Ventilation: The Only Way That Passive Vents Will Work

A version of this article appears in the January/February 2016 issue of Home Energy Magazine.

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Exhaust-only ventilation is one of the most common strategies for single-family and multifamily buildings alike. In multifamily structures, exhaust is pulled out of the apartments, often in bathrooms and/or kitchens, and fresh air is intended to replace it. Where this “fresh air” comes from can be a mystery. It may come from cracks or intentional vents in the exterior envelope, or it may come from other apartments or the hallway through cracks or corridor door undercuts. Often it comes from several of these sources, depending on where the apartment is located in the building and on wind, temperature, mechanical system balance and operation schedules, and occupant behavior. It really is difficult to say.

Part of the problem is that it is so difficult to hold all of these factors constant or to correct for them during a study, especially in occupied buildings. Steven Winter Associates (SWA), my former employer, did a research project for the Building America program under the National Renewable Energy Laboratory that attempted to get a better picture of what is going on, and to identify best practices for multifamily ventilation. While we didn’t have enough equipment, time, or money to determine conclusively where fresh air comes from even most of the time, we did learn some things about ventilation systems and what makes them work (or not). Some of the experiments were even fun to do.

First, we tested as many things in the apartment as we could to identify potential leakage pathways. For instance, we tested the door to the corridor. In order to do this test, we devised all sorts of contraptions, including a foam board capsule to fit over the door and a duct tester to measure leaks (see Figure 1). We also needed to measure the leakage from passive vents in the exterior envelope. In most of the tested buildings, these were so-called trickle vents, because they are meant to provide a steady trickle of fresh air to occupants.

Through our testing, we quickly found that the corridor door was often by far the largest single air exchange pathway in the apartment. Every building was different, but the leakage coming from a door ranged from as little as 20 CFM₅₀ to more than 200 CFM₅₀. For very tight apartments, this could be more than 50% of the total leakage area of the apartment. Note that a typical apartment blower door test does not take door leakage into account, because that’s the door most often used for the test! This practice tends to ignore one of the defining characteristics of the apartment envelope and airflow patterns.

Testing often showed defects in many of the passive-vent installations. Still, even if they were working as designed, consistent negative pressures of up to 20 Pa are needed to draw air from the vents at the rates desired. The chart in Figure 2 shows that a reasonable target of 7.5 CFM per trickle vent would require 20 Pa of negative pressure.

Because knowing the amount of leakage alone won’t tell you how much fresh air comes through the apartment door, we monitored pressures using pressure transducers and data loggers. This allowed us to see whether an apartment was consistently under negative or positive pressure. In this case, we wanted constant negative pressure, because an exhaust-only ventilation design calls for fresh air to be pulled from passive vents or other desirable places.

Unfortunately, in most buildings the apartments just weren’t tight enough to consistently hold a negative pressure and pull air from the passive vents. Instead, pressures in the apartments and the corridor fluctuated widely with changes in weather, wind, and particularly occupation of the building as a whole. More often, air flowed into or out of cracks in the corridor door. Although the doors were often well weather-stripped, they and the myriad other leaks in the apartment dwarfed the leakage area of the passive vents. The result was that the vents provided little or no ventilation under typical negative pressures.

Airtight Apartments

We wanted to see what would happen if we did have truly airtight apartments, and we got some help from UC Davis, which agreed to apply an aerosol sealing technology on several apartments. ASHRAE 62.2-2013 recommends apartment airtightness of 0.30 CFM₅₀ or less per square foot of apartment enclosure. We sealed one apartment to 0.27 CFM₅₀ per square foot by manual means, while UC Davis sealed another to 0.08 CFM₅₀ per square foot with aerosol. The difference in operation between these two apartments was dramatic. Figures 3 and 4 show how each apartment operated under six different conditions.

In both cases, the first condition shows the apartment with no systems running—exhaust fans off, with passive vents closed. The second condition shows the maximum pressure likely—exhaust fans on, but passive vents still closed. The third condition shows the apartment with the vents open; this is normal operation as designed. The fourth condition shows the apartment with the corridor door cracked open; this was done to help interpret anomalies from the long-term data logging. The fifth condition shows a return to normal operation, and the sixth condition shows the apartment with all systems off again.

There is a stark difference between the two apartments. In the relatively leaky apartment (Figure 3), turning the fans on and opening and closing the vents has some impact, but not much. In the end, air is as likely to come from other apartments or the corridor as it is from the fresh-air passive vents. The more airtight apartment (Figure 4) could dramatically alter its pressure by changing the operation of the vents and fans. In this apartment, air is much more likely to come from the passive vents than it is in the other apartments. But is this good enough?

SWA has tried to determine just how airtight an apartment must be in order to get most of its air from passive vents. It is practically impossible to get all makeup air from passive vents, because it is impossible to get perfectly airtight apartments. But considering that meeting ASHRAE 62.2-2013 requires fresh air, and that we want to minimize the influx of air into a dwelling from unknown sources, an airtightness of less than 0.1 CFM₅₀ per square foot would be advisable. How achievable is this, generally speaking?

Effective Exhaust Only Ventilation

SWA has maintained a database of apartment blower door tests done over the past several years in buildings that have participated in some sort of green or energy efficiency program. Figure 5 shows the results of these tests. Eighty-eight percent of the apartments tested passed the ASHRAE 62.2-2013 threshold of 0.3 CFM₅₀ per square foot of enclosure, but only a tiny fraction were able to reach 0.1.

In order to effectively use exhaust-only ventilation, and to reduce the influx of makeup air from undesirable sources such as other apartments and hallways, apartments must be sealed to levels not commonly seen even in the best green buildings. Since exhaust-only ventilation is one of the commonest forms of ventilation, this represents a problem. In addition, the significant negative pressures required for consistent operation of the passive vents may introduce other unintended consequences for the building envelope.

One remedy is to deliver fresh air directly to apartments by means such as balanced ventilation, and this probably makes the most sense from an indoor air quality point of view. But balanced ventilation comes with a higher first cost. Innovations in small, low-cost heat recovery ventilators and energy recovery ventilation are needed. In general, all ventilation designs, whether exhaust only or balanced, require airtight apartments to function well.

Sean Maxwell was senior energy consultant at Steven Winter Associates and now resides in Australia. His nearly seven years of work at SWA ranged from dozens of multifamily energy audits to research for DOE’s Building America program.

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