Passive House Air Quality

Passive House buildings rely on a constant supply of filtered fresh air, provided by a heat or energy recovery ventilator (HRV/ERV). Fresh air is delivered to the primary living areas, and stale air is exhausted from the “wet” areas—bathrooms, kitchens, and laundry rooms. (Note—the kitchen exhaust is not a replacement for a properly-sized range hood.)

There are several important characteristics of this equipment:

  • Fresh air is delivered and stale air exhausted in roughly the same amounts throughout the building. This is called balanced ventilation.
  • The heat recovery efficiency is typically at least 75% (it can be as high as 90%). This significantly reduces the heating load during the heating season, and can somewhat reduce the cooling load under certain summer conditions.
  • The incoming air is filtered with at least a MERV 13 filter, promoting high indoor air quality. HEPA or activated charcoal filters can be added if more stringent filtering is desired.
  • Electricity consumption is typically very low, minimizing the cost and environmental impact of running the equipment. 
  • In dry climate conditions, an energy recovery ventilator can help maintain acceptable indoor humidity levels.

The airflow levels are typically determined using the Passive House Planning Package (PHPP). The equipment manufacturers, or mechanical contractors then design the system to deliver those airflows. Once installed, the system is commissioned to make sure that it is operating as expected.

One underlying assumption about Passive House ventilation is that the incoming fresh air gets circulated throughout the building. The interaction between the supply and exhaust areas is done using transfer zones—parts of the house with neither supply nor exhaust air. The figure below shows a typical residential layout. Supply zones are in green, exhaust zones in red, transfer zones in blue. 

Passive House design principles require providing pathways to and from the transfer zones. Supply and exhaust areas may have doors that interfere with the air flow into and out of the transfer zones. These obstacles can be overcome by undercutting doors, but that can increase inter-room noise. There are ways to create invisible air gaps around door trim, but that’s not a common practice in the United States. You can also install transfer grills or jump ducts between rooms, which can also promote inter-room noise.

Unlike a typical ducted forced air heating and cooling system, ventilation airflow rates have very low velocity. The air doesn’t easily move from place to place. A two-person bedroom with closed doors can accumulate carbon dioxide (CO2) throughout the night, even with adequate transfer zone pathways and properly-commissioned flow rates. You might think that the exhaust register in the master bath in Figure 1, for instance, would keep the CO2 level in the master bedroom at acceptable levels. How do you know?

The answer is to use an indoor air quality monitor. There has been a renaissance in good-quality consumer monitors in the last few years, partly in response to houses getting tighter and tighter. There are several devices costing roughly $300 or less. The simplest ones usually measure temperature, humidity, and CO2. Some measure as many as eight air quality parameters, including nitrogen dioxide (NO2), air pressure, and ozone.

For a good introduction to some of these devices, presented by professionals that have installed them in their homes, you might want to take a look at “IAQ Devices – Where are They Now and Who is Using Them and for What?” from the RESNET 20201 conference. This excellent roundtable was compiled by Healthy Building Scientist Joe Medosch, cofounder of the Healthy Home Environment Association. 

Regardless of the type of house you’re living in, it’s a good idea to know what’s in your air. The only way to do that is to test it. You might be surprised at what you find. 

Steve Mann

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