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Ventilation & Air Pressure

Air, and any pollutants it contains,  move through openings in building envelopes only when driven by pressure differences.  The air pressure differences are commonly caused by winds and unbalanced air circulation systems.

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The safest, most comfortable, and energy efficient buildings combine the following features:

National Ventilation and Indoor Air Pressure Standards

Beginning in 2004, ASHRAE Standard 62.1 "Ventilation for Acceptable Indoor Air Quality" requires proper ventilation and a slightly positive pressurization of buildings. Maintaining a small positive air pressure, relative to the outdoors, limits the entrance of outdoor moisture and is a very common strategy to prevent mold and mildew formation in a building.

The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) develops national consensus standards. ASHRAE's Internet address is www.ASHRAE.org

Monitoring and Control of Indoor Air Pressure

"Limiting house pressure is important for safety, health, durability comfort, and energy-efficiency. Exhaust air from chimneys, exhaust fans, and clothes dryers create a slight vacuum in the house. Acting alone or in combination, these exhaust devices can depressurize a house, causing backdrafting or flame roll-out. Backdrafting is a threat primarily to combustion appliances that have their combustion chambers open to outdoors, and which also have naturally drafting chimneys. The depressurization limit for a combustion zone is often set at -3 pascals with reference to outdoors.

Negative pressures greater than -5 pascals may cause flame roll-out (a fire that burns outside the appliance) in open-combustion appliances such as standard furnaces and water heaters. Flame roll-out is a common cause of house fires.

Negative air pressure in a building can also pull soil gasses (including water vapor, radon and pesticide residues) into the building.

Chimney Safety

Backdrafting of chimneys is a threat primarily from combustion appliances that have naturally (non-powered) drafting chimneys and which also have their combustion chambers open to indoors. The depressurization limit for a combustion zone is often set at -3 Pascals (.012 inches of water column) with reference to outdoors.

"To reduce depressurization, technicians provide combustion-air inlets and makeup air to exhaust fans. Passive make-up or combustion air openings may be ineffective unless they are large enough. Fan-powered combustion and make-up are recommended by some experts."
                     — John T. Krigger, Residential Energy (4th Edition), page 81, ISBN 1-880120-12-7.
 

Soil Gasses

The removal of indoor air during use of chimneys, exhaust fans, and clothes dryers creates a partial vacuum (negative indoor air pressure) in buildings. A partial vacuum causes unfiltered air to be pulled into buildings through all the openings in the building shell. A partial vacuum can also pull radon, water vapor, sewer gasses, and soil insecticide fumes into a building through pores and cracks in concrete floors and walls.

To minimize entry of radon and water vapor from the soil through pores, joints and cracks in the walls and floors of the lowest rooms, the U. S. Environmental Protection Agency recommends:

  1. Avoid negative air pressure in rooms with floors built directly above the soil.

  2. Whenever possible, keep the air pressure in the lowest rooms about 0.5 Pascals (.002 inches of  water column) higher than the pressure of gasses in the soil.

Mitigating Radon and Reducing the Costs of Air Conditioning and Dehumidification

Sealing air leaks in a building makes it easier to maintain a slightly positive air pressure in the lowest rooms to minimize entry of radon, water vapor, pesticides, herbicides, sewer gas, etc.

The entry of water vapor can increase the costs of air conditioning and increase the odds of growing smelly molds that can trigger allergy and asthma symptoms in some individuals. See:

Preventing Mold Growth in Building Cavities

Excessive positive or negative pressures can move large quantities of moisture-laden air through holes in a building's shell, leading to condensation and mold growth within building cavities. Moist insulation is less efficient. Building experts have observed pressure-derived moisture problems at only ± 1 Pascal (enough to raise a column of water .004 inches) during severe humidity conditions."
                     — John T. Krigger, Residential Energy (4th Edition), page 81, ISBN 1-880120-12-7.
 

Infection Control

Indoor air pressure control is used in hospitals to help prevent the spread of infectious airborne microbes. See: Hospital Air Pressure .

Monitoring Indoor Air Pressure

To reliably control indoor air pressure at the very low level recommended by the Environmental Protection Agency, you must have a reliable means to monitor the pressure. We manufacture affordable and reliable  indoor air pressure sensors that have only one moving part and never require recalibration. These air pressure sensors can detect positive or negative air pressure as low as 0.5 Pascals (enough to raise a column of water 0.002 inches).

Additional Sources of Information:

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