The Physics and Thermodynamics of Various Roof Types

Infrared thermography is essentially a process of pattern recognition, so the best roofs for surveying are those with highly absorbent insulations and emissive surfaces. However, this is not always the case because when weather is optimal, various factors on the roof can affect the collection and interpretation of data.

These factors include:

  • The existence of non-absorbent insulations
  • Reflective coatings
  • Heavy flood coats
  • Old roof patches
  • Ponding of water on the roof
  • Ballast buildup
  • Moisture between layers
  • Heat producing equipment under the roof
  • Air leaking out of the roof

An experienced thermographer understands the difference between an impossible-to-perform successful infrared survey and one that is merely difficult under good conditions. Roofs that are notoriously difficult to infrared survey include:

  • White reflective roofing systems (snow-blinding)
  • Systems installed over the top of foil-faced insulation
  • Roofs that are heavily ballasted
  • PMA roofs
  • Roofs retaining moisture from installation over lightweight concrete decks

Some impossible-to-survey roof types include:

  • Inverted Roof Membrane Assembly roofs
  • Metal roofs – when not conducted Under-Roof
  • Shingled roofs – when not conducted Under-Roof
  • Aluminum coatings that are highly reflective
  • Roofs that aren’t insulated
  • New or newer roofing systems with foam glass or closed-cell foam insulation

A Consideration of Technique and Methods
While physics and thermodynamics applies equally to all methods, techniques still may vary. For instance, when a thermographer is standing on a roof, even a high resolution thermal imager is incapable of shooting a large puddle or wet striation. Taking multiple pictures may be possible, but stitching them together can be laborious, if not impossible.

In other words, you may not be able to see the forest for the trees, so while marking the roof may be easy, it can also be very time consuming on a problem roof. This is because accurate CAD drawings are quite difficult to create from painted marks on a rooftop, and building drawings supplied by the owner are often outdated or non-existent.

What about Drones?
Infrared roof surveys are possible because wet spots on the roof have a higher mass, and greater thermal conductivity. Heat absorbs and dissipates from wet areas at a different speed than dry areas because heat is absorbed by the roof and retained for a longer period of time by the wet areas.

When heat is also coming from the inside of the building, a survey can be conducted using the difference in temperatures from inside to outside. This is seldom possible because the conditions are extremely difficult to anticipate. In other words, it’s far easier to let the sun do its job.

Understanding Energy Efficiency
In reality, very few roofs contain much saturated insulation – usually less than two percent. So, even if there is an increase in heat loss from wet insulation, the amount of lost energy is actually quite low. When you factor in the cost of insulation combined with the waste of perfectly good insulation going into landfills, it takes more energy to replace it than it does to use infrared to find the wet insulation and surgically replace affected areas.

This is ultimately a great savings of time and money for building owners.

Condensation and Air Leakage
Reflective (white) single-ply membrane roofs often get cold quickly after the sun sets, especially during the winter months. This is because these roofs have little mass, are better insulated, and perpendicular to the night sky, so depending on the dew point, condensation forms quickly. Still, when there is slight positive pressure in the structure, warm and wet air leaks up through the board insulation and condenses on the bottom of the membrane. Condensed moisture then drips back down onto the face of the isoboard, and can fool an amateur roof thermographer into thinking the insulation is wet when it is actually bone-dry.