Air Infiltration & Leakage (Part 2)

portrait photo of Michael

Michael W. Dickenson, P.E., CFEI, CVFI

High Energy Bills? Understand and Improve Energy Performance: Air Changes per Hour 

Air infiltration (air that comes in) and air leakage (air that goes out) negatively impact energy usage and costs in conditioned spaces – for homes and businesses alike. We’ve talked about what infiltration and leakage are and how to combat them at a basic level, but to really fix the problem for the long term requires measuring and improving your building’s Air Changes per Hour, or “ACH.”

Can Air Infiltration and Leakage be Measured?

The quantity of air which is exchanged between the indoor (conditioned) space and the outdoor (unconditioned) space can be determined by a blower door test. Professionals perform a blower door test by using a fan, usually located at the front door, to pressurize the building envelope. Controls which are connected to the fan are used to measure pressure difference between the indoor and outdoor conditions. The test results in a number for air exchanged between the indoor air to the outdoor air, referred to as air changes per hour (ACH).

What Does an ACH Number Mean?

ACH ranges between zero and ten. In general, ACH below two is correlated with air-tight construction and is the ideal state for maximum energy efficiency and heating, ventilation, and air conditioning (HVAC) unit effectiveness. Common residential construction ranges between 2 ACH and 4 ACH; ACH values greater than 5 ACH should be evaluated.

EXAMPLE: For the sake of differentiating the cost between what would be considered “air-tight” construction and “leaky” construction, some assumptions are made:

  • One-week time period
  • Winter with outdoor temperatures averaging 20 degrees Fahrenheit
  • “Leaky” and “air-tight” constructions are heated with all electric heating
  • Target indoor temperature (setpoint) is no less than 60 degrees
  • Each construction contains 1200 square feet of conditioned space
  • Average electric utility charge is $0.09 per kilowatt-hour (KWH)
  • “Leaky” construction results in $160.73 additional heating costs from air infiltration/leakage
  • “Tight” construction results in $45.46 additional heating costs from air infiltration/leakage

How Can ACH Numbers be Improved to Reduce Energy Waste/Costs?

Here are some steps home or business owners may take to accomplish better energy effectiveness and realize a cost savings by improving ACH:

  • Foam sealant can be purchased at most hardware stores and is sold in pressurized cans. Applying the sealant, as directed, to areas where air can be felt entering or exiting the building will reduce air infiltration and leakage. One example where applying foam sealant can have a positive effect is relating to conductors and bore holes within a crawlspace. A home or business owner can seal the annular space around the conductor to prevent air movement within the bore hole. Piping penetrations of a building envelope should be sealed in a similar manner.
  • Examine and replace exterior door seals and thresholds which have deteriorated. Ensure the new seals provide an air-tight seal when the door is shut, and that no air movement is felt around the door jamb.
  • Ensure windows are locked, and the seals prevent air movement around exterior windows. If air movement is felt with a window seal, consult the window manufacturer regarding the proper procedure to replace a seal.
  • If floor registers are used, verify no air movement exists between the supply air ductwork and the floor opening. If air movement is felt, sealing gaps around the supply air ductwork floor connection is recommended from the crawlspace/basement.
  • Examine bathroom exhaust fans. If air movement is felt at the center of the decorative grille, air may be leaking past the backdraft damper and the damper should be examined/replaced. If air movement is felt around the edge of the decorative grille, the grille may need to be removed to seal gaps/spaces around the exhaust fan housing.

Local utility providers or government institutions may provide assistance to home or business owners, such as tax rebates or other incentives, when steps are taken to reduce energy waste, such as those discussed in this article.

What is the Real Importance of ACH?

ACH is really an indicator of a home or building’s overall energy performance, related to its HVAC:

  • If ACH is low, costs and energy consumption are lower. 
  • If ACH is high, costs and energy consumption are higher. Furthermore, machine/equipment lifespans are dramatically reduced. 

In short, whether your home or business has noticed increased energy costs, poorly functioning HVAC units, or a home/business owner simply wants to improve energy efficiency and system effectiveness, there are steps you can take to improve ACH. Primarily, home and business owners can reduce energy costs related to air infiltration and leakage by sealing gaps and cracks where these problems occur. However, it’s also important to recognize where at-home-fixes end and professional-expertise begins. Remember to contact manufacturers and experts when it comes to working safely with equipment and products. Getting the job done right (and, again, getting the job done safely) is the best way to ensure long equipment life, improved energy costs, and more sustainable energy consumption.