Why surfaces warm up and cool down differently

01 / Thermography

Temperature is a snapshot. Heat flow is the story.

A thin metal flashing and a thick concrete wall might have similar surface temperatures at one moment and behave very differently an hour later. Temperature tells us about the state of the surface. It does not, by itself, tell us how much energy is stored in the entire object, where that energy came from or how quickly the object will change.

Heat is energy transferred because of a temperature difference. The direction and rate of that transfer depend on the materials, geometry and surroundings. When reviewing imagery, ask what happened before the frame: Was the surface in sunlight? Did the equipment begin operating? Was the material recently wet? That history can explain more than a single color.

Source: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference)

02 / Thermography

Conduction: energy moves through a material.

Conduction transfers energy through matter. A metal bracket can provide a different heat-flow path from the insulation around it. The resulting surface pattern may follow a connection, frame or change in construction. That makes geometry useful: a repeating line deserves a different set of questions from an isolated irregular patch.

For building work, the camera usually observes an accessible surface rather than the hidden source. A condition inside an assembly can influence that surface, but thickness, layers and elapsed time affect the response. A visible pattern is therefore an invitation to compare the image with the construction details and other evidence, rather than an automatic identification of what is underneath.

Sources: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference) · Teledyne FLIR: What thermal imaging can and cannot see through ↗ · Infrared Training Center / Scott Wood: Thermography for Building Science Applications ↗

03 / Thermography

Convection: moving air changes the surface.

Air flowing across a surface exchanges heat with it. Wind can reduce a temperature difference that would otherwise be easier to observe, while ventilation or a nearby exhaust can create a local influence. A sheltered wall and an exposed wall may behave differently even when their construction is similar.

Record the surroundings along with the thermal frame. A plume, a fan, an open door or a gusty period may help explain a pattern or limit a comparison. A useful flight plan considers safe standoff and the subject, rather than assuming that the closest possible view always gives the best evidence.

Source: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference)

04 / Thermography

Radiation: energy crosses the space between objects.

Radiant energy can travel without direct contact. Sunlight is a familiar source of surface heating. Surfaces also exchange infrared radiation with their surroundings. A roof can lose energy to a clear sky, while a nearby warmer object can contribute energy in another part of the scene.

Keep that exchange separate from a reflection in the image. Radiation absorbed by a surface can change its temperature; radiation reflected toward the camera can change its apparent temperature without producing the same change in the surface itself. Viewing angle and material finish help a trained reviewer investigate the difference.

Sources: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference) · Teledyne FLIR: How emissivity affects thermal imaging ↗

05 / Thermography

Heat storage and evaporation add a time dimension.

Mass and material properties influence how much energy an object stores and how quickly its temperature changes. After a period of heating, two roof areas may cool at different rates. Under suitable conditions, moisture within an assembly can affect that response. Construction differences, repairs and other materials can produce differences too.

Evaporation removes energy from a surface and may produce a cooler pattern. Moisture can therefore be associated with warming or cooling behavior in different circumstances. There is no reliable rule that every wet area must appear as the hottest region. A roof investigation needs the assembly, weather history, surface condition and an appropriate confirmation method.

Sources: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference) · Teledyne FLIR: How to Detect a Water Leak with Thermal Imaging ↗

06 / Thermography

Use the mechanisms to ask a better question.

Imagine two roof areas photographed just after sunset. One remains warmer. Before naming a defect, compare material, thickness, sun exposure, equipment operation and the recent weather. Ask whether the same area can be located in the visible image and whether a qualified roofing reviewer should investigate it on site.

That process is the practical value of thermal science. It helps narrow the next step, improve the capture window and avoid a persuasive but unsupported conclusion. Fly Maxwell plans the imagery around that next decision so the thermal record can work alongside the rest of the project information.

• Identify the surface and the comparison area.
• Record the conditions and what changed before capture.
• Consider more than one physical explanation.
• Preserve the observation separately from the confirmed finding.

Source: Infrared Training Center: Level I sUAS Thermography, ITC 107-RM [1.00], July 1, 2023 (private training reference)