Airtight construction saves energy and improves the durability of buildings. While an energy efficient home can never be “too tight”, since it is so airtight, it is necessary to provide a clean, healthy indoor environment. This involves three steps: reducing or eliminating indoor pollution sources, installing automatic, controlled ventilation, and providing air filtration. A good way to address all three steps is to participate in the US Environmental Protection Agency’s Indoor airPLUS program. Following good design and construction practices, a zero energy home is much healthier than any conventional home.
Minimize Indoor Pollution Sources
Unfortunately, many building materials include toxic chemicals, such as formaldehyde, volatile organic compounds (VOCs), flame retardants, and more — all of these should be avoided. Government regulations fall short of what is needed to fully protect consumers from exposure to these harmful chemicals, and it is left to builders and consumers to carefully select safe materials. Start by avoiding toxic chemicals from the Red List.
Wherever possible use healthier materials, such as low- and no-VOC paints and formaldehyde-free cabinetry, hardwood, and tile flooring or non-toxic carpeting without glue. These resources can help you find non-toxic materials: Healthy House Institute, Indoor airPLUS material guide, Healthy Building Materials Research, and EWG’s Healthy Living: Home Guide.
While materials are a significant issue, it’s also necessary to exclude all combustion devices from the living space. Although sealed-combustion, gas-fired furnaces and water heaters greatly reduce the potential for indoor air pollution, they are not risk free and are unnecessary in zero energy homes. Gas cooking ranges have been shown to generate a significant amount of carbon monoxide, oxides of nitrogen (NOx), particulates, and other hazardous pollutants and should be avoided in favor of clean, induction cooktops. It’s best to use only electric appliances and equipment in zero energy homes, because they don’t produce pollution in either the indoor or outdoor environments.
Automatic, Controlled Ventilation
All homes that have been air sealed below 4 air changes per hour (ACH) need a mechanical ventilation system that operates automatically. This ventilation brings in fresh filtered air and expels stale air to maintain superior air quality for the health of the occupants. To reduce energy loss in zero energy homes, heat from the ventilated air can be captured rather than wasted by installing heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs). These ventilators bring fresh air into a home and expel stale air, while retaining about 70% to 90% of the heat from the discharged air and returning it to the incoming air. In addition, HRVs and ERVs each help manage moisture in the air in different ways.
Both types of ventilators use a heat exchange core to pass heat from one airstream to the other. The core in an HRV can be aluminum or plastic. This core prevents indoor moisture and odors from crossing into the flow of incoming air. ERVs operate the same as HRVs except the core of the ERV is made of coated paper or perforated plastic, which allows water vapor to pass back into the house, but keeps odors out. Most manufacturers offer both HRVs and ERVs. To select and install an HRV or ERV, follow these six steps.
HEAT RECOVERY VENTILATION (HRV) SYSTEM
Efficiency Versus Cost
The efficiency of any HRV or ERV depends on the core material and size. ERVs can take advantage of the energy contained in the water vapor they recover, giving them a slightly higher efficiency than HRVs. Both types of ventilators have fan motors that can run continuously, requiring considerable energy. Look for ones with electrically-commutated motors (ECMs), which have greater efficiency and the benefit of variable speed operation.
For a more energy efficient HRV or ERV system consider the UltimateAir RecoupAerator 200DX ERV, the Venmar AVS E15 ECM ERV or the Zehnder products. However, highly efficient units like these are more expensive. If cost is a primary concern, consider less efficient, lower priced units such as Life Breath or Panasonic. Or, save even more with balanced ventilation without heat recovery for mild climates where heat recovery is less cost effective. To select ventilation equipment that is appropriate for your project, consult the Home Ventilating Institute’s list of equipment specifications. To estimate the cost-effectiveness of heat recovery and ventilation options, you can perform an energy model on the building design.
It’s important to choose equipment that matches the needs of the project. The most efficient system may not be the best overall choice. For most applications, a moderately efficient unit will be sufficient. For example, homes under 1,500 square feet may not need a large amount of airflow, and may not have spaces for ducts to run. They might be better served by a smaller, somewhat less efficient, system such as the Panasonic Whisper Comfort Spot ERV. A newly available type of ERV, designed to install through the wall, is most appropriate for small spaces or those without good access for ductwork.
HRVs and ERVs have two duct systems. One system removes air from high moisture areas, such as bathrooms, kitchens, and laundry rooms. The other supplies outside air to bedrooms and living spaces. This arrangement helps to balance the air flow and mix air throughout the home. To keep duct runs short and inside the conditioned space, it’s best to pick a central location for the main unit, which houses the exchange core and fans. Because all ventilation systems make a modest amount of noise, take care not to place the unit near bedrooms or minimize potential disturbance by using sound-abatement methods.
Try to maintain indoor relative humidity between 30% and 50%. Most climates will benefit from removing small amounts of indoor water vapor with an HRV. In dry climates, homes need to retain some water vapor in the living area, which can be done with an ERV. And in warm humid climates, ERVs help keep excessive humidity outside. Just moving the air regularly with either type of ventilation system will help prevent mold and keep indoor air pleasant. Learn more about moisture management in homes.
Replacing Exhaust Fans
Bathroom exhaust fans can cost several hundred dollars each when installed by an electrician. Let the HRV/ERV do the work of exhaust fans and save money. Most HRV/ERVs can operate at multiple speeds. They are generally sized so that the lowest speed meets the home’s basic ventilation requirement when operated continuously. Boosting to high speed can clear bathroom moisture and odors, making bath fans unnecessary. The cost of installing bath fans is avoided, and helps pay for a portion of the HRV/ERV installation. This has the added benefit of reducing building air leakage by eliminating penetrations for exhaust fan ducts to the outside.
Placing an ERV or HRV vent in the kitchen, instead of a vented range hood, eliminates another penetration of the sealed building envelope and avoids venting heated air directly out of the kitchen. The exhaust grille should be at least 6 feet from the stove in order to prevent grease buildup from cooking. Check with the building code officials in your jurisdiction before replacing the range hood with an HRV/ERV to be sure this approach is allowed. If needed a recirculating range hood can be used in conjunction with an HRV/ERV for kitchen ventilation.
Filter The Air
Even with a home built with non-toxic materials, cooking odors, cleaning agents, and toiletries can all release gases that pollute the indoor air. In addition to bringing in outside air to dilute indoor sources of air pollution, it’s also necessary to clean or filter the indoor air. Furthermore, outside air isn’t always as healthy as it should be. Vehicle exhaust and industrial emissions pump toxic chemicals and particulates into the air. In some regions, wildfires fill the sky with smoke for weeks at a time. So in addition to diluting the indoor sources of pollution, it is often necessary to filter the “fresh” air coming in from the outside. So, most ERVs and HRVs filter the incoming and recirculating air.
Air filter effectiveness is expressed as the median efficiency reporting value (MERV) rating ranging from 1 to 16. A typical furnace filter has a MERV rating of about 8. Zero energy homes should use a filter rated at MERV 13 or greater on their ERV or HRV. High-efficiency (MERV 13+) filters can create a high degree of resistance to airflow so check with the ERV/HRV manufacturer beforehand to be sure the unit you intend to purchase can accommodate a MERV 13 filter or better. The filter that comes with HRVs and ERVs may not have a filter efficiency necessary to capture small enough particles to meet everyone’s needs.
People with allergies, special health concerns or those living in areas with high outdoor air pollution should consider a high efficiency particulate air (HEPA) filter. These high-efficiency filters create too much resistance for most ERV/HRV units. In that case, it’s necessary to add an in-line self-powered filter system, such as the Lifebreath Turbulent Flow Precipitator (TFP) or the stand-alone Honeywell True HEPA Air Purifier.
Because zero energy homes are tightly sealed and highly insulated they protect against unfiltered polluted air from the outside. At the same time, tightly air sealed homes also demand extra precautions to protect the health of occupants from indoor pollutants. Using pollutant source control, fresh air ventilation and high-efficiency filtration zero energy homes deliver both high performance and a much healthier home.