Tag Archives: exhaust fan

How Do I Ventilate My Barn’s Attic?

How Do I Ventilate My Barn’s Attic?

Right up there with curing condensation issues is how to properly ventilate a pole barn’s dead attic space.

Reader CURTIS in TRENTON writes:

“I have a 40’x60’ outbuilding that doesn’t have soffit vents for fresh air intake and the ridge cap has solid foam closures along the length of both sides of it so I have no air exhaust either. Basically just whatever air leaks are within the building. My plan is to fully finish the inside of my building. (Insulated walls and OSB sheathing and a metal ceiling with a blown in r-38) I plan on making it as air tight as possible. This building will be heated some over the winter as well. My roof panels also have the “drip X” felt like material that is attached to the underside of the panels. I believe this acts as a vapor barrier to keep the roof from condensing. My question is since I don’t have soffit vents or a vented ridge cap do you recommend adding 2 gable vents? If so, what size? I know there’s a formula for sizing them. I believe it is the square footage of the building, (40×60=2400/300=8 sq ft) so would be adding a 4 sq ft gable vent on each end of the building be sufficient? 4 sq ft of intake and 4 sq ft of exhaust totaling 8 sq ft of ventilation? Or should I do a gable vent on the west side of my building for air intake and a shutter exhaust fan wired to a humidistat/thermostat on the east side (away from prevailing winds) to pull hot air from the attic. I believe there’s a formula for this too to determine the fan’s cfm rating and the gable vent size. (40×60=2400×0.7=1680CFM) 

Add an additional 15% (1680×1.15=1980CFM) for a darker colored roof. My roof is forest green. So an exhaust fan with a minimum rating of 1980CFM/300=6.44 sq ft, which would determine the gable vent size on the west end of the building. Examples: 30”x32”=6.65 sq ft or 26”x36”=6.48 sq ft) My question is are these formulas correct and which one do you recommend using? Two gable vents, a gable vent and a shutter exhaust fan, something different or nothing at all? I just want to do this right and make sure my attic has sufficient ventilation once it’s enclosed. I hope to hear back from you. Thanks! 

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. “

Mike the Pole Barn Guru responds:

As long as your four square feet (576 square inches) of NFVA (Net Free Ventilation Area) is located in the upper half of each gable endwall, your formula is correct and should provide sufficient ventilation. Actual vent size is not the same as the NFVA – so look closely before investing in any particular vent.

A vapor barrier would only be needed if you have over 8000 heating degree days. One of those vents on each end should do the trick. 

You can look up your heating degree days here: https://www.huduser.gov/portal/resources/UtilityModel/hdd.html

Proper Ventilation, Condensation Control, and Post Treatment

This week the Pole Barn Guru answers reader questions about ventilation, how to control condensation on fifteen year old building, and best practices for treated posts in ground or on specialty brackets.

DEAR POLE BARN GURU: I have a barn that doesn’t have soffit vents but it does have a ridge vent. I installed reflective insulation which seems to have dropped the temperature quite a bit. I also have a gable powered fan that does 2000 CFM. However it is about 8 to 10 feet lower than the ridge vent. Are these two things fighting each other do you think or should I take a pipe and vent it up to my ridge vent and to my fan to take the hot air out from the top or should I turn my gable fan off completely.
I could always try to install some soffit vents as well. KEN in GREENVILLE

DEAR KEN: Your ridge vent is a natural air exhaust point, as warm air will rise and exit through it. Any vents lower in your building should be air intakes (whether gable or soffit vents) and ratio of net free ventilation area between the two should be roughly equal (intake can be up to 10% greater area than exhaust, but should not be greater).

 

DEAR POLE BARN GURU: I have a 30x56x10 Menards package with the inside finished off with drywall built 15 years ago. The roof was installed without any condensation control to the underside of the steel. The amount of condensation this past year created truss heave. All ceiling penetration’s are sealed and fiberglass blow in was installed to a r49 rating. what would be the most effective way to eliminate our condensation issue? Thank you for your time and knowledge. LOUIE in AMBOY

DEAR LOUIE: Drives me absolutely bonkers when providers or builders make no true efforts to advise customers on perils of condensation and how to prevent it. So easily, and inexpensively, done at time of construction.

Have two inches of closed cell spray foam applied to the underside of your roof steel. Make sure your dead attic space is adequately vented, ideally with soffit vents and ridge vent, or (if no sidewall overhangs) gable vents and ridge vent. If you do not have a vapor barrier under your concrete slab, apply a high quality sealer to it.

DEAR POLE BARN GURU: What are the pro’s and con’s for 6″x6″ posts to be set on a drilled whole with footer at the base then back filled VS drilled whole with footer poured to grade and using CBS/CBSQ column bases that are submerged into the footer while pouring so the 6″x6″ can be above grade.

Column bases are made by Simpson Strong-Tie on page 76 and 77 they also come in stainless steel. I attached the Simpson booklet for you to see the Column bases, I’m looking to build a 40’x40’x14′ pole barn with gable roof, one man door, 16′ wide x 13′ high insulated garage door, and a cement floor. I live in North East Ohio so I get a far amount of snow load.

Kind regards, DENNIS in CHESTERLAND

DEAR DENNIS: Properly pressure preservative treated columns, embedded in ground are unlikely to decay within lifetimes of anyone alive on our planet today https://www.hansenpolebuildings.com/2020/09/pressure-treated-post-frame-building-poles-rot/.

Simpson brackets you reference are not designed to withstand moment (bending) loads, just uplift and downward (gravity) loads. If your concern is properly pressure preservative treated columns will decay prematurely and do not want wood in ground, we can furnish your building kit package with Code compliant brackets and properly engineer concrete piers. Bracket mounting will increase your investment, somewhat, however if it results in your sleeping better at night, then they are well worth it.