Air leakage, also called air infiltration, is the unintentional introduction of outside air into a building. It is often associated with penetrations in the building envelope such as windows, doors, wiring, plumbing, and ducts. Air sealing your home, combined with proper ventilation, can reduce your energy bills, eliminate unwanted drafts, and improve your homes’ air quality.
We start with a short introduction highlighting the main reasons you should consider reducing your home's air infiltration. Then we discuss the most common air infiltration measurement technique, the stack effect, and air infiltration reduction practices. Then, we introduce the air infiltration tests included in the energy audit. Lastly, we examine the cost and payback of reducing air infiltration along with a case study of a homeowner who conducted a energy-audit, and his experience in doing so.
A tighter house will result in the following:
Lower heating and cooling bills
25-40% of the energy used for heating and cooling is associated with air leakage.7 By decreasing air infiltration, you will lose less energy to the outside environment.
by decreasing your air infiltration, you will reduce unwanted drafts.
Reduced risk of mold and rot
infiltration in the summer can bring humid outdoor air into your home. Conversely, in winter air infiltration can result in warm moist indoor air moving into cold envelope cavities. In either of these cases, condensation may occur in the structure, resulting in mold or rot, which can be detrimental to occupant health.
Enhanced performance of the ventilation system
fewer leaks result in less uncontrollable drafts within your home, thus allowing for better control of your home's ventilation.
Smaller heating and cooling loads
by decreasing your energy losses, you may have the potential to downsize your heating and cooling equipment.
Causes of air leakage
The primary cause of air infiltration is the stack effect. The stack effect occurs because warm air in a home moves upward due to its lower density than the cooler air in your home. The rising warm air escapes out of the upper levels of the building (typically through the attic), and reduces the pressure in the base of the building, forcing cold air to infiltrate through doors, windows, or other openings. It happens in both the summer and winter, but is most prevalent in the winter since indoor-outdoor temperature differences are greatest during that time of year. Special attention should be given to recessed lights, attic hatches, and plumbing vents to reduce the stack effect.
Other causes include but are not limited to air leakage through small gaps on walls and around windows, doors, and openings for wiring, plumbing, and ducts.
Image source: www.yourhome.gov.au
Air infiltration measurement
Air leakage is commonly referred to in air changes per hour (ACH). Poorly sealed homes could have a natural ACH score greater than 1.5. An energy-efficient home typically has an ACH value of 0.35 - 0.5 under normal weather conditions. A home with an ACH less than 0.35 can lead to air quality concerns that must be addressed by adding additional ventilation.
Though many refer to a home's ACH, a home's air flow rate is measured in cubic feet per minute, per square foot of a home (CFM/ft2). The conversion between the two are:
ACH = CFMair x 60 (minutes/hour) / [sq. ft. (home) x height (rooms)]
CFMair = ACH * [sq. ft. (home)] x [height (rooms)] / 60 (minutes/hour)]
To test your home's air leakage, it is recommended that you conduct an energy audit of your home, which includes a blower door test and thermographic scan.
Blower Door Test
For a blower door test your home is depressurized using a large fan while all windows, vents, and doors are closed from the outside. This causes the outside air to come through the small openings in your house, and the air infiltration rate is measured in your home in units of cubic feet per minute per square foot of your house (cfm/ft2). According to the Department of Energy's Building America's performance criteria, your home should have an air leakage rate of less than 0.25 cfm/ft2 (1.3 L/s*m2) for a building enclosure surface area at 50 Pascals air pressure differential to be considered an energy-efficient, safe, and comfortable home.4
A thermographic scan measures the surface temperature of exterior walls using an infrared camera. It is a powerful tool to detect locations in the envelope with leakages or poor insulation.
Air infiltration reduction products
There are simple-the-shelf products that can reduce air infiltration, such as house wraps, sealants, foams, and tapes. If you would like to further reduce air infiltration, you should request a professional home energy audit.
Reducing air infiltration is by far the cheapest and most effective method to reduce your home's energy consumption. Energy savings resulting from air sealing typically takes 0-2 years to be offset the initial auditing and sealing costs.2 Check out our energy audit page to learn how to locate professionals and start saving today!
- "Blower Door Basics" by Green Building Advisor - A general overview of the blower door air infiltration analysis.
- "Air Leakage Guide" by the Department of Energy Building Energy Codes Program - Comprehensive air infiltration guide providing: introduction, infiltration codes, planning, testing, ventilation, HVAC sizing, and case studys.
- "Blower Door Testing" by David Keefe
- "Setting Airtightness Standards" by ASHRAE - An in-depth article examining the air infiltration standards and includes recommendations for whole-building air tightness.
- "Blower Door Test" by the Office of Energy Efficiency and Renewable Energy - The basics to the blower door test.
- "Air Sealing Your Home" by the Office of Energy Efficiency and Renewable Energy - Tips on how to effectively air seal your home.
- "Air Sealing - Building Envelope Improvements" by ENERGY STAR® - Benefits of having a tight building envelope
- "Tight Construction" by ENERGY STAR® - Reference from and a brief introduction on how you can reduce air infiltration in your home.