Gathering Data for A Wise Future
Over this past year, I have been (and continue to be) spending a lot of time poking around in the attics and basements of some of the oldest institutional buildings in New England, right here in Middlebury VT. As Middlebury College begins to look beyond the Covid pandemic, its Energy2028 initiative is gathering steam (see https://bit.ly/MiddleburyCollegeEnergy2028). Part of the initiative is the stated intention to reduce building energy consumption (heating, cooling, hot water, and electricity) on its core campus by 25% by the year 2028. In order to measure this goal as accurately as possible, Middlebury College is gathering a lot of data about the current energy use of its large campus buildings. One of my roles in this broad initiative is to measure the amount of air leaking through the exterior walls, ceilings, and floors (collectively, the building envelope) of many of the campus buildings. Air leakage is a direct and preventable cause of winter heat loss and in a leaky commercial building, leakage may account for over 40% of heating energy use!
So, how do I measure the leakage in a 35,000 square foot building with a building envelope surface area of almost 60,000 square feet? It’s the good old blower-door test, just scaled up to institutional size. Instead of a single fan, I’ll use three or more fans (see picture 1) to move enough air to accurately measure the building’s leakage. Instead of taking a reading at one pressure differential (usually 50 Pascals difference between inside and outside for a single-family home), I’ll take readings at as many as a dozen different pressures to gain a more accurate building leakage curve (see picture 2). There are other factors to consider in a large-building blower door test. One example is outside air temperature. Very cold air is much denser than warm air, so flow and pressure measurements have to account for this difference and weather conditions have to be within acceptable parameters to conduct the test. Same thing goes for total building height (stack effect gets pretty intense when you get taller than three stories) and outside wind speed (Bernoulli’s principle means that air moving past the measurement instruments outside will temporarily change the measured pressure, throwing off the consistent pressure differentials needed for testing).
Once the conditions are correct, the fans are set up, and the reference pressure manometers are stationed on all sides of the building and connected to the test computer via closed Wi-Fi mesh network, then the testing can commence. Once measurements at all necessary pressures are complete, I have a very important number—namely the air infiltration rate of the whole building, expressed in cubic feet per minute at 75 Pascals (CFM75). Once I have this number, I’ll leave the fans on cruise control and walk through the entire building, searching for and documenting air leaks. All of this information gets packaged into a report that contains hard data on the current performance of the building against which to measure improvements, helping the College prove delivery on its Energy2028 initiative intentions.
Nate Gusakov is a building envelope consultant and AeroBarrier installer for Zone 6 Energy.