Indoor Safety and Comfort by Jon Traudt      09/26/2007

Concentrations of gases, particles, and microbes are commonly 2 to 10 times greater inside homes than outside, according to scientists at the U. S. Environmental Protection Agency and the American Lung Association.  Microbes, mold spores, and other respirable particles are invisible to the naked eye. Carbon monoxide is only one of the invisible, odorless, and dangerous pollutants commonly found in homes.

Superior indoor air quality is found in buildings where the air leaks have been sealed and fresh filtered air is consistently provided by a mechanical ventilation system.

Contents:

• What Can Make Even A Leaky House Dangerous?
• Finding and Sealing Air Leaks
• Potential Legal Liabilities for Weatherization Personnel
• Identifying Solutions
• Conclusions
• References

What Can Make Even A Leaky House Dangerous?

1. Infiltration of Pollutants from Outside and from the Soil:

A leaky house allows the wind to blow dust, pollen, and other particles in through unfiltered openings. The President of the American Lung Association cited a recent study which estimated that about 60,000 Americans — most with lung or heart disease — die prematurely each year from inhalation of particles alone.

Chimneys, clothes dryers, and other air exhaust systems create negative indoor air pressure that pulls odors, microorganisms, moisture, and radon from the soil through cracks, joints, and pores in the floors and walls that contact the soil.

A ducted air circulation system will cause negative air pressure in basements if it removes more air from an enclosed basement than it supplies to the basement.

During cold weather, a leaky house can act like a chimney, causing negative indoor air pressure in lower rooms as warm air leaks up and out through walls and attics. Negative indoor air pressure can pull radon in through floors and walls that contact the soil.

Radon as a serious national health problem according to the American Lung Association, Centers for Disease Control, the National Academy of Science, the Surgeon General, the American Public Health Association, and the American Medical Association have all identified . The EPA estimates that radon causes more than 20,000 new lung cancer cases in the USA each year.

2. Prolonged Periods of Inadequate Ventilation:

With passive ventilation, the amount of fresh air entering a house depends on the size of openings, the stack effect, and the wind speed. Consistently safe, but not excessive, ventilation can only be achieved by sealing openings and using mechanical ventilation.

The United States EPA reported that most American houses have uncontrolled leakage that averages between 70% and 100% of an air change per hour (see "The Inside Story. A Guide to Indoor Air Quality"). This typical "passive" air leakage averages 100% to 200% more than the 35% of an air change per hour recommended for homes by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (see ANSI/ASHRAE Standard #62-1999 "Ventilation for Acceptable Indoor Air Quality").

The EPA and the American Lung Association both agree that indoor air pollution is one of the biggest health threats facing Americans today. Most houses have polluted outdoor air leaking in through unfiltered openings. Addition of gasses and particles from materials used indoors then makes indoor air more polluted than outdoor air. The average American is at home 70% of the time, inhaling about 5.6 million times each year. Even very low levels of some common air pollutants in a home can gradually cause health problems. The American Lung Association cites studies showing that airborne pollutants account for about 80% of the pollutants absorbed by a typical person in the USA. Some Medical Doctors believe that air pollution levels commonly found in houses are strong enough to damage the human immune system.

A group of EPA researchers used sophisticated equipment to study exposure of householders to indoor and outdoor air pollutants. The researchers were very surprised because no matter how clean or dirty the outdoor air, indoor air pollution levels were much worse than outdoor levels. Indoor air pollution can cause symptoms such as headache, lethargy, nausea, or eye irritation, and may show neurological or behavioral effects, such as impaired manual dexterity or reduced ability to perceive time intervals accurately. Some pollutants can initiate genetic damage that affect future generations.

Since humans lack the ability to see or smell many the dangerous air pollutants commonly found in homes, most local building codes recommend providing enough ventilation to prevent the indoor air pollutants from becoming dangerously concentrated. Practical methods and materials are available for use in new and existing homes to simultaneously protect the health and safety of occupants, protect structures from moisture damage, and minimize energy requirements.

3. Negative Indoor Air Pressure (partial vacuum):

It is commonly assumed that a leaky house cannot have a dangerously negative indoor air pressure. Therefore, preventive measures are seldom taken until after a serious problem occurs, like the one that nearly killed a family in a house that meets all building code requirements in Lincoln, Nebraska.

Ron Wilhelm, along with his wife and three children suffered severe carbon monoxide poisoning after the fire in the living room fireplace burned down to embers and the chimney became cooler. After midnight, when the family was asleep, the furnace turned on. As air went up the furnace chimney, replacement air was pulled down through the fireplace chimney where it picked-up carbon monoxide from the smoldering embers. Carbon monoxide entered the living room and was quickly circulated into the bedrooms by the furnace blower. Early the next morning, after the family began to wake-up, the baby fell unconscious while nursing. Fortunately, the family sensed a problem in time to leave the house. The two other children fell unconscious in the driveway. A slightly higher concentration of, or longer exposure to, carbon monoxide could have resulted in permanent brain damage or fatalities.

You may be surprised to learn that the Wilhelm house has more total leakage area than most existing houses of the same size. A blower door test of the 2,000 square foot house indicated the shell of the home leaked 2,600 cubic feet per minute at a pressure of 50 Pascals (0.2" of water column). This is about 60% leakier than many houses that have been professionally weatherized. After the nearly fatal incident, a carbon monoxide (CO) detector was installed to provide a warning in case a chimney back drafted again. The CO alarm has sounded several times each year. A strongly burning fire in the open fireplace removes indoor air at about 400 cubic feet per minute and sometimes creates enough negative indoor air pressure to back draft the furnace. After the fire in the fireplace has burned down to embers, operation of the furnace removes indoor air and sometimes creates enough negative indoor air pressure to back draft the fireplace chimney when embers are glowing in the fireplace and producing carbon monoxide.

Finding and Sealing Air Leaks

Weatherization professionals typically stop sealing air leaks when blower door instruments indicate as little as 1,000 CFM air leakage @ 50 Pascals. The air infiltration resulting from the remaining air leaks has been widely assumed to passively provide adequate ventilation and prevent dangerously negative indoor air pressure without the use of a mechanical fresh air supply system. Several problems with this method were frequently found during tests of homes in the USA and Canada:

Energy Waste due to Excessive Air Leakage:

The air leaks intentionally left in hopes of providing adequate "natural ventilation" typically allow highly excessive air infiltration during cold and/or windy days.

Energy Waste due to Wet Insulation:

When moist air is allowed to leak out through cold walls and attics during the winter, moisture can condense in the insulation and make it less effective. The leakage is typically driven by warm air rising (stack effect), wind, and/or an unbalanced air distribution system.

Health Threats due to Wet Insulation:

Moisture condensed in walls and attics allows mold, bacteria, and other microorganisms to grow in wet insulation and then migrate in air leaking into rooms through remaining air leaks.

Damage to Wood and Exterior Paint:

Moisture condensed within exterior walls can make wooden siding wet enough to cause blistering and peeling of exterior paint. Moisture condensed in attics can damage roofs. In extreme cases, mold can weaken wet wooden siding and structural members.

Energy Waste When Windows are Open:

When a house is very tight, and lacks a mechanical fresh air supply system, occupants tend to open windows when indoor air begins to feel stuffy. Some occupants acquire a habit of leaving windows open all day and/or all night. The amount of air flowing through an open window is usually not measured and can often be much more than necessary for protection of health.

Backdrafting of Chimneys:

The most common, and least safe, kind of exhaust system is a "natural draft" chimney. A surprisingly small amount of negative indoor air pressure (equal to about .004 inches of water column) can prevent a typical chimney from safely exhausting combustion gasses. Serious lung damage can gradually develop when inhaling particles plus gasses such as nitrogen dioxide (NO2) that are produced by combustion of fuel. NO2 can permanently burn and scar sensitive lung tissue. Long-term NO2 exposure can worsen breathing problems, weaken the immune system, and increase lung infections.

Frequent and Prolonged Periods of Inadequate Ventilation:

The remaining air leaks do not assure adequate ventilation except when sufficient pressure differentials are being caused by wind, stack-effect and/or unbalanced air circulation systems.

Entry of Odors, Microorganisms, Moisture, and Radon from the Soil:

Use of exhaust-only ventilation systems causes makeup air to enter through leaks in the building shell. After a building is tightened, a stronger negative indoor air pressure results during operation of air exhaust systems because it is harder to pull-in the makeup air. Negative indoor air pressure allows pollutants to leak in through joints, cracks, and pores in floors and walls that contact the soil.

 

Potential Legal Liability for Weatherization Professionals:

The United States Department of Energy recognizes the need for elimination of health and safety hazards, elimination of which is necessary before, or because of, installation of weatherization materials.

Potential Changes After Weatherization Professionals Complete Their Work:

• Unbalancing of HVAC system. (Many people reduce the amount of air flowing into their basements through air ducts from the central heating/cooling system, but leave the air return fully open, thus causing negative indoor air pressure in the basement whenever the furnace blower is on. This can cause chimney backdrafting along with entry of radon and water vapor from the soil.)

• Further tightening of building shell. (Problems resulting from inadequate ventilation are often made worse by homeowners who continue to tighten their homes by adding insulation, replacing siding, caulking, weather-stripping, etc.)

• Installation of additional air exhaust systems.

• Opening of windows on downwind side of house. This tends to cause negative air pressure in a house.

• Blockage of chimneys by debris or nests.

• Separation of flue pipes.

Identifying Solutions:

1. Control the Fresh Air Supply

2. Use Power-Vented or Closed-Combustion Systems

3. Avoid Flow of Moist Air into Walls and Attics

During cold weather, it is important to prevent excessive indoor air pressure. Otherwise, significant amounts of moist air may leak into cold walls and attics where the moisture may condense on cold surfaces.

4. Block Entry of Pollutants and Water Vapor from the Soil

Odors, water vapor, radon, insecticide fumes, and insects enter many homes from the soil through openings and pores in the floors and walls that contact the soil. Complete sealing can help if it is done thoroughly and properly maintained. When effective sealing is impractical, prevention of negative air pressure in the lowest rooms can be very helpful. The EPA says that negative indoor air pressure, relative to soil gas pressure, is the main reason why radon leaks into buildings from the soil.

The United States EPA recommends sealing air leaks in the shells of buildings to reduce energy costs, allow for improved environmental control, and minimize the amount of outdoor air needed to maintain a slight positive indoor air pressure. “Measurements in existing schools show that a slight positive air pressure equal to the pressure of as little as 0.001 inches of water column (.25 Pascals) relative to sub-slab and outdoor air pressure, reduces indoor radon levels by preventing radon entry.”

Reference: Radon Prevention in the Design and Construction of Schools and Other Large Buildings, January 1993, US Environmental Protection Agency, Office of Research and Development, Washington DC 20460, EPA/625/R-92/016, p 25.

 Use of soil gas pressure under a building as a reference is particularly useful because it is highly shielded from the wind and enables users to accurately maintain just enough indoor air pressure to minimize entry of radon and other pollutants from the soil. A simple and reliable indoor air pressure monitor is available from the Health & Energy Company.

Avoiding negative air pressure in rooms with floor drains helps avoid entry of sewer gasses if water traps in the drains evaporate.

Common causes of undesirable negative air pressure in ground contact rooms include:

• Use of air exhaust systems (clothes dryer, bathroom exhaust fan, kitchen exhaust fan,...)
• Operation of an improperly adjusted air circulation system.

• Leakage of warm air up through attics during cold weather due to "stack effect".

• Fireplace or furnace chimney that removes air from ground contact rooms.
• Exit of air through a downwind window.

Note: Many homes have a basement with a door between the upstairs and basement  rooms. If the basement door is closed, then basement air pressure can become undesirably negative if an unbalanced air circulation system removes more air from the basement than it supplies to the basement.

5. Controlling Indoor Air Pressure

Adjust HVAC systems to avoid creating more than 1 Pascal of positive or negative air pressure in any room relative to other rooms and to the outside. Maintaining indoor air pressure within a desired range is simplified by use of an indoor air pressure monitor.

During the summer, keep all rooms slightly pressurized to minimize entry of moist and unfiltered air.  When the inside surfaces of outer walls are cool during the summer, if humid outside air leaks in through the outer walls, some condensation may occur within the walls. Fungi, bacteria, and other microorganisms can grow in moist dark places and then enter living areas through air leaks.

During the winter, maintain air pressure in the lowest rooms at about .004 inches of water column (1 Pascal). For a simple and reliable means to monitor very low air pressure, see Controlling Indoor Air Pressure.

6. Use Carbon Dioxide (CO2) Detectors

A carbon dioxide (CO2) detector is useful for determining when the CO2 level exceeds the 1,000 parts per million (PPM) limit recommended by ASHRAE.

CAUTION: ASHRAE Standard 62-1999 recommends consistent ventilation even when CO2 is below 1,000 PPM, so that other indoor air pollutants will not become dangerously concentrated.

7. Use Carbon Monoxide (CO) Detectors:

Every home where a fuel-burning appliance is used indoors should be equipped with a carbon monoxide (CO) detector. Power vented and sealed combustion systems can put CO and other toxic gasses into homes if the systems are improperly installed or if they malfunction. When the indoor level of carbon monoxide starts building up, a carbon monoxide detector can provide a timely warning.

CAUTION: A CO detector should not be used as a primary defense against fumes from gas stoves or other fuel burning appliances, since it may not always work. Even when working, a CO detector may not detect carbon monoxide at a level that is low, but that may be sufficient to gradually cause serious and permanent damage to health. Burning of fuels typically produce particles, nitric oxides and toxic fumes in addition to CO. Some carbon monoxide detectors retail for less than $50.

8. Seal Most of the Accessible Air Leaks in Shell of House:

CAUTION: This step is recommended only if a mechanical ventilation system will be used to assure adequate ventilation under all weather conditions. Only mechanical ventilation systems with high-efficiency filters can provide convenient automatic control of ventilation rates and effectively remove dust, pollen, microbes, etc. from incoming fresh air.

It is very important to seal air leaks that would otherwise allow moist air to leak out through cold walls and attics during the heating season. In cold climates, sealing of air leaks is normally done from the interior. During cold weather, vapor permeable materials on the exterior allow moist air to escape through walls and attics to the outside. The least troublesome openings are very small ones that leak almost no air and the openings that do not allow moist indoor air to flow out through interior of cold walls or attics. Typically these leaks are located at:

• Corners of doors and windows.
• Perimeter of dampers in air exhaust systems.

The EPA says that sealing the cracks and joints in walls and floors that contact the soil typically reduces entry of radon from the soil by less than 50%. Ordinary concrete looks solid to the naked eye, but is porous enough to allow some radon gas to enter from the soil. Controlling air pressure in lower rooms is essential to prevent pulling radon and other pollutants indoors from the soil.

9. Control Indoor Humidity

• Keep humidity above 25% to protect your respiratory system.
• Keep humidity below 50% to minimize growth of dust mites and mold

Conclusions:

Adequate insulation and a tight building shell, combined with proper control of ventilation and indoor air pressure, can make a home safer, more comfortable, and more energy efficient.

References:

1.       Strunk, Peter R.  and Kinney, Dr. Larry.

"Air Leakage and the Effects of Mechanical Ventilation: Measurements in Fifty Newer Homes in New York State". SYN TR 93-516, April 1993. Prepared for The New York State Energy Research and Development Authority.

 

2.       Wallace, L. A.  "Total Exposure Assessment Methodology (TEAM) Study:

Summary and Analysis, Volume 1" (Washington, D. C.: U. S. Environmental Protection Agency, 1987).

 

Sources of Information on Ventilation

• Mechanical Ventilation for the Home — Home Energy Magazine
• Air Pressure Sensors
• Ventilation and Air Quality in Offices — U. S. Environmental Protection Agency
• Indoor Air Pressure and Ventilation
• Heat Recovery Ventilators
• Energy Recovery Ventilators
• Controlling Indoor Air Pressure

Additional Sources of Information:

• Home, Sick Home — Johns Hopkins Science & Technology
• Backdrafting of combustion gasses — Metropolitan Utilities District
• Weatherization and Indoor Air Quality
• Sealing Air Leaks
• Indoor Air Quality Solutions