by Mike Barcik

Indoor air quality (IAQ) problems in homes can take many forms but the main way to look at problems is to remember the three “P’s” of IAQ: “Pollutants need a Pathway to People.” If you get rid of any one of the 3 “P’s,” the problem goes away.

For example, getting rid of pollutants is the best approach. Or, separating pollutants by blocking their pathway to the occupants is another viable alternative. Finally, having sick people move out of their polluted home is another, albeit fairly drastic, option.

To further refine priorities, if a moldy crawlspace is the primary source of my allergy problem, I can first try to get rid of the mold, second try to seal off the moldy air from me, and finally try to dilute the mold with fresh outside air. This leads to the following hierarchy for fixing IAQ problems:

1. Source Control – Don’t bring pollutants into the home
2. Separation – Keep any pollutants sealed away from people
3. Dilution – Provide fresh, outside air to reduce pollutant concentrations
4. Filtration – Catch some pollutants via a filter

What does this mean? The most important approach is to not bring the pollutant inside the home; the least effective way is to try and capture it with a filter.

Enough theory! Give me application! What are the pollutants?

When building or renovating a home, it important to make good choices in terms of not bringing pollutants in homes. Many times the sources of pollutants are the materials we choose to build with – in fact chemically sensitive individuals must be extra careful with their material selections.

Some common pollutants found in homes and ways to prevent or lessen their impact on the occupants will be discussed: VOC’s, mold, dust (pollen, dust mites, insulation fibers, etc.), Carbon Monoxide and other combustion products, radon, pesticides and household chemicals.

VOCs (Volatile Organic Compounds) – these are the off-gassing of chemicals (think of “that new car smell”) typically found in many glues and adhesives as well as finishes such as paints and carpets and press-board materials such as particle board or MDF. Many times there are low or non-VOC options available for many of these materials that contain, amongst others, urea-formaldehyde (a common VOC).

Almost every major manufacturer carries a line of low-VOC paints at little or no cost premium. Several carpeting options are available with different glues or that are made with natural fibers, such as wool. Caulks and adhesives are also commercially available as are press-board products made with no added VOC glues. And while many solid surface countertop options are available, tight budgets can still utilize laminate surfaces if the substrate has low VOCs or is sealed to reduce off-gassing.

Mold - Mold is a common pollutant that requires mold spores, a food source, typical room temperatures, and high moisture content (above 70% Relative Humidity). Because spores and food sources are everywhere, mold growth can only effectively be prevented by controlling moisture.

Mold prevention ties directly with proper moisture management, particularly exterior drainage and flashing details. Certain exterior claddings systems, such as brick veneer and vinyl siding, are intentionally designed to have a drainage space behind them, effectively creating an escape route for water that gets beyond the exterior surface of the cladding. This drainage plane concept can be applied to virtually any cladding system. Acceptable “behind the cladding” water shedding surfaces are felt, housewrap, and foam sheathing.

When a mold outbreak does occur, it is important to react quickly and to document your efforts in case of liability concerns. Generally, except in cases where required by litigation, mold testing is not recommended. If you see mold, you have mold (remember, mold spores are everywhere). Instead, spend the money and effort on finding and fixing the moisture problems and then on remediation of the mold.

Mold remediation rules and guidelines vary depending on the degree of exposure, physical size of the outbreak, and materials affected. Porous and absorptive materials, such as drywall and carpet, must generally be removed while wood studs and joists can often be wiped down with a mixture of mild soap and water or water and TSP. Visit www.epa.gov/iaq for more information.

If the source of the mold is an uninhabited portion of the home, such as an attic or crawlspace, careful air sealing to prevent that air from mixing with the conditioned space is crucial. While efforts should be made to prevent mold growth in those locations, the physical pathways made by plumbing, electrical, and HVAC penetrations should be sealed to eliminate the connection to the living space.

Since ductwork is often located in attics, crawlspaces, and basements, it should be carefully sealed with mastic (not tape!) to prevent pressure differences that cause pollutants to enter the home. Besides saving energy, sealing leaky ducts is one of the most effective means of improving air quality.

Dust (pollen, dust mites, insulation fibers, etc.) - Dirt and dust can be comprised of many possible different materials. A great deal of larger dirt particles are often brought into a home on people’s shoes, so a mud room area or an effective mat can be useful. Most of the dust that creates respiratory problems is of a smaller particle size, which means that it is lighter and more airborne and thus more difficult to capture.

While pollen, insulation fibers, and other particles can usually be controlled by air sealing their entry pathway and separating them from the occupants, dust mites are commonly generated internally. Dust mites are microscopic critters that feast on dead skin cells and their population flourishes when the indoor moisture content exceeds 50-60% RH (relative humidity). Tiny, dead dust mite body parts and poop actually make up a portion of the airborne dust in homes.

The best solution is to try and maintain the RH at or below 50% to keep the dust mite population from thriving. Properly sized and installed air conditioning equipment (consider variable speed equipment for greater moisture control), tight ductwork, and good exterior moisture details are all part of the effort to controlling moisture. In some cases, a separate dehumidifier may be necessary for a moisture problem home.

Dust and fibers can somewhat be contained by filtration. Since most homes come standard with a 1” fiberglass filter that is designed mainly to protect the equipment, the amount of filtering actually done is fairly small. While better filters are available in a 1” thickness, their increased resistance to airflow could create greater problems with duct pressures. Changing filters regularly is critical.

Better quality, thicker, pleated media filters are available as an upgrade that will typically require modification to the duct system and careful understanding by the HVAC professional as to their impact on the air distribution system. Since generally leaving the air handler on full time costs a lot and can worsen duct pressure problems, this is not a recommended practice unless the blower is variable speed. However, filters can only catch pollutants that are blown across them so the bottom line is that filters are only a partial solution, at best, to an air quality problem.

Carbon monoxide and other combustion products - Since burning any fuel requires oxygen, air must be provided for combustion to be sustained. A good rule to follow is that combustion air and people air should never be mixed and never be allowed to mix. Following that rule means that combustion appliances like water heaters, space heaters, and furnaces should always be located in a non-conditioned space or be sealed combustion, direct vent.

For example, a home with a small closet with a water heater in it may have a louvered door connecting the house air with the closet air. This scenario is dangerous in that any negative pressure could prevent the flue gasses from going up the flue pipe causing backdrafting to occur. Since one of the common results of backdrafting is the production of carbon monoxide, this is a dangerous and potentially deadly possibility.

A better approach would be to seal and isolate the closet from the occupied portion of the house and provide combustion air directly to the closet via high-low vents (as per code). Another option is sealed combustion, direct vent equipment that, due to its double pipe venting, provides combustion air and exhaust venting as part of the installation. A final option would be to relocate the water heater to an unconditioned garage or vented attic (in milder climates). Each of these approaches separates the water heater from the conditioned space air.

It may seem obvious, but flue pipes are absolutely necessary to vent combustion products (such as water vapor, carbon dioxide and carbon monoxide) for safe operation of combustion appliances. That said, many gas-log fireplace inserts are unvented and should be avoided. Choose a wood or gas-log fireplace that has both a flue pipe as well as combustion air ducted to the firebox. Better yet, select a sealed combustion, direct vent gas-log with a solid glass front.

A final note, one common source of carbon monoxide is attached garages. At a minimum, careful air sealing to separate the garage air from the conditioned space is crucial. A better approach would be a garage exhaust fan controlled with an occupancy sensor. The best option is a detached garage or open air carport. Avoid locating air handling units in attached garages due to duct leakage concerns and NEVER add a supply duct to the garage from the home’s HVAC system.

Radon - Radon is a colorless, odorless gas naturally caused by the breakdown of radioactive materials in the earth. It is also the second leading cause of lung cancer (after cigarettes!) and seeps into homes through cracks and seams in the foundation. Since it is not practical to know if there will be a radon problem until after the home is built, radon prevention should be a consideration during construction and the home should be tested for radon upon completion.

Test kits are commonly available and mitigation is generally warranted for levels of 4 pico-Curies/litre or more. Proper air sealing of the foundation plus a sub-slab depressurization effort may be employed to mitigate high levels. A passive vent system is generally not expensive to install during construction and a continuous duty exhaust fan can be retrofitted if testing deems it necessary.

Pesticides and household chemicals - It doesn’t make sense to follow building science principles and construct a tight, well insulated home with properly sealed ductwork and then bring in all kinds of nasty chemicals to the living space. Make safe choices about cleaning solutions, pesticides, and other household products to avoid the hazards of creating a toxic indoor air “soup.”

If home projects require the use of strong chemicals, attempt to relocate the effort outdoors or at least open all windows and run fans for safety. Read and follow all safety labels for proper use of chemicals and try not to store them in the living space if possible.

V is for Ventilation - Sealing up a home and the ductwork is important for energy and pollutant entry reasons. Operable windows are always provided to allow the occupants the ability to vary the connections to the outside air (although they often remain closed). However, most homes, regardless of how leaky they are, should have some degree of regular outside air ventilation provided.

One simple method of introducing outside air is to duct a small amount (~50 cfm via a 4-6” diameter duct) of outside air to the return plenum. A motorized damper and controller can make this process even more efficient and consistent and prevent over- ventilating on extreme temperature days.

Exhaust fans in the kitchen and baths that are ducted to the outside are also vital. Ducts must be routed carefully and terminate through to the outside and not into an attic or crawlspace. Quiet, efficient, Energy Star fans are available that are more effective than the standard quality, noise-maker variety.

Many excellent factsheets and articles on any of the above topics can be found for free from the www.southface.org website.

Mike Barcik is Director of Technical Services at Southface