The Building Envelope and Energy Loss
When discussing energy loss, it is important to understand the different types of heat transfer.
Energy Loss by Radiation: Heat transfer that is detected by a thermal imager is radiation. All objects emit energy and depending on the makeup of the material the object is composed of some objects emit more energy than others. Understanding these emittance properties is imperative when performing a thermal survey.
Energy Loss by Heat Transfer: Heat will also transfer when two objects of different temperatures come into contact. The heat will transfer until both objects reach the same temperature. For example, a pan is placed on the heating element of an electric stove, the heat transfers from the heating element to the pan, causing the pan to get hot.
Energy Loss by Convection: Convection is the third type of heat transfer. Convection is the transfer of heat by a gas or liquid. In a forced air furnace, the heating source heats up tubes inside the furnace and then the furnace blower forces air over the tube. As the air passes over the tubes it picks up the heat from the tubes and transfers it into the structure.
The Importance of Air Leakage
A major cause of energy loss in a building envelope is due to air leakage.
Exterior air can leak into the building or conditioned air can leak out of the building at numerous locations.
Natural Air Leakage: Natural is the process in which warm air rises in a structure creating a small negative pressure in lower areas and a small positive pressure in higher areas. The negative pressure will draw in exterior air where the positive pressure will force interior air out.
Forced Air Leakage: Forced air leakage can be caused by wind or a mechanical device such as a fan or blower.
With the proper weather conditions and site conditions, an infrared inspection can identify areas of leakage regardless how insignificant they may seem.
To identify areas were air is leaking into the building envelope, the infrared survey will be performed on the interior of the building.
To identify areas where interior conditioned air is escaping from the building, the survey will be done from the exterior of the building.
Using a blower door during a thermal inspection for air leakage, the building can be pressurized to exaggerate exterior leakage or depressurized to survey interior leakage. A blower door is a device that seals off a door and uses a controlled fan to blow air in or draw air out of a structure.
Insulation failures and energy loss is not just a problem with building systems but they include applications where hot and cold processes are involved.
There are low temperature insulations such as fiber glass found in the wall of a building or around the HVAC systems; and, there are high temperature insulations such as refractory used to insulate very high temperature equipment.
The Building Envelope and Moisture
Moisture intrusion is not only responsible for wasted energy but will cause significant component and structural damage if not detected.
A thermal imager will quickly detect possible moisture intrusion due to the thermal properties of water.
When water is present in materials it will heat or cool at a different rate than the other surrounding material so a wet area will appear at a different temperature on a thermal image. As the moisture evaporates it will appear on an image as a cooler area.
Also, wet insulation loses its insulation value because it’s ability to transfer heat in or out also increases.
Roofs are of particular interest for infrared inspections.
A small water leak if left undetected can cause a catastrophic failure. Depending on the type of roof construction, a leak may be difficult to detect. There are specific methods for doing a thermal survey of a roof system under specific conditions to get the most accurate results.
When a possible area of water intrusion is identified during an infrared inspection, further tests should be conducted to verify the presence of water. Additional testing may include the use of a moisture meter or taking a sample of the roof material.
When an area or a building component is located that has an abnormal thermal pattern or thermal anomaly an infrared image will be produced of the area, combined with an explanation of the problem and other information that would be necessary to correct the problem.