Improved Indoor Air Quality (IAQ) in an Energy Efficient Demonstration House

This paper describes a model demonstration house that was built to be extremely energy efficient and, at the same time, have minimal indoor air pollution. The house was designed to be suitable even for chemically sensitive individuals. With the building envelope constructed in an airtight manner, and the use of a mechanical ventilation/filtration system, occupants are given optimal control over the indoor air. The design parameters, as well as the construction materials and techniques that were used to construct this house, are discussed.

Introduction

During the energy crunch of the 1970s, homeowners added insulation, caulking, storm windows, and weather-stripping to their homes in order to make them more energy efficient. This resulted in structures that were not only more energy efficient, but also more airtight. Soon, there were dozens of stories in the media equating energy efficiency with poor indoor air quality. While it has since been shown that the two concepts are not at odds with each other, the connection is still often made by the public.

 

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Chemically sensitive people were described as early as 1962. Individuals so affected often have difficulty tolerating common building materials due to their outgassing characteristics, therefore housing for them must be as pollution free as possible. This paper will describe an extremely energy efficient house that was designed and built to maintain the high degree of indoor air quality required by chemically sensitive people. The single story three bedroom two bath house contains 1568 square feet and it is located in a rural section of southern Indiana. The features of this demonstration house are described in depth in a recently released book and video tape.

Goals

Several goals were enumerated early in the design phase of this project. They were:

Air Quality

• Inert materials should be used wherever possible.
• The use of toxic pest control chemicals should be avoided.
• The house should be built in an airtight manner for controlling the infiltration of polluted air.
• When inert materials aren't readily available, polluting materials should only be used if they can be isolated from the living space.
• A method of ventilating the whole house should be employed to provide fresh air to all rooms, and additional more powerful exhaust fans should be used in rooms where pollutants or moisture are generated.
• The house should have a mechanical air filtration system to remove any minor pollutants generated indoors by the occupants or brought indoors with the ventilation air.

Energy Efficiency

• The house should have superb insulation levels.
• Windows and doors should be energy efficient.
• The house should be built in an airtight manner for controlling infiltration of unconditioned air.
• The mechanical ventilation equipment should be energy efficient.

General Requirements

• The construction materials should be readily available.
• Difficult construction techniques should be avoided.
• The house should not look unusual.
• The house should be affordable.

Healthy Design Principles

Pollutant source strength reduction has come to be regarded as the most effective method of reducing indoor pollution. This was accomplished by utilizing three basic design principles: eliminate, separate, and ventilate. eliminating potentially polluting materials means utilizing construction products that are as low-polluting or inert as possible. As major pollutant sources, wall-to-wall carpeting, resilient floor coverings, and synthetic wall covering materials were avoided because of their potential to release volatile organic compounds (VOCs), as were manufactured wood products capable of releasing formaldehyde. Low odor paints, caulking, sealants, etc. were chosen among products often recommended for chemically sensitive people. Since a non-toxic insulation was not available locally, the second principal of separate was used to completely isolate the insulation from the living space. The principle of ventilate was used to provide fresh air and at the same time to remove stale air from the house.

Foundation

A concrete slab-on-grade foundation was selected as being the most cost effective way of achieving a healthy floor system. The foundation has R-20 perimeter insulation and R-20 under-slab insulation that extends inward approximately 72" from the exterior walls. All foundation insulation is extruded polystyrene. The slab was strengthened with reinforcing steel to minimize cracking, and all plumbing penetrations were sealed with caulking.

 

The monolithic concrete slab, poured on top of a polyethylene vapor retarder, forms an effective barrier against soil gases such as radon. However, a sub-slab piping system was installed as a means of mitigating high indoor radon levels should they ever occur. It was decided that the low cost of the piping was inexpensive insurance against a possible future problem.

 

For sensitive occupants concerned about outgassing from the polystyrene insulation or the polyethylene vapor retarder, it should be pointed out that they are well separated from the living space by a 4" thick concrete slab. The slab was eventually covered with glazed ceramic tile. The tile was attached with a conventional thin-set mortar, and the grout between the tiles was made on the jobsite from a mixture containing sand, Portland cement (Type I), mason's mineral colorants, and water.

Building Envelope

A double wall system was used to achieve R-33 insulation levels. The ceiling is insulated to R-38. Fiberglass insulation was used above grade. There have been literally hundreds of papers written on the potential cancer causing ability of fiberglass, but negative health effects have shown up with other insulating materials as well. A cementitious foam insulation made from magnesium silicate is being marketed as a non-toxic alternative for chemically sensitive people, and natural cork insulation has been in use for centuries, but both can be quite expensive, and availability is limited. The decision to use fiberglass insulation was made after it was determined that airtight construction techniques could be used to form an effective barrier between the insulation and the living space. Insulation installers, who are theoretically at greater risk than homeowners, are encouraged to wear respiratory and skin protection.

 

The walls of the house were framed out of standard steel studs and channels. Widely used in commercial construction, this material is unfamiliar to many residential builders. However, it is not difficult to work with and, as lumber prices rise, home builders are finding it an attractive alternative. The primary reason for using steel framing in this project was because it is termite resistant. The avoidance of chemical termiticides is often mentioned in construction guides for sensitive individuals. A secondary reason for using steel framing involves the fact that chemically sensitive people often react negatively to the naturally occuring terpenes given off by conventional softwood framing lumber.

 

Aluminum siding was used for the exterior of the house and sheet metal was used for the roofing because their baked-on finishes are generally well tolerated by chemically sensitive people. It should be pointed out that exterior cladding materials only minimally affect interior air quality, but it was decided to use materials that were as inert as possible throughout the project. Entry doors are insulated steel units with magnetic weather-stripping.

 

The windows have thermally broken aluminum frames and triple glazing. The house will be partially heated by the sun through passive solar design techniques. The 148 sq. ft. of glazing in the house is equivalent to 9.4% of the floor area, and 82% of the glazing is south facing. The roof overhang is constructed to shade the south facing windows in the summer, yet allow them to be exposed to the sun in the winter.

Airtight Construction

The drywall in the house is foil-backed to provide both a vapor barrier and a pollutant barrier. An airtight building envelope has better energy efficiency, fewer hidden moisture problems, and it allows the occupants to have more control of the indoor air through the use of mechanical ventilation equipment. With airtight construction, the living space is effectively isolated from the insulation and the outdoors. A technique called the Airtight Drywall Approach (ADA) was used in conjunction with caulking, gaskets, and airtight electrical boxes to achieve virtually an airtight structure. ADA involves relatively simple techniques that builders can use to construct an airtight envelope.

Filtration and Ventilation

A packaged filtration unit was installed to remove pollutants from three sources: those brought indoors by the ventilation system, those released indoors by occupant related activities, and those generated by the occupants' metabolism. The filtration unit contains a 1" polyester prefilter, a 2" thick activated carbon filter, a HEPA filter, and a two speed (300/600 cfm) blower in a sheet metal cabinet. The ductwork for the system was designed to pull stale air from all of the closets, baths, and the kitchen, filter it, and blow the cleaned air into the living room. This circulation pattern provides the cleanest air to the living areas but also results in closets not becoming stagnant. Interior doors are undercut approximately 1" to allow for air circulation when doors are closed.

 

A heat recovery ventilator (HRV) having removable dual aluminum crossflow cores was used to provide approximately one air change per hour (200 cfm) in an energy efficient manner. It exhausts stale air and brings in fresh air simultaneously. When operating, it will also keep indoor humidity levels down in order to control the growth of mold and other microorganisms. The outdoor air passing into the house through the HRV is ducted into a plenum ahead of the filter unit. At the same time, the HRV exhausts air to the outdoors from the kitchen and master bathroom, thus keeping the air pressures in the house balanced. The HRV used was tested at 77% sensible recovery efficiency at -13 degrees F in accordance with the Canadian Standards Association Standard C439M.

 

The filtration unit and the HRV are activated by a single 12 hour manually operated crank timer. Since the filter moves more air than the HRV, even on low speed, whenever the system is operating, air will be circulating throughout the house. If outdoor air pollution levels are unusually high, the HRV can be shut off, and the filter can continue to recirculate and clean the air in the house if desired. The one air change per hour capacity of the HRV provides more fresh air than is typically needed during normal occupancy, especially since low pollution materials have been used throughout the house, so it only needs to be run for 6-8 hours per day to supply sufficient fresh air for the occupants.

 

The kitchen range hood and the exhaust fans in the two bathrooms are used to remove air pollutants, or high levels of moisture, quickly. The laundry is located in a closet in one of the bathrooms, thus a single exhaust fan in the closet serves both the bathroom and the laundry area. Because of the airtightness of the structure, during operation the exhaust fans tend to depressurize the entire house, resulting in make-up air entering through the HRV ducting. Since exhaust flows during depressurization are approximately half of rated flows, the fans were oversized to compensate. The clothes dryer and central vacuum also depressurize the house, so the homeowners are advised to open a window a small amount for optimal performance. In conventional construction, depressurization can cause chimneys and flues to backdraft, or cause radon to be drawn indoors, but this house has no chimneys or flues, and the concrete floor slab prevents the infiltration of radon.

Heating and Air Conditioning

With the low heat losses in winter (13,914 BTU/hr. @ TD=65 degrees F) and gains in summer (6,242 BTU/hr. @ TD=15 degrees F), the high installation and equipment costs associated with very efficient forced air systems could not be justified. Instead, low polluting low temperature hydronic electric baseboard heaters and a small ductless split system air conditioner were selected. The air conditioner uses an HCFC refrigerant rather that a more ozone-destroying CFC refrigerant.

Interior Finishes

Construction materials directly exposed to the interior space have the greatest potential to pollute that space, therefore they must be chosen with care. In this demonstration house an environmental paint, clear finish, and drywall finishing compound were selected that chemically sensitive individuals often tolerate. Kitchen cabinets are all-steel with a baked enamel finish. The cabinet manufacturer has recently begun to offer a "chemically reduced" cabinet that uses metal stiffeners inside the doors rather than mastic and wood. They will also degrease the drawer rollers and door hinges on special request. Kitchen countertops and shower surrounds are stainless steel. Lavatory tops are vitreous china and bathtubs are porcelainized steel. Interior doors and woodwork are solid hardwood [yellow poplar (Liriodendron tulipifera)]. Softwoods were not used in order to avoid the smell of terpenes indoors. Closet organizers are made of wire mesh with a baked epoxy finish.

 

The house contains an energy efficient dishwasher, radiant glass top electric range, and a central vacuum system. Water supply lines are copper, soldered with lead-free solder. A water filter, with sediment and activated carbon cartridges, processes all of the water entering the house.

 

In order that the occupants can separate potentially bothersome possessions from the living space, three built-in cabinets (pantry, book case, and display case) are fitted with glass or wood doors.

Costs

Several features make this house more expensive than conventional housing. For example, the cost of extra insulation and air sealing are added directly to the house's selling price. However, they will result in low heating bills over the entire life of the house. Similarly, ceramic tile floors are more costly than wall-to-wall carpeting, but they will last indefinitely. When all of the costs of carpeting are calculated, its life-cycle cost is high compared to other floors.

 

It has been estimated that indoor air pollution results in lost productivity and health care costs in the United States of up to 100 billion dollars per year. If living in less polluted air results in healthier occupants, then medical expenses and health insurance premiums may be decreased.

 

Some healthy construction strategies are actually lower in cost when compared to conventional construction (e.g. solid pine floor vs. carpeting and padding over particle board) and some are significantly higher (e.g. cementitious foam insulation vs. fiberglass). The former would tend to be not only cheaper, but provide more health benefits in the way of reduced exposure to VOCs and microorganisms.

 

The house described in this paper, being a demonstration project, contains a wide variety of healthy features, hence its selling price is approximately 20-25% higher than conventional custom built houses in the vicinity. However, it is the author's opinion that houses can be built with fewer health related features, at lower costs, and still have much improved air quality. The resulting life-cycle cost reductions, increased productivity, and improvements in occupant health would seem to offset the minor increases in up-front costs.

Results

In order to determine the actual tightness of the completed house envelope, a blower door test was performed. A flow rate of 99 cfm (equivalent to 0.45 air changes per hour) was measured at a 50 Pascal depressurization. From a practical standpoint, the structure is almost airtight, so the occupants will have maximum control over the quality of the air in the house through judicious use of the ventilation/filtration system.

 

Once the house was completed, a short term charcoal canister radon test was performed and the indoor radon level was found to be comparable to the outdoor ambient level (>0.5 picocuries/liter). The monolithic concrete floor slab forms such a good barrier to soil gases that it was deemed unnecessary to attach a radon suction fan to the sub-slab piping system.

 

Badge-type formaldehyde monitors were placed in the kitchen and master bedroom for 24 hours. The results were 0.01 ppm for each location, the same as for a control monitor placed outdoors. Since there are no formaldehyde sources indoors, similar indoor and outdoor readings were expected.

 

Several chemically sensitive individuals have visited the house, and found the interior space to be very comfortable and tolerable. Since these people typically react to very low levels of a wide variety of air pollutants, this test, though anecdotal, would seem to indicate that the quality of the indoor air is quite good.

 

 

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