Figure 1. Common air leak sources. (Source: DOE

The first step in evaluating a building’s energy use involves an energy audit. This consists of various home performance tests that identify opportunities to reduce energy use. More HVAC contractors are starting to offer this service because of its direct impact on a home’s heating-cooling costs.

Once the audit is complete, various weatherization techniques are performed to improve the energy efficiency of the building, often called “weatherizing.” Weatherization is commonly associated with the U.S. Department of Energy’s (DOE’s) Weatherization Assistance Program, which provides low-income families with the opportunity to reduce energy bills by having their homes audited and weatherized.

Although the DOE’s program is meant for low-income households, virtually all property owners can benefit from having an energy audit performed and the necessary repairs made.


Use and Loss
Heating and cooling the conditioned or living space accounts for almost 45 percent of a typical home’s energy use. Property owners can realize significant savings if proper measures are taken to control unwanted air leakage, as air leaks relate directly to heating and cooling costs.

According to Energy Star®, sealing and insulating the envelope or shell of a home - its outer walls, ceiling, windows, doors, and floors - is often the most cost-effective way to improve energy efficiency and comfort.

Air leaks and insulation issues in homes often go unnoticed, simply because they cannot be seen - unless infrared or thermal imaging is used. Thermal imaging has become more widely accepted as a must-have tool for energy auditing and weatherization.


Energy auditors and weatherization professionals use thermal imaging because it provides a fast and easy way to pinpoint and document exact locations of problems.

In an industry where speed and accuracy are vital, infrared allows for faster inspections and more detailed documentation. Many traditional auditing tools provide a general idea of where problems are, but may fall short of pinpointing them.

Perhaps the most valuable part of infrared inspection is the ability to document and report findings. For example, Fluke said its thermal imagers’ IR-Fusion® technology combines an infrared image with a visual image for enhanced identification, analysis, and reporting. By incorporating the visual reference image, contractors tasked with making repairs can relocate problems after an audit is complete.

Thermal imaging can also be used to validate the effectiveness of repairs and improvements such as caulking, filling voids with spray foam, and adding insulation, by performing a follow-up infrared inspection.


Controlled air exchange is necessary for occupant safety, but most structures waste significant energy through excessive, uncontrolled air leakage. Remedies for leaks can be simple, but finding them without the use of infrared technology remains a challenge.

According to ASTM E1186, for best air leak inspection results, a temperature difference, or Delta T, of at least 3°F from the inside to the outside of a structure should exist (the larger the difference, the better). For this reason, it’s easiest to conduct inspections during periods of intense heat or cold.

Significant air leaks tend to occur near attics and basements due to the stack effect. This occurs when warm air rising in a home creates an area of low pressure at the lower levels and high pressure near the roof. These pressure differences force warm air to escape from the top and cold air to enter near the bottom.

Air leak inspections are greatly enhanced by the use of a blower door. Auditors and inspectors have long used them to measure the overall air exchange rate or airtightness of a structure. Blower doors create a pressure difference (usually negative) from the inside to the outside of the structure. By creating a pressure difference, air leaks are exaggerated and the effect the moving air has on the surfaces around the leaks is exaggerated as well.

When used in conjunction with blower doors, thermal imagers detect air leaks more easily because there is a greater difference in temperature on surfaces surrounding the air leak source. This exaggerated temperature difference also allows infrared inspections to take place more often throughout the year, as the blower door reduces the required Delta T.


Problems with insulation that lead to energy loss often include missing, inadequate, settled, and/or wet conditions. All reduce the effectiveness of insulation and can lead to thermal and/or air bypass.

According to ASTM C1060, for best insulation inspection results, a Delta T of at least 18° from the inside to the outside of a structure should exist (again, the larger the difference, the better).

It’s much easier to interpret findings if the type of insulation is known. Knowing the insulation type enables an auditor to prepare for issues commonly associated with certain insulation types. For example, blown-in insulation is notorious for settling over time.

Moisture and condensation often go hand-in-hand with air leaks in a structure, as air can provide a means for moisture to travel. Moisture, if not properly remedied, can lead to building damage, reduced insulation effectiveness, and mold growth. Thermal imagers are effective tools for identifying moisture.

Water has a high thermal capacitance, meaning that it efficiently absorbs and stores energy. The thermal capacitance of water or the effects of evaporative cooling (usually a 2°-5° surface temperature difference) help reveal the extent of moisture damage, even when the surface feels dry to the touch. All suspected moisture should be validated with a moisture meter.

For more information, contact Fluke at 800-760-4523 (U.S. only) or 425-446-5500 (outside U.S.);

Sidebar: Helpful Links

More information on the topics in the article may be found at:

Affordable Comfort Inc. (ACI) -

American Society of Home Inspectors (ASHI) -

America Society of Test and Measurement Standards (ASTM)

Building Performance Institute (BPI)

Department of Energy (DOE)

Residential Energy Services Network (RESNET®) -

U.S. Department of Energy Building America Program

Sidebar: Procedures

Below are inspection procedures to follow when using a thermal imager:

• Knowledge of building methods and materials is critical. Infrared audits are best performed by someone who understands how buildings work and how they are built.

• Thermal inspections can be performed in both warm and cold weather. By utilizing the HVAC system, adequate Delta T can be mechanically influenced. However, always ensure inside temperature stabilization by turning off the HVAC at least 15 minutes before beginning an inspection.

• To ensure a thorough inspection, work systematically. Follow a route and make sure to scan both interior and exterior walls. Also, it is always best to record appropriate visible, voice, or written annotations during the process, to ensure that you have adequate information for the final reporting.

• Thermal imagers can be operated in auto or manual temperature scaling modes. For best results and to ensure all issues are identified, use the manual scaling mode. Keep the span narrow and adjust the level as necessary.

• Solar loading and wind are environmental factors that must be considered. Solar loading occurs when one or more sides of a structure are uniformly heated by the sun, causing temperature differences to be masked over. Similarly, wind moving over a structure can wash away thermal signatures, or create unexpected pressure differences, which can leave some problems undetected.

• Thermal sensitivity is a key factor to consider when purchasing a thermal imager for building inspections. It should be at least 0.1°C (100 mk) at 30° or better. The more sensitive the imager, the easier it will be to identify anomalies. Highly sensitive imagers are more effective for conducting inspections throughout the year, or when minimal Delta T is present.

Publication date:06/20/2011