How To Interpret Thermal Images? (2024 Guide)

Do you want to learn how to interpret infrared thermal images?

Thermal cameras are a great way to analyze a house for energy loss and water leaks, but you have to know how to interpret the imagery.

In this guide, I will go over...

  • Manipulating the environment so water lights up like a christmas tree
  • Understanding the color spectrum of thermal images
  • Finding missing or inadequate insulation

Let's get started with this guide!

how to interpret thermal images


Thermal imagery is a great way to see what is going on in your home that the human eye cannot see. With an infrared camera, you can spot missing insulation, water leaks, air infiltration, stud locations, electrical hot spots, and more.

As home inspectors, we are almost always looking for just two things: water leaks and missing insulation.

And when you are interpreting thermal images, it is important to understand that it isn't x-ray vision...

Read Also: What Are The Best Thermal Imaging Cameras?

Thermal Imagery Shows Surface Temperature

With an infrared camera, you aren't looking through walls. You are actually looking at the surface temperatures, and the thermal picture is a visual map of different temperatures on the surface.

If you took a laser temperature meter, and pointed it at the different colors associated with the thermal image, each color will correspond with a particular temperature.

Warmer surface temperatures will be in the red or yellow color range. And cooler surface temperatures will be in the blue or purple color range.

When I am inspecting the interior of a house, water leaks or moisture will almost always be cooler than the surrounding material and will show up as blue.

However, if I am inspecting a flat roof on a hot day, and the sun just went down, then water will show up as red or orange. You have to think logically about the color differences. The long and the short of it is this: cool areas are blue and warm areas are red.

Read Also: What Are The Best Thermal Imaging Cameras For Android?

Understanding Water

You must also understand that water has one of the highest thermal capacities out of any substance. This means that water will take the longest to change temperature.

As an example, about 70% of the human body is water, so when you take a thermal image of yourself, you will light up like a christmas tree. In fact, water emits about 2-3 times more infrared radiation than wood, concrete, insulation or drywall.

In contrast, air, insulation, or drywall can quickly change temperature. And this is why changing the environmental temperature can make water leaks stick out like a sore thumb.

Read Also: How To Use A Moisture Meter On Drywall?

Roof Leak From A Home Inspection

A couple years ago I was doing an inspection on a home where there was a water leak in a bedroom closet, and the owner wanted me to find the source.

So I went up on the roof with my thermal camera, not long after the sunrise, and I pointed the thermal camera to an area of metal roof flashing.

Water leaking through roof flashing shows up colder as a blue color

how to interpret thermal images

The problematic roof area had two long metal flashings that came together into a roof valley with one large metal flashing. As soon as I took the infrared picture, I knew this area was the problem, because the thermal image made the water stick out like a sore thumb, and showed up as a blue color.

Overnight, the roof cooled, but as soon as the sun rose, it quickly warmed up the asphalt shingles, but the water remained a cool blue. Again, water has one of the highest thermal capacities of any material, and always takes the longest to warm up (or cool down).

The owner was very grateful that I finally found the source of the roof leak.

Read Also: What Are My Top Home Inspection Tools?

Why Change The Enviromental Temperature?

The most important thing to understand is that to see anything, you must create a temperature difference or thermal gradiance.

Let's say you are looking for a water leak in a thermal image. But if the water in the wall has attained equilibrium or the same temperature as the rest of the room, then you won't be able to 'see' anything.

This phenomenon is called thermal blindness. Since the water is the same temperature as the wall, the thermal image will all be the same color.

There are really just a handful of ways of manipulating temperature. The easiest way is either to turn the HVAC thermostat up or down ten degrees.

If you are inspecting the exterior of a house, then the best thing to do is take infrared images just after sunrise, or just after sunset. If you just wait for sunset or sunrise, it is equivalent to turning the HVAC up or down ten degrees. You also need to be careful of taking thermal images on a windy day because that can bring the total surface temperature down and create thermal blindness.

How To Find Missing Insulation?

As a home inspector, sometimes the owner asks me to find areas of missing insulation (or wet insulation).

If it is hot outside, then all I have to do is lower the HVAC temperature by about 10 degrees. This creates a thermal gradiance with the exterior, so any areas of missing insulation should light up as reddish/yellow areas in the thermal imagery.

In the picture below, you can clearly see that there is no insulation around the rim joist. The rim joist is at the top of the basement/foundation walls, and these gaps in-between the floor joists are frequently missing insulation or have inadequate insulation.

You can see the gaps in the joists showing up as a bright yellow color which means the heat from the outside is infiltrating into the basement. You can also see the drain line showing up as a dark blue color because the water flowing through it is a cold/cool temperature.

Read Also: What Are The Best Rated Moisture Meters For Home Inspectors?

how to interpret thermal images (missing insulation) (1)

Missing insulation in basement

Final Thoughts

The most important thing is to think logically about what you are seeing in a thermal image.

If you manipulated the environmental temperature, you will have to ask yourself if what you are looking for will be cooler (blue) or warmer (red). If I am looking for a water leak, and I turn the HVAC temperature up ten degrees, then the water will stay 'cooler' because it is the last thing to warm up. And so I will look for bluish areas.

And if I want to find areas of missing insulation on a hot day, I will turn down the HVAC temperature ten degrees, and look for 'hot' areas that are red or yellow.

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