An air tight barrier in a high performance home is critical to making the home perform at a high level of energy efficiency. To ensure that we are meeting our own expectations for air tightness, I perform a blower door test on the house at least once during construction to identify areas that could use a little extra attention before the house is 100 percent completed. Once the house is finished, a third party (Efficiency Vermont) will perform the final blower door test and provide the official results.
A blower door test is a simple test performed with hi-tech equipment that measures the tightness of the air barrier of a home. A fan is mounted in an exterior door frame, which, when running, sucks the air out of the interior of the home, depressurizing it relative to the exterior. Since areas of high pressure are always trying to move toward areas of low pressure, any holes in the air barrier of the house will reveal themselves when the fan is running because air can be felt entering the home through the holes!
Some areas that are notorious for being difficult to seal include electrical fixtures, windows and doors, and the box sill (the area where the floor system sits on the foundation).
When the blower door is operating, the manometer (the device that measures air pressure) will spit out a number to help us gauge the air tightness in the home. To accomplish this, I need to calculate the volume of air in the home, plug in a few multipliers along with the measurement from a manometer…and voilà, we have a calculation that we can use as a point of reference.
That number is called ACH50, or “air changes per hour at negative 50 pascals” (pascal is a unit of measurement of air pressure). Simply put, we depressurize the house to negative 50 pascals (the left number in the picture) and that tells us how many times per hour the interior air would be exchanged with exterior air entering through holes in the air barrier.
The 2012 International Energy Conservation Code® (IECC) states that all new homes need to have an ACH50 equal to or less than 3. This is relatively easy to achieve if your contractor spends even an iota of time air sealing the home. That said, many contractors do not invest that time. I’ve seen some new homes with ACH50 ratings that fall far outside of meeting this requirement.
So how did this home measure up? Per my calculations, this house is currently at .93 ACH50, and it should tighten up further as we put the finishing touches on it. The nice part about conducting this test before construction is complete is that we can identify any areas that need a little more attention. The light fixtures in the ceiling were leaking a little bit, even though they are encapsulated in spray foam, and the door that we mounted the blower door in is only a temporary door and we could see daylight around the edges.
Our goal on any new home is to achieve an ACH50 reading of 1 or less with practical construction techniques that don’t result in oodles of added labor (translation = increase in construction costs). It’s nice to know that we’ve already achieved it on this house!