Air Leakage: Air Infiltration & Air Exfiltration


Air Leakage represents 5-40% heating & cooling costs. Controlling air leakage is one of weatherization’s most important functions and is often the most difficult. Ideally, an effective air barrier surrounds the house on all side. Most homes have flawed air barriers that can be corrected by air sealing

Information about reducing home air leakage is presented here as follows:

  • Current State of House Air Leakage
  • Contributing Issues to Air Leaks
  • Diagnosing Air Leakage
  • Air Sealing Recommendations
  • Air Pressure Principles
  • Customer Performance Targets for Air Sealing
  • Reducing Air Leakage

Current State of House Air Leakage

There is no established standard for home air leakage.

Air leakage tends to range from 0.25 -1.50 air changes per hour (ACH)

Energy efficient houses leak less than 0.35 ACH Newer homes average about 0.50 ACH Older homes leak 0.75 ACH or more

Air changes per hour represent the number of complete changes of conditioned air in a house in one hour. An old, leaky house typically loses 4 times as much conditioned air than an energy-efficient one. A new house uses 1.4 times as much.

The United States Environmental Protection Agency (EPA) says 20% of heating & cooling costs, or about 10% of an energy bill, can be saved by first air sealing the building and then insulating it.

Contributing Issues

Until the oil embargo in 1973, houses were built air leaky. No one complained because energy was cheap.

Ignorance. As a result of the oil embargo, as a Nation, we began taking building air leakage seriously. We started caulking our windows & doors, which helped some, but we found out there were a lot of other “hidden” air leaks, way bigger than seams.

Hidden Air Leaks. Large building air leaks are usually located in unconditioned spaces, such as the attic, crawlspace, and basement. They include mechanical chases, cabinet soffits, stairways, and attic kneewalls.

Forgetfulness. Since then, either the architect or builder forgets to complete the air barrier through walls, ceilings, and floors: known as the building shell or building envelope, which also has windows and doors in it.

Air Barrier Defined. The air barrier is the outer layer of the building shell. The building envelope has two barriers: an air barrier and a thermal barrier (usually insulation) right up against it.

Debate. Also, there is significant debate whether we should be depending on natural air leakage to provide our indoor fresh air needs. Some building professionals have the attitude that houses have to breathe, so steps required by the building code are not done unless a building code official enforces it.

Home Energy Audit: Diagnosing Air Leakage

Tip: Diagnosing air tightness is based on understanding Air Pressure Principles

Together with insulation, reducing air leakage is the most important thing to do for making the house more energy efficient. Before doing so, moisture, ventilation, and combustion issues must be corrected.

The level air tightness achieved directly impacts decisions about improving the heating & cooling system. It’s an essential first step toward using solar energy because it reduces the amount of energy needed to heat & cool the house.

Reducing Home Air Leakage

The process for reducing air leakage follows this path:

Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.

As they relate to air sealing, the following questions are asked during the homeowner interview:

How often do you need to dust?
Who in your family has allergies?
Who in your family has asthma?
Are there any hot or cold rooms?
How long do strong cooking odors linger?
Is there anything that looks to be mold or other microbial growth?
Is there anything else you feel is relevant?

Answers to these questions help us evaluate the system in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.

Conduct Visual Assessment. The expected sources of air leaks found in most homes can easily be verified by doing an air infiltration (air leakage into the house through walls, floors, and ceilings). The major air leaks though are hidden in places like the attic, basement, crawlspace, and garage. A major source of air leaks is duct leakage, which is discussed separately.

Expected Air Leaks. Predictable air leaks occur around windows, doors, plumbing, outlets, lights, bath area fans, rangehood exhaust fans, and attic openings. All these are found, and felt, in the living space.

Hidden Air Leaks. Unexpected air leaks occur around soffits over cabinets, behind interior wall board, through bands around floors, open walls and chases in the attic, gaps around chimneys, under attic bonus room floors, where porch roofs connect to walls, under cantilevered floors, stairways on exterior walls, behind tubs, gaps in subflooring, and a bunch of other construction flaws.

Sneaky Air Leaks. Some of the air leaks don’t necessarily leak air into the house, but allow air to rush along walls, floors, and ceilings, such as under attic bonus rooms over a garage. Without air leaks felt inside the house, they may not be discovered, especially by people who don’t know what to look for.

Perform Diagnostic Tests. Air infiltration testing is done to help find air leaks, determine the location effective air barriers, and measure air infiltration.

Test Names. Other names for this air testing are blower door testing, air door testing, and air blower testing. It’s done by a blower door tester or air leakage tester, otherwise known as your home energy consultant.

Test Process. Air is intentionally drawn into the house to make finding air leaks easier. The amount of air pulled in to keep the house a predetermined pressure is used to calculate air leakage in a variety of ways for various purposes.

The easiest measurement to understand and is used most often is the number of air changes per hour under natural conditions (ACHnat). 0.35 ACHnat is a measure of a moderately tight house

Test Uses. Major air leaks need to be fixed and the right air barrier needs to be sealed, such as foundation vs. floor. Air leak measurements are also used to size heating & cooling equipment, as well as set home performance improvement priorities.

Air Leakage Test Tools. Air infiltration testing is done with a blower door (air infiltrometer). They are commonly known as blower door tests.

Air leaks are easily found with blower door tests because they can be felt. Sometimes a smoke pen is used to find where air is leaking in from outside, such as the garage.

Air movement across walls, floors, and ceilings can be picked up with an infrared scanner, but only if the temperature difference between inside and out is significant. After doing a few assessments with an infrared camera, even hidden conditions can be predicted with relative ease during a visual assessment.

Air Barrier Location. At lot of time can be spent air sealing the “wrong” air barrier. For example, is the roof the air barrier or is it the ceiling? Maybe, it’s both. The same is true for foundation and floors.

The goal is to seal the air barrier that will also be insulated. This decision is not simple because combustion air needs, moisture management, and improvement cost ultimately dictates where it needs to be. However, before making any decisions, it’s useful to know where the tightest air barrier currently is. This is done with pressure testing.

Air Sealing Recommendations

Select Air Sealing Options. The results of the visual assessment and diagnostic tests are used to set home performance improvement priorities. Recommendations are made, based what else needs to be done. In terms of air sealing, the decisions that need to be made are as follows:

Where is the air barrier and its insulation located?
What are the priorities for air sealing?
What objectives must be met?

The choice of the air barrier location is based on many factors. It is primarily driven by combustion safety concerns and installation costs.

Priorities and objectives for air sealing are the basis for what to do until established thresholds are met. Typically, they include the building tightness limit or financial limit.

Building Tightness Limit. Our preference is to seal up a house tighter than a submarine and provide mechanical ventilation from controlled sources.

However, until houses drop below 1 air change every three hours or small houses have a lot of people living in them, supplying fresh air is not usually required. When fresh air ventilation is not provided, air sealing needs to stop before air leakage drops below safe levels for indoor air quality.


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