Conscious Choice: Building for Indoor Air Quality

January 2001

Building for Indoor Air Quality

by William Hurrle

What does quality housing mean to you? Most home builders and buyers focus on cosmetically correct homes in the right location. And these, of course, are important elements in a place where you will spend both waking and sleeping hours. But health, safety, and structural durability are far from secondary matters. The recent increases of allergies and asthmas testifies to the failure of the looking-good approach: Indoor air quality is a prime cause of these ailments.

Indoor air quality is in the toilet, so to speak, because changes in the construction industry have not been matched with awareness of what they mean. Builders today use as their reference point the houses that were built in the sixties. But construction has changed since then, and so have building materials. Houses have gotten tighter and myriad chemicals are used in raw building materials and consumer products. So, even as people spend more time inside, indoor air quality has declined.

Mold

Humidity-related problems offer one clear example. Air in houses was dry in winter, so people added moisture with humidifiers. Now many homeowners see water condense on their windows. That circumstance is more than an aesthetic inconvenience; greater-than-fifty percent relative humidity (rh) — allows molds and dust mites to thrive. We don’t know asthma’s cause, but we do know that mold spores and dust mite feces are certain triggers for asthmatic episodes. Neither can live at humidity levels less than fifty percent rh.

Obviously, precipitation has to be kept out of a house. Building scientists want a continuous drainage plane from roof to foundation footings. Since moisture can get under siding, outside walls are protected with a house wrap. Gutters, downspouts that drain away from the house, and a positive slope away from the foundation are part of the system. So is drain tile around the footings, gravel fill next to the foundation, dampproofing on it, and a vapor retarder under the slab.

These measures, along with modular construction, good windows, vapor retarders, densely blown fiber insulation, foam, caulk, and sealed can lights, produce homes with less than 0.3 air changes an hour (ACH), much less than the 0.5 to 0.75 ACH of older homes. While this increases energy efficiency, it’s not great for ventilation.

After all, not all sources of moisture come from outside. Showers, cooking, plants, animals, aquariums, stored fire wood, and human respiration daily produce quarts of water vapor. Open sump pits, damp foundation walls and back drafting combustion appliances can add more. In homes with discontinuous, imperfect vapor retarders and shoddy insulation detailing, warm, moist interior air slips through air bypasses. When it hits something with a temperature less than the dew point, it condenses like beads of sweat on a summer beer. This condensation hides in the walls, under rugs on concrete floors, behind insulation in sill boxes, and on the back of roof sheathing in attics. That’s where wood rots and mold grows.

There are other sources of indoor moisture, too: plumbing leaks or condensate dripping off cold water pipes. Any source of moisture is a potential breeding ground, and if organic matter finds its way into the mix, mold makes itself at home. In old houses, the risk was not as high, because high inside rh was reduced by cold, low rh outside air coming through air leaks.

Dust Mites and Humidity

Dust mites are spiders, smaller than we can see. When humidity drops they stay in cocoons tightly attached to fibers in carpets or a couch pillow cover. If rh continues below fifty percent, eventually they dry out and die. But in tight, ill-ventilated homes humidity is often above 50 percent rh. The best way to keep air moving is to ventilate moisture at its source. Range hoods, for example, should terminate outside, not just recirculate air minus larger grease particles. Bath fans, which often are ineffective rattlers with flex duct outlets thrown in the general direction of the soffit, should be ducted through the roof. The most central bathroom should have a dehumidistat control set to forty-five percent rh. There are multi-point systems with one fan and ducts that run to baths, laundry rooms, and kitchens. Sump pits need gas-tight lids with gaskets for the drain pipe and power cord. If measured air flow through the shell doesn’t supply enough air, controllable vents should be installed in bedroom walls.

The above measures are the least you should do to ensure healthful indoor air. Better is a heat recovery ventilator (HRV). Like the multi-point fan, an HRV pulls stale air from several places. But it also supplies fresh air in the wintertime. In this system, cool, fresh air from outside passes by warm, outgoing stale air, recovering about 75 percent of the heat.

The 80-120 cubic feet per minute fans in HRVs can’t push air through ducts designed for 1,000 cfm furnace blowers, however. If the HRV is tied into furnace supply ducts to avoid the expense of a separate duct system, the furnace fan must be on to distribute air. A control called a FanRecycler turns the furnace fan on at a set time per hour, and remembers if it has run.

Dust and Pollen

Dust and pollen trigger allergies, but this risk is easily minimized with furnace filters. A combination of fiber furnace filters and pleated media filters removes particles 0.1 microns and larger: animal dander, bacteria, pollen, plant spores, lint, asbestos, and lung damaging dust. Left are smaller household dusts, tobacco, wood, and cooking oil smoke. Electronic air cleaners get much of this, down to 0.01 microns, but viruses and some dusts remain. The low-tech practice of removing shoes at the door is a good precaution.

When hot, humid air from the Gulf of Mexico comes north, the best way to keep rh less than 50 percent is with a whole-house dehumidifier. Air from outside comes through that, then on to the house via ducts. If the building shell is not tight, humid air defeats the mechanics. Air conditioners also remove humidity, if they run long enough. Often, however, they are oversized in relation to the room in which they are placed. Then they run on a short cycle, producing a cool, but clammy house.

Exhaust

Humidity is the lead indicator of problems, but there is more to indoor air quality, including pollution from various kinds of exhaust. Carbon monoxide (CO) for example, is poison. Folklore has CO coming from cracked furnace heat exchangers, but this is a small source of problems, at around 0.05 percent. Responders say 60 percent of calls regarding exhaust fumes are due to CO from attached garages. People open the door, start the vehicle, back out, and the door closes. The open door is a large hole; but there is no driving pressure moving air toward it, so CO remains in the garage.

It doesn’t stay there. Basements inhale garage air CO, and the furnace fan distributes it. The solution is to seal garage-house connections. Tape and fill garage drywall seams. Also caulk the sill box assembly and gasket switch and receptacle boxes in the common wall. Weatherstrip the service door carefully. Then check to see if the efforts work.

Other sources of CO include dryers; gas ranges; and inadequate, poorly installed, and corroded flues. If flue pipes are rusty, if there is not much upward slope, or if there are two appliances into one flue, there may be trouble. Wood-burning fireplaces also can be CO sources. As the fire dies in a tight house, it may not generate enough heat to keep a draft established (this is a more likely scenario if an exhaust appliance comes on). The house becomes the chimney. Non-vented combustion heaters are promoted as safe, but prudent people never use them in a closed space.

In fact, all flamed appliances — furnaces, water heaters, and fireplaces — should have sealed combustion. Their doors should be gasketed and built to shut tight. Even so, it is a good idea to have a CO detector on each floor. Their sensors get coated with crud over a few years, so treat them as replacement items. Know that the low limit is set high enough to prevent mortality, but higher than the EPA says is okay, nine parts per million.

Outgassing

Construction materials and even construction sites can carry inherent risks. For example, radon, ionizing radiation like an A-bomb’s, is often part of the ground on which a house is built. It enters with other soil gases, like methane, through openings in foundation masonry. The American Lung Association blames radon for about a quarter of lung cancers. Householders can test for radon with a charcoal canister and then mitigate, eliminating that risk.

Building materials often come with risks attached, too. Vinyl siding and PVC pipe don’t seem harmful, but their life cycles produce dioxins and other killer organochlorides. Organochlorides are evil toxins that cause cancers (breast, bladder, testicular), disrupt hormone signals, reduce sperm counts, cause birth defects, suppress the immune system, cause endometriosis, pregnancy failure, and cognitive/behavior deficits. Plywood, pressed wood, and formica countertops are treated with formaldehyde, which then outgasses even after the house is built. Sensitive and environmentally aware consumers can choose to avoid these hazardous products and, instead, buy products marketed as "green."

Then again, householders can be their own worst enemies. They smoke. They store toxic cleansers under sinks; they let paints sit and harden as the volatiles hammer indoor air quality. Art, hobby, and beauty products add to the burden. Candles, for example, produce millions of minute, greasy particulates. Outgassing plastics abound. Pesticide applicators come on monthly schedules, and people dose their own lawns and gardens with herbicides and fungicides, turning a healthful lawn into a deathtrap.

In fact, your lawn can be a starting point for creating a healthful home. Whatever your home environment at the moment, you can improve it with plants. Indoor plants — and the microbial life in their soils — reduce indoor toxins. Trees outside provide shade, wind shelter, carbon sequestering, and oxygen production. Native plants in the landscape minimize turf monocultures and require less intrusive care than conventional lawns.

It is impossible to create an island of health in an ocean of hazard. But building science, home performance technicians, green building, and proper landscaping can create healthful homes with a cost increase of only 3 to 5 percent. You can save that in energy use, over time, and be content in the knowledge that your house has healthful indoor air.

Resources

Building Science Corporation

Health House, American Lung Association of Minnesota

Energy and Environmental Builders Association, 10740 Lyndale Ave. S., Bloomington, MN 55420, publishes Builder’s Guides, the bibles, for how to do it. Chicago wants the cold climate edition.

Shelter Supply, 800-762-8399, sells HRVs and materials to build an efficient, healthy home. They are not at local building supply outlets.

Energy Federation, Inc., 800-876-0660, sells hardware for weatherization, ventilation and efficient lights.

Therma-Stor Products, 800-533-7533, sells whole-house dehumidifiers.

RadonAway has radon testing and mitigation products.

No Regrets Remodeling: Creating a Comfortable, Healthy Home that Saves Energy by the editors of Home Energy magazine, a technical publication for building scientists, weatherization agencies, government bureaucrats, research, and utility people. The book presents technically correct information for the general public.

Energy Star appliances and home programs

Pandora’s Poison: Chlorine, Health, and a New Environmental Strategy by Joe Thornton, is authoritative, comprehensive and terrifying.

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