News from American Institute of Building Design, California Society

Indoor Smog

While there's been widespread concern about air pollution and it's impact on health for decades, the focus has been mainly on outdoor air. Only relatively recently has attention been paid to the fact that indoor air is often more toxic than the worst outdoor smog. In fact, of the hundreds of airborne pollutants monitored by the U.S. Environmental Protection Agency (EPA), only two are more prevalent outdoors than in. With Americans spending an average of 90% of their lives inside, indoor air quality (IAQ) is an important health concern.

Materials Matter

The impact of building materials on indoor air quality seems widely publicized, with formaldehyde (from sheet goods), vinyl chloride (from PVC and vinyl flooring), styrene-butadiene latex (from carpet backing) and benzene (from paints, solvents and tobacco smoke) among the "hit parade" of Volatile Organic Compounds (VOC's) to be minimized or avoided. In fact, the EPA itself had its issues with indoor air quality: After they installed new carpeting in their offices in 1987, dozens of employees reported problems including burning lips, sore throats and flulike symptoms. A few workers could not enter the building without becoming intensely ill.

Green Materials Are Not Enough

The good news is that the market abounds with "healthy" or "green" building materials which can be substituted for polluting products. What is less widely recognized is the necessity of controlled ventilation to provide for good indoor air quality. Even if a home is built or remodeled entirely with the very greenest, lowest VOC finishes and fittings, there are pollutant sources which cannot be eliminated unless the house is left vacant. The occupants of a home produce carbon dioxide, water vapor (which can foster mold growth), and numerous other airborne pollutants on a continual basis. There's no way around it, we need indoor air exchange to be healthy.

Residential Ventilation Requirements are Coming

While the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the building code has long recognized the importance of controlled mechanical ventilation in commercial buildings, standards and requirements for residential buildings were not offered until recently. Nonetheless, numerous countries and at least a couple states (Minnesota and Washington) now have requirements for residential ventilation and this year, Title 24 will require mechanical ventilation for new homes constructed in California.

Air Leakage Nostalgia

Some people look at this problem and are wistful for old, leaky houses, which provided "random" rather than "controlled" ventilation. The key issue is that random ventilation (in addition to being extremely energy inefficient) provides ventilation in random amounts, at random times, from random locations, of random temperature and of random quality - relying upon random ventilation is "gambling" with indoor air quality. Controlled mechanical ventilation can allow for astounding improvements in energy efficiency and provide a consistent supply of filtered, comfortable, quality air.

Contrary to the previously mentioned nostalgia for leaky houses, so-called "tight" houses are never "too tight" but they have been often "under-ventilated." In fact, "tight" houses are the only houses which CAN be ventilated in a controlled manner. Leaky houses often leak so much air of questionable quality that a mechanical system is overwhelmed. Any concerted effort to provide controlled ventilation starts with control of air leakage.

Fix the Leaks First

How does one provide for controlled ventilation? The first step, as mentioned above, it to minimize air leakage (infiltration) in the home to be ventilated. There is excellent information for both new and existing homes available from the websites listed below:

- The U.S. Department of Energy Office of Energy Efficiency and Renewable Energy (EERE): www1.eere.energy.gov/consumer/tips/

- Energy Star: www.energystar.gov/index.cfm?=new_homes_features.hm_f_reduced_air_infiltration

- The Southface Energy Institute: www.southface.org/web/resources&services/sf_resources&services.htm

Mechanical Ventilation Options

Once air leakage is under control, the next step is to select, design and install a mechanical ventilation system. There are three basic types of mechanical ventilation systems.

1) "Exhaust Only," or Negative Pressure Ventilation

This is the least expensive option, but also the least efficient and least effective at providing clean, comfortable air, as it relies upon air leakage for the air supply.

Typically, an energy efficient and quiet fan (or fans) is installed and dedicated to air exchange, or a bathroom fan can be dual purposed. By drawing air out of the building, the interior pressure is reduced, which causes air to enter from outside through leaks and/or intentional openings. These fans may run continuously or on a scheduled cycle. While it's best to use a quiet fan for ventilation (especially since it's on more often than in a standard bathroom application) no fan is SILENT. Toward that end, there are fans which are mounted inline with ducting, so that the fan can be located well away from the living space. In fact, these fans can be used to ventilate more than one bathroom, saving money and materials and drawing exhaust air from multiple locations, which can increase fresh air distribution.

Note that "Exhaust Only" Ventilation relies on depressurization to ventilate the house. If there is insufficient infiltration to supply the makeup air, depressurization will result, and exhaust gases and other dirt air can be drawn into the house. In these cases, care must be taken to ensure that exhaust gases and dirty air are not drawn into the house (test, identify and plug leaks, use only direct vent gas appliances in the house, and ensure that there is adequate controlled infiltration to balance the exhaust with a "passive inlet vent.") Furthermore, in areas of high outdoor humidity, depressurization may cause excessive moisture to be drawn into the building's framing and insulation, giving rise to moisture and mold problems.

2) "Supply-Only," or Positive Pressure Ventilation

This system works in an opposite way to the exhaust-only approach - it relies on pressurization by forcing air into the house. This system has the advantage of controlling the air supply at it's source, so it can be drawn from a "clean" area and filtered for pollen and dust. Either a self-contained supply ventilator is installed, or a fresh air inlet duct is integrated with the forced air furnace/air conditioner, generally coupled with a controller which runs the forced air fan sufficiently, even when heating are air conditioning are not being used, and closes the vent when not required. If the ventilation system is implemented using the furnace fan, it is important that the furnace has a high-efficiency, variable-speed "brushless DC" or "ECM" motor. Otherwise the large fan of a typical furnace with produce uncomfortable "winds" and large electric bills.

As "exhaust only" ventilation systems are prone to moisture problems in areas where the humidity outside is greater than inside, the opposite is true for "supply only" ventilation systems - in areas where the air inside the home is considerably higher in humidity than outside (cold climates in Winter), "supply only" ventilation systems can force moisture from inside the house into the building's insulation and framing.

3) "Balanced," or Pressure Neutral Ventilation

A more elaborate system, which avoids the pressure imbalance issues of the two previous approaches, and can also greatly reduce the energy loss from incoming air, is a "balanced" system with a dedicated ventilator with at least partially dedicated ductwork. The balanced ventilator both draws air out of, and forces air into, the house. In homes without a ducts and a forced air system, and in areas with high heating and/or cooling loads, it often makes sense to install a dedicated ventilator. There are several basic types: Air to Air Heat Exchangers (AEVs), which simply draw air in and out of the house, Heat Recovery Ventilators (HRVs), which also transfer heat between the incoming and outgoing airstreams without mixing the air together, and Energy Recovery Ventilators (ERVs), which exchange heat and moisture between the two airstreams. ERVs are typically recommended for climates with high outdoor humidity and heavy air conditioning loads and very cold climates with extremely low outdoor humidity.

Energy Consumption for Ventilation

While electricity is required for mechanical ventilation, as long as the equipment is powered by efficient motors, the space-conditioning savings more than compensate for the ventilator's energy use. Building a tight home to today's standards can cut the overall energy consumption for heating and cooling by 25 to 50%. An air-to-air heat exchanger typically recovers 60 to 75% of the energy in the exhaust air, and returns it to the home. HRVs typically use about 100 to 200 Watts per hour of electrical energy, compared to the 2000 to 4000 Watts per hour of heating energy typically saved by building a tight home instead of a leaky, drafty home. There is a growing movement from Europe known as "Passive House," which takes this concept to its logical extreme - extremely airtight, well-insulated houses that "recycle" most of their internal heat gains and require almost no additional heating or cooling at all. Passive Houses use 80-90% less energy for space conditioning than typical homes. We will explore this topic in detail in the future.