Tag Archives: CFM

Blower Door Testing Your New Barndominium Part II

Continued from yesterday’s blog;

Aside from code compliance or indoor air quality concerns, another reason to get a blower door test is to properly size your furnace or air conditioner. How leaky or tight your barndominium is can change how much heating/humidification or cooling/dehumidification you need. This then ties into how carefully your mechanical system is designed. If in doubt, ask your HVAC designer whether and how they use air leakage metrics in their load calculations.

Envelope leakage is measured in terms of air volume per time unit or CFM (cubic feet of air per minute). From this number, a standard metric called ACH50 (air changes per hour at a 50 pascal standard test pressure) is calculated. ACH50 indicates how many times building interior air volume changes with outside air under test conditions, correlating to air leakage under normal or “natural” conditions. This ACH50 number is how leakage across different homes is compared. CFM per floor area square foot and CFM per building envelope square foot may also be used.

Blower door testing is often done near the building process, when paint is done,  doors and windows are in place, and all weather stripping is installed.

This is a great time to find out your final numbers, but not such a great time to try and fix any issues. Doing leakage testing at different construction stages can help diagnose issues and fix them while the primary air barrier is still accessible. Caveat to this early testing is all window or door openings must be sealed, otherwise you won’t be able to pressurize the building enough to look for leaks.

To find someone to conduct blower door testing Google for a testing company using keywords such as: energy code testing, HERS Rater, or energy auditor. Ideally, you’ll find someone who is RESNET- or BPI-certified, meaning they should have right equipment and experience to help get a leakage number and also identify where leakage is coming from.

One can even make their own equipment with a box fan or two, and some ISO board to seal up the door opening. This will not get any actual leakage numbers, but it can go a long way in helping find leaks. When pulling air in through leaks by blowing air out of your barndominium, you can usually feel where air is coming in.  Depending on temperature difference, you might also be able to see it with an infrared (IR) camera. Another great tool to use is a theatrical fogger to help make air movement visible. 

Energy amount saved by air-sealing your barndominium depends on many factors: what climate zone you live in, interior temperature, wind speeds, etc. International Energy Conservation Code (IECC) required a 7 ACH50 building envelope leakage in 2009, but now 2018 code requires 3 and 5 ACH50 in most areas. This downward trend in leakage requirements indicates building codes will continue to get more stringent over time as builders get used to these standards, and as products and technologies improve. Achieving a 3-5 ACH50 is more than doable. These days, Passive House projects are required to achieve 0.6 ACH50. All of this is done to save energy on a large scale.

My Pole Barn Needs Ventilation

My Pole Barn is a Sauna in Summer- and needs ventilation!

“Hey there Pole Barn Guru, got a question about ventilation.

Just bought a house with a pole barn on the property. I believe it’s only about a year old. 30 x 32.  It has no soffits or windows, only a standard garage door and walk-in door.

Metal siding and roof, and the underbelly of the roof has a vapor barrier. There are also two ceiling fans in here.

I don’t care that it’s cold inside the building in winter, but it’s like a sauna now in the summer.  I was thinking of an exhaust fan to pull out the heat, but I don’t know if that’s waste of money. How does one ventilate this thing without having to bulldoze it and start over?



Mike the Pole Barn Guru responds:

Since you cannot increase the amount of venting in your soffits (as you have none), you’ll need help from power vent fans.

Attic vent fans can be hard-wired and equipped with a thermostat and/or humidity sensor so they automatically cut on at a preset moisture level or temperature. You could also install solar-powered attic vent fans, though it has been found most solar models aren’t powerful enough to be very effective.

To determine what size power vent fan(s) you need for your attic, you first need to know the size of your attic in square feet.

Attic Size

To determine the size of your attic, multiply the width by the length of the attic floor in feet. In your case 30′ wide x 32′ long = 960 square feet of attic space.

Vent Fan Size

Next, multiply the square feet of attic space by 0.7 to get the minimum number of cubic feet of air per minute the fan should be rated to move. 960 sft x 0.7 = 672 CFM minimum fan rating.

Add an additional 20% (CFM x 1.20) if you have a steep roof, and 15% (CFM x 1.15) for a dark roof. Attic vent fans are commonly rated from 800 to 1,600 CFM, making one fan suitable.

Vent Fan Location

Install gable mounted fans on the gable vent at end of the building facing away from the prevailing winds.

Intake Air Vents

It’s also important to have plenty of soffit or gable vents for the fan to draw air into the attic. To find out if you have enough vent space, divide the cubic feet of air per minute the fan(s) is rated for by 300 to come up with the minimum number of square feet of intake vent space needed for that size fan. 672 CFM ÷ 300 = 2.24 sq. ft. intake vent area

If you prefer the answer in square inches rather than square feet, multiply the answer by 144 and round to the nearest inch (2.24 x 144 = 322.56 sq. in. vent area).