Healthy Air for Tight Homes

The superior energy efficiency of ICF walls is a major factor driving industry growth. This efficiency is due to three factors: the higher R-value of the foam, the thermal mass of the concrete core, and the airtightness of the walls. While airtight walls are positive from an energy standpoint, they can be detrimental to indoor air quality.

It’s possible that an ICF structure may be so tight that levels of moisture, radon, carbon dioxide, pollen and allergens, etc. rise to unhealthy levels. This is especially true if the home includes an attached garage.

Nikki Krueger is the marketing manager at Ultra-Aire Whole House Ventilating and an expert on home ventilation. She says, “In tight homes, indoor air can actually become much more polluted than the outdoor air and create indoor environments that are not only uncomfortable to the occupants, but unhealthy as well.” Groups such as the EPA, American Medical Association and American Lung Association have all voiced concern about indoor air quality.

Clearly, to maximize the health of the building’s occupants and the efficiency of the ICF structure, careful attention needs to be paid to how the building will be ventilated.

When Is Additional Ventilation Needed?
Whether whole-house mechanical ventilation is needed has been hotly debated in the ICF community for some time, and ultimately the answer depends on the climate, lifestyle, and construction of the home.

Blower-door tests, now required by many building codes, can easily verify how tight the home is.

Code requirements for residential construction is referenced in ASHRAE standard 62.2. It specifies .03 cubic feet per minute (cfm) per sq. ft. of living space, plus 7.5 cfm per occupant. For commercial buildings, schools, and offices, the U.S. standard is .35 air changes per hour (ACH), or 15 cfm per person, whichever is greater.

For comparison, older wood-frame homes typically measure between 5 and 12 ACH. ICF construction with a wood frame roof usually come in between 1.0 and 1.5 ACH. However, ICF homes with concrete roofs consistently measure as low as 0.15 ACH.

Because most air leakage comes through the windows, builders have used a simple rule of thumb devised by energy expert Richard Rue for years. This guideline states that if the building has at least a 10% to 30% ratio of windows (operable glass) to wall area, there’s enough leakage to ensure healthy air.

In recent years, more precise measuring techniques have become common. Code requirements for tighter homes—regardless of construction—have made “blower door” tests affordable and widely available. Back in 2009, the International Energy Conservation Code (IECC) required new homes have not more than 7.0 ACH, and allowed for a visual inspection to measure air tightness. But the 2012, the IECC set the maximum allowable whole building leakage for residential buildings at the equivalent of 3 ACH, and requires it to be verified with a blower door test. (For commercial buildings, the maximum allowable is 0.40 cfm/ft².)

Even if the ICF home meets the required minimum of 0.3 ACH, additional ventilation may be desired. Combustion appliances (furnace, dryer, gas stove) may require changes to the ventilation strategy. So might occupants’ sensitivity to allergens such as pollen and VOCs.

Dehumidification is also sometimes needed to ensure the drying of building materials and to handle humidity. Krueger points out, “Cooking, showering, breathing, and other day-to-day activities of the home’s occupants also generate a moisture load that needs to be removed.”

It should be noted that many jurisdictions require ventilation, a trend that is becoming more common. Minnesota has required mechanical ventilation since 1999, and Canada has required air exchangers since 1995.

Traditional Options
The easiest response to an overtight home—opening a window—does provide ventilation, but defeats the purpose of an energy-efficient shell, since the building’s conditioned air will escape and the building’s HVAC system must compensate.

Another low-cost solution (less than $500) is “ventilation on demand.” Similar to an automatic bathroom fan, these systems are coupled to a humidistat (to monitor moisture levels) or a CO2 sensor for minimal additional cost. If air quality or humidity reaches an unacceptable level, it will trigger the fan. These systems are exhaust-only devices, so like the open window, they allow conditioned air to escape without capturing the thermal energy it contains.

Krueger cautions, “Exhaust ventilation is not recommended, especially in humid climates, because of concerns that air entering the structure will cool off as it works through the structure and into the home, potentially resulting in condensation within the wall cavity.” She notes that for every cubic foot of air exhausted, a cubic foot of air will move into the home through the various cracks and seams, and there is no way to filter, dry, heat or cool that air before it is introduced into the living space.

Balanced Air Systems
For high-efficiency buildings, a better solution involves mechanically controlled ventilation that uses the exhaust air to pre-condition the incoming stream.

Balanced air systems, such as heat recovery ventilators (HRV) and energy recovery ventilators (ERVs) use the energy and moisture contained in the exhaust air to pre-condition the incoming air stream.

Heat recovery ventilators (HRV) and energy recovery ventilators (ERVs) both do this. The primary difference is that HRV systems transfer heat only, while ERV systems efficiently transfer both heat and moisture content. When properly engineered, these systems can provide the optimal solution: better climate control, high quality air, and maximum energy efficiency. Most ERV systems currently on the market recover about 70% to 80% of the energy in the outgoing air.

One major consideration for these systems is the cost. Prices typically range from about $500 to $1,700, not including installation. According to the U.S. Department of Energy, they’re “most cost-effective in climates with extreme winters or summers, and where fuel costs are high.” In mild climates, they may consume more energy than is saved.

Another option is EZ Breathe, an innovative balanced air system that mixes the fresh supply air directly with conditioned interior air, instead of using a heat core, enthalpy wheels or membranes. Tim Chapin, director of research and development, says, “We started getting questions from contractors, especially in Canada where they were building tighter homes, about a more effective balanced-air system. HRVs don’t address internal moisture issues typically found in basements and crawlspaces.”

The EZ Breathe system puts the exhaust fan unit in the lowest level of the home, typically the basement or crawlspace. The intake unit is located adjacent to the living area, and mixes two parts of inside air with one part outside air to create an acceptable temperature and humidity of about 35-40%.

Like ERVs, the intake and exhaust typically pull from opposite sides of the building so the air streams don’t mix, but there are key differences. Chapin continues, “This is really low maintenance compared to HRV. The outside filter is easy to wash in the sink. And it’s extremely efficient.”

To verify efficiency outside the lab, the company tested six homes in the middle of the Minnesota winter. All had traditional HRVs, and an EZ Breathe. Chapin reports, “The exhaust stream coming from the EZ Breathe units was always four to six degrees cooler than the HRV exhaust. The explanation is simple: we are always exhausting air at the coldest dampest place—the basement or crawlspace floor—whereas HRV/ERV systems typically are drawing air from the conditioned living area.”

He continues, “The system is easy to install, virtually maintenance free, and the end cost of the system to homeowners can be as little as one-third of an HRV or ERV system.”

EZ Breathe’s balanced air system mixes the fresh supply air directly with conditioned interior air (below). It always exhausts air from the basement or crawlspace floor— typically the coldest, dampest place.

Supply Ventilation
Other ventilation experts, including Krueger, recommend “supply-side” ventilation, where only the intake air is mechanically driven.

Supply ventilation is considered the best approach for ventilation in most climates because it keeps air moving from the inside of the home toward the outside, removing the concern about internal condensation. Supply ventilation systems provide make-up air for spot ventilation fans such as bathroom exhaust range and range hoods. The pressurization of the home also reduces the amount of total ventilation a home will experience due to other pressures on the home, such as wind.”

A simple supply ventilation system can be inexpensive to install. Since air is coming into the home from an outside duct, supply ventilation provides the ability to filter, dry, heat or cool outside air before it is introduced. In hot, humid climates such as much of the southern U.S., this means cooling and dehumidifing the intake air. To address this concern, EPA’s Building America program says a supplemental dehumidification system should be installed in hot/humid climates as best practice.

Krueger’s company makes Ultra-Aire, a combined dehumidifier and supply-side ventilation system. “Unlike a typical HRV or ERV, this provides the ability to mechanically remove water from the ventilated air until a specific set-point is reached. It can provide fresh air ventilation along with year-round humidity control. This results in an indoor environment that is healthy, comfortable and provides the ideal conditions conducive to protecting and preserving the building and its contents.”

Regardless of the system you choose, remember the advice given in U.S.
Department of Energy guidelines: “ ‘Seal tight, ventilate right’ is the recommended approach to house ventilation. Houses should be tightly sealed to reduce infiltration, and a whole-house ventilation system installed to provide fresh air and remove pollutants.”