass. I only plan on heating the building to about 55-60 degrees and that’s only if I’m out there working. The thermostat will be set to 45-50 if I’m not out there. Also, this outbuilding doesn’t currently have air conditioning but I may consider adding it down the road. You’ve been a big help so far. Thank you and I look forward to your response. “
