Air Movement in Buildings

By Kenton Shepard

The building envelope consists of those parts of the floor, wall and roof assemblies designed to control the loss of conditioned air. Conditioned air is air which has been warmed, cooled or had moisture added to or removed from it.

Building science is the study of how moisture, heat and air move through buildings and how their movement affects human health, comfort and the cost of operating homes. Air movement is an important influence on home indoor environments because it commonly moves across the building envelope. We generally want to keep control of indoor air quality, and air movement across the building envelope has the potential to affect the quality of indoor air dramatically.

Circular air movement happening within a building envelope is called circulation. Air movement between the building interior and the outside of the building’s conditioned-air envelope, such as the exterior, crawlspace, attic or unheated basement is called infiltration if air is moving into the conditioned space and exfiltration if it’s moving out.

Air movement in a home can be a concern when it creates undesirable conditions. These conditions can be uncomfortable moisture or temperature levels, or the introduction into the home of dust, pollen, mold spores, radon or other pollutants or health hazards.

INDOOR AIR MOVEMENT

Air movement is affected by the following…

• Differences in air pressure as air moves from areas of high pressure to areas of low pressure.
  • Circulation- heating and cooling equipment both use blowers to distribute conditioned air throughout buildings. Depending on how well the system is balanced, this can establish air pressure differences in various areas of a building which can cause air to move in or out through the building envelope.
  • Ventilation fans for bathrooms, laundries and range hoods all vent conditioned air to the outside which must be replaced. In the past, this make-up air has come from air infiltration around doors and windows and through other gaps in the building envelope.
  • Combustion processes- appliances such as boilers, furnaces, heating stoves and water heaters pull air from the home interior as they exhaust the products of combustion to the exterior.
• Differences in temperature
  • Thermal buoyancy- describes the action of air as it is heated. Because heated air is less dense it rises, moving from a cool, high-density area to ward a warm, low-density area.
  • Stack effect- describes the action of warm air rising through a building. As warm air rises, it pulls cold make-up air into the home through the lower building envelope and pushes warm air out through the upper building envelope. Stack effect can have a significant effect on homes, pulling undesirable hot or cold air, moisture or environmental pollutants and hazards (radon) into the home.
  • Convection currents- The movement of cooler air moving in to replace rising warm air will establish convection currents any place in the home in which temperature differentials exist, with main areas of concern being the living space and attics. Supply and return registers are key points of temperature differentials and also key points of pressure differences caused by heating and cooling system hair handlers.

OUTDOOR AIR MOVEMENT

How does wind effect buildings?

As wind blows against a home, a high-pressure area builds up next to the upwind exterior wall and roof surfaces.  Air pressure is lower on the other (interior) side of this wall, so on the upwind side of the home air will be sucked into the home.

As air blows past the home, air flowing next to the walls and roof (laminar flow) can’t turn sharply enough to cling to the downwind side of the building and so a vacuum is created on the downwind side of the building. On the downwind side of the home this vacuum will suck conditioned air out of the home. This process can lead to unwanted heat loss or gain depending on whether it’s the heating or cooling season.

Make-up Air

As air is exhausted from the home by the methods mentioned above, it must be replaced by make-up air. Unless ventilation devices are deliberately installed to provide make-up air, it will be pulled into the home through the building envelope. Uncontrolled make-up air may carry with it excessive moisture or heat (or lack of heat). It may infiltrate from the exterior, the crawlspace or the attic. In extremely tightly-built homes, make-up air has been supplied from sewers after water was sucked out of the plumbing traps. Installing a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV) offers more control over the supply of make-up air, allows for more efficient use of heating and cooling equipment and reduces heating and cooling costs.

Heat and Energy Recovery Ventilators

Inspecting HRV’s and ERV’s lies beyond the scope of a General Home Inspection, but inspectors should be able to recognize them.

HRV’s use a heat exchanger to transfer heat between home exhaust-air and make-up air without allowing the two airstreams to mix. This exchange pre-warms (or pre-cools) make-up air, which in turn, lowers heating and cooling costs.

ERV’s perform the same function but in addition, they also transfer moisture. Systems are available in different sizes in order to maintain as closely as possible an ideal 3.5 air changes per hour.

HRV’s and ERV’s are typically installed in-line with the home heating/cooling ducts and may include filtration devices such as High Efficiency Particulate Air (HEPA) filters.

One type of HRV

An HRV with a HEPA filter

All photos by Kenton Shepard

Sources for More Information 

Building Science Corporation- One of the top resources for home inspectors for a variety of building science subjects.