A bedrock principle of zero energy home construction is super sealing the building envelope to minimize air leakage. In the last decade, dedicated builders have slashed air leakage to astonishingly low levels. While a conventional home built to current building codes may have an air leakage rate of 4 to 7 air changes per hour at 50 Pascals (ACH50), zero energy homes often go below 1 ACH50.

Super airtight homes need a way to expel moisture, odors, and harmful air and replace it with fresh outdoor air — making a balanced mechanical ventilation system a necessary companion to airtight construction. Together, tight envelopes and balanced ventilation systems have a significant benefit: allowing greater control over indoor air quality by regulating how much outdoor air comes into the home and where it comes from. Unfortunately, outdoor air is often polluted to a degree that can overwhelm these systems. Air tight homes with balanced ventilation systems can and should address the possibility of extreme pollution events by upgrading their level of air filtration.

Fresh Air Systems

The fresh air systems in zero energy homes expel a specific amount of stale indoor air and draw in about the same amount of fresh air — while allowing 70 to 95% of the “waste” heat to be recovered. This provides healthy air quality by addressing problematic indoor sources, such as odors, volatile organic compounds and moisture, while bringing in and filtering fresh air. To accomplish this most builders use a heat recovery ventilator (HRV) or energy recovery ventilator (ERV). For simplicity, let’s use the term HRV for both. These fresh air devices draw outside air from a single point, presumably a location free from outdoor pollutants –  carefully avoiding locations with poor air quality, such as roofs, driveways and dryer or bathroom vents.

Not all HRV filters are created equal. The factory installed filter is often intended to strain larger dust particles from the incoming air that would clog the heat exchange core. These filters generally have a particle removal efficiency rating of MERV 8, which works well enough under normal conditions. But what about times and places of extreme outside air pollution?

Extreme Outdoor Pollution

Poor outdoor air quality is increasing and presents a challenge to indoor air quality in most homes. Some urban areas may have emissions from industrial or transportation sources that are excessive or exacerbated by temperature inversions. Increasingly, wildfires are fouling the air over wide swaths of the country. Standard homes are the most vulnerable because they allow a lot of polluted outside air to seep in, while super tight homes are designed to keep most of the polluted air out. However, there are times when the standard filtration system on HRVs can become overwhelmed by unusually high levels of outdoor pollutants.

When this happens, it may be best to shut down the system for a few hours at a time. You will still be better off in your air tight home than in a standard leaky home. But there are better options.

Perhaps the most important outdoor pollutant to block is particulate matter that is 2.5 microns or less, known as PM2.5, which is associated with a variety of negative health effects. Capturing these tiny particles requires a high efficiency particulate filter with a MERV rating of at least 13, which can remove around 65% of the PM2.5 particles. Better yet is a MERV 16 filter, which can remove up to 95% of these particles or a HEPA filter, which can remove 99.97% of particles with a size of 0.3 microns, as well as removing many bacteria and viruses.

Design for High PM2.5 Filtration

If you’re planning a new zero energy home or adding a new HRV to an existing home, you should take the opportunity to design high-efficiency filtration into the ventilation system from the beginning. This provides the best overall solution for healthy air quality. In addition to expelling problematic indoor sources, such as moisture, odors, and volatile organics compounds with ventilation, adding advanced filters will handle more extreme outdoor pollution events resulting from wildfire smoke, vehicle or factory emissions, and other sources of PM2.5.

If the HRV is integrated with a central ducted heat pump forced air heating and cooling system, a high-efficiency filter can be included in the air handler. IQAir offers a MERV 16 filter designed to work with any air handler. Several HRV manufacturers now include high-efficiency models Merv 13-16 or HEPA filters along with sufficient fan power to handle the added resistance created by denser filters. Check out products such as the Broan ERVH100S, Zehnder filters and VENMAR EVO5 700.

If your existing HRV doesn’t have built-in advanced filtration, you can install a high-efficiency filter (MERV 13-16 or HEPA) in the duct that brings outside air to the HRV. However, there is a complication. Filters resist air flow and many small HRV blower motors may not have the power to pull air through a high-efficiency filter. To address this issue, there are two good options for installing an in-line, high-efficiency filter in the HRV supply duct.

The Fantech FB6 filter box contains a MERV 13 filter that is designed for low-flow ventilation systems. A slim shape and six-inch duct collars help the system fit in tight spaces. Another solution is a self-powered filter, which is a box containing a filter with its own blower. Lifebreath offers a product designed to work with HRVs that contains a HEPA filter and blower. And they also have an HRV that seamlessly integrates with this self-powered filter.

Free Standing HEPA Filter

Existing zero energy homes don’t often have a good way to add an inline filter to an existing HVAC or ventilation system. In this case, one or more stand alone filters can be purchased. Choose a HEPA model for maximum removal efficiency. There are a wide variety of these products so be sure to compare the costs and benefits. These are commonly available from home centers and internet vendors. IQair and Honeywell would be good places to start your research.

Because standalone filters are not connected to the HRV duct system, they do not filter the air as it enters the home. You can think of these as room-by-room air cleaners, and most homes would need more than one. It is best to have one in spaces where you spend the most time. Not being physically attached to the duct system does have an advantage. If outdoor air quality is so bad that the HRV must be shut down for hours at a time, the stand alone filter will continue cleaning the air. While temporarily shutting down the ventilation system may be necessary, outside air will be eventually needed. It’s best to restart the HRV when the outside PM2.5 count drops to a safe, or at least, less harmful level. Even with this inconvenience, the indoor air in an airtight home will still be better than that in a conventional home.

Combine High Efficiency with Activated Carbon Filtration

High-efficiency, mechanical filters remove particles, but not harmful gases, such as nitrous oxides, carbon monoxide, and volatile organic compounds. Activated carbon filters cling to these gas molecules through a process known as adsorption. Some stand alone air purifiers have activated carbon/HEPA options, such as the Whirlpool Whispure Air Purifier, IQAir, and several others. The Honeywell air cleaner comes with a thin activated carbon pre filter, which may be sufficient for everyday air purifying. In cases where a high-efficiency particulate filter already exists, a separate activated carbon pre filter can be added to an existing system.

The Zehnder HRV has an option for adding on an activated carbon filter to their standard MERV13 filter. The HVACquick CFB filter box with a carbon impregnated pleated filter is offered with a range of duct connection sizes that can be attached to HRVs or central air handlers. Any of these arrangements effectively remove both particles and gaseous chemicals and should be considered in areas vulnerable to extreme outdoor air pollution.

A New Standard

By combining super tight envelopes with balanced ventilation, zero energy homes provide air quality that is much healthier than that found in code-built homes. However, as extreme outdoor air pollution becomes more prevalent — even in airsheds once considered pristine — it’s time to add high-efficiency particulate filtration to the package with MERV 13-16 or HEPA filters along with activated carbon filtration. For new homes, these measures must be included in the design requirements, while existing homes can be equipped with add-on systems. This is rapidly becoming the new standard for all zero energy homes.

Super air sealing plus a robust ventilation system that expels stale air and filters incoming air is the best path to health. It pays off in energy savings and well being. And now with advanced filtration, it can protect residents from the worst pollution events, making zero energy homes even more resilient.