Testing Building Envelope Systems Using Infrared Thermal Imaging

By Josh L. White

Introduction to Envelope Systems part 1

Buildings of all sorts, from homes to factories to high rises, can be troubled by problems related to design, construction and maintenance that can be difficult to diagnose and resolve. The major problems found in buildings include:

* Excessive energy use due to missing or damaged insulation, insulation that is performing inadequately, and excessive air-leakage across the thermal perimeter
* Moisture damage due to leaks or condensation, especially in the walls or roofs
* Ice damage to sloped roofs
* Poor hvac distribution or performance
* Inadequate verification of construction details or structural performance delaminations of façade materials
* "Sick building syndrome," mold growth and other health related issues

Often the problems - as well as their causes and consequences - simply cannot be seen until after costly damage has been done. At that point the only recourse may be extensive, costly reconstruction. For example, a commercial building, a nursing home, had extensive problems resulting from a poorly designed roof/ceiling insulation system; these included excessive heat loss during the winter months and, as a result, extensive ice damage due to snow melt. Hundreds of thousands of dollars were spent in various, poorly planned attempts to correct the underlying causes. In the end the owners simply lived with the continued problem because corrective actions were deemed too expensive. Despite this unfortunate resolution, the thermograms helped the owners negotiate a financial settlement with the architects and contractors.

When properly used, thermal imaging enables building owners, architects, contractors and inspectors to verify building performance, identify potential problem areas and validate corrective solutions.

A great value of infrared thermography is that it provides a means of seeing the invisible thermal signatures related to many of these problems in building. When properly used, thermography enables building owners, architects, contractors and inspectors to locate problems, verify building performance, and validate solutions. When people act on this information, significant savings result and buildings are more comfortable! All surfaces radiate invisible heat energy.

You've felt this energy emitted by the sun or a stove burner. Infrared cameras are specially-designed electronic devices that detect thermal radiation. They convert this radiation into thermal images, or thermograms, which visually portray temperature differences as small as 0.05°C. These portable, battery-operated instruments record the thermal data either as still, digital images or on conventional videotape or digital video. The image is displayed live in a viewfinder or on an LCD view screen.

Different radiant temperatures are shown as different colors or shades of gray. Although it may sometimes be useful to display temperature values, this is often not required in building work. Rather, the temperature differences are normally of most interest. Given the right conditions most buildings exhibit characteristic thermal patterns that can be interpreted by a qualified person. The infrared systems themselves are quite easy to operate and, thus, a number of thermographers conduct building inspections. The tasks of interpreting the imagery, understanding the root cause problems, and finding solutions are all more difficult. Because of this, thermographers often work closely with a team consisting of building specialists, architects, and contractors.

The key to using thermography successfully is understanding what thermal patterns are associated with the problems being studied and knowing when those patterns will become visible in the infrared image.

Building Applications for Thermography

Thermography has been used since the mid-60s to solve building problems. During the late 70s and early 80s, a time when fuel prices rose dramatically, thermography was embraced widely as a tool to help determine building performance. Since then other applications have been developed and refined, especially related to verification of structural performance. The major building-related applications for the technology are detailed below.

Insulation Checks

Missing, damaged or non-performing insulation will stand out clearly in a thermal image when there is at least a 10°C (18°F) stable temperature difference between the conditioned space and the outside air. It is often possible to do work with less of a temperature spread due to differences in the thermal capacitance of the building materials. The inspection is typically done from both inside and outside. Often the best results are gained from inside because of fewer influences, but a better overall understanding of the building can often be gained from larger views of the outside elevations.

Missing, damaged or non-performing insulation will stand out clearly in a thermal image when there is at least a 10°C (18°F) stable temperature difference between the conditioned space and the outside air.

It is essential to know the type of insulation in the building and construction details, including how the insulation was installed. Insulation may be in place but not performing; often a destructive evaluation is warranted to establish baseline conditions or understand the exact construction detail. Each type of insulation has a characteristic thermal pattern.

A soft foam insulation is susceptible to shrinkage and cracking when poorly installed. Many factors impact the image you will see. When work is done in the daytime or early evening, the impact of solar loading must be considered. The affects of the sun can easily last 6-8 hours on both the inside and outside after a wall has been exposed. This often results in the direction of heat flow being reversed, making for confusing images and misdiagnosis. Wind must also be reckoned with, as it can both quickly eliminate the thermal difference on a surface as well as enhance others.

If building problems are wind-related, i.e. "we are cold on windy days," then it is wise to conduct the inspection with a wind load. The costs of poor performance of insulation are huge. In addition to excessive energy consumption, there may be costly freeze-ups of water pipes or fire sprinkler systems; health issues associated with mold growth in cold spots, damage to roofs and interiors caused by ice dams, condensation, and water intrusion.

Air Leakage Location

Excessive air leakage can account for up to half of the energy consumed to condition buildings. Of course adequate air exchange is essential for the occupants' health and safety, but most buildings have a far higher rate of air exchange than is necessary.

The root cause is often poor design and/or construction which allows air to move across the thermal perimeter. The problems can be as straightforward as a failed door weather seal or as complex as an air pathway through a plumbing chase in an interior wall or ceiling plenum. The leakage pathway is often complex and, without infrared, extremely difficult to visualize. Air leakage inspections are best conducted when air flow is directed and controlled.

This can be accomplished with exhaust fans, specialized blower fan door, or, in larger buildings, by temporarily altering the HVAC system, to create a negative pressure inside. During the heating season the resulting sites of air infiltration appear cooler. The work can be done any time of year as long as the indoor/outdoor temperature difference is greater than a few degrees. Blower door fans can also be used to quantify air leakage rates.

This technique is invaluable in predicting building performance and monitoring air sealing work. Most types of insulation are not effective at reducing air movement through the thermal perimeter. Good construction practice includes interior air sealing; if this is not in place effectively, air can move through the interior and exterior surfaces and through the insulation.

Unfortunately, fiberglass is particularly susceptible to this problem. Thus, while the insulation may be present, it does not perform as expected when the building is under a pressure gradient. This will typically go unnoticed until the fuel bill is paid.
Continued