This question comes up quite a bit, particularly in affordable housing. Using a blower door to depressurize a rowhome means that air is being pulled not just from the outside, but also from adjacent units through the common or party walls of the home. And while the air that leaks into one home from another may be conditioned (heated or cooled) and not a real issue from an energy-efficiency perspective, there may very well be indoor air quality concerns related to combustion safety, radon, smoking, etc.
In 1987, the Canadian R-2000 program evaluated 5 different ways of blower door testing units within a 6-unit rowhome building in British Columbia (“Methods for Airtightness Testing of Multi-unit R-2000 Housing;” Sheltair Scientific Ltd, 1987; this report is not readily available on the Web).
Detached-Unit (DU) Method: Shared surfaces are treated as part of the exterior shell of the row home being tested. Ideally, windows and doors of the adjacent units are open. This method involves one blower door.
Pressure Equalization (PE) Method: Depressurize adjacent units at the same time and to the same values. That means 3 blower doors and permission of neighbors to set up. This method cancels out air flow between adjacent units and yields just the air leakage of true exterior surfaces.
Pressure Drop (PD) Method: Depressurize the test home with a blower door while also measuring pressure differences in adjacent units to estimate the air flow associated with adjacent units. This requires just one blower door, three pressure gauges (manometers), limited access to adjacent units, and does involve some assumptions and additional calculations.
Whole Building (WB) Method: Treat the multi-unit building as a single detached unit and assign air leakage proportionally based on total exterior surfaces. This approach does not really work for long strings of rowhomes that don’t really have common spaces.
Single Point (SP) Method: This method is a variation of the Pressure Equalization Method, except that rather than testing at a range of pressures, testing of the unit and adjacent units is just at one pressure, presumably 50 Pascals (Pa). This method does save some time compared to the PE method, but if you are going to set up 3 blower doors and gain access to the adjacent units, might as well take full advantage and run the PE method.
Which approach works best?
Without a doubt, the approach that works the best is the Pressure Equalization Method. It provides detailed information on exterior air leakage alone, and, by changing the pressures of adjacent units, provides information on shared-surface leakage. This method is, however, the most difficult to accomplish because it involves three blower door setups and access to both adjacent units.
David White of Right Environments used the Pressure Equalization Method on a recent Passive House rowhome project, the Park Slope Townhouse. “The Passive House Institute approved testing the airtightness of the home using this method,” says White. “And we did vary the pressures between units to learn more about leakage between the units.” When I asked David how they succeeded in gaining access to the neighbors’ homes, he replied, “We pretty much just asked politely.”
This method might be the best, but it simply is not reasonable for weatherization or affordable housing programs.
Which approach is the most practical?
The most practical method is clearly the detached-unit method, particularly if you luck out and test while neighbors happen to have windows open! While you won’t be able to get quantitative information on air leakage originating from common walls relative to exterior surfaces, you can use a smoke stick (or a Wizard stick – see photo at bottom of page) to identify air leakage along the common walls.
When I asked Elliott Seibert of Every Day Green about blower door testing of rowhomes, here is what he had to say: “For practical reasons we use the detached-unit method. In our experience testing both new construction and gut-rehab row homes, attached units can perform tightly if the extra effort is made to air seal the common walls. For instance, exposed brick walls can result in a lot of infiltration in gut-rehabs if not repointed. Yes, this testing method marginally overestimates outdoor leakage. But when the test shows serious leakage, it almost always means big problems in the exterior envelope because that’s where the holes get cut. The method is not perfect for estimating precise energy usage, but it is still useful as a metric of relative leakiness and for identifying major leakage failures.”
NOTE: Rowhomes are just one particular type or configuration of multi-family building; there are lots of others. If you are interested in a comparison of approaches to blower door testing other configurations of multi-family buildings, take a look at this 1999 Master’s Thesis by Sebastiano DePani..