RADIANT FLOOR HEAT AND THERMOGRAPHY

A Geeks Guide To The Radiant Electromagnetic Spectrum

While radiant heat can be felt when you are on a radiantly heated floor, it usually can not be heard and can not be seen. This makes it hard to visualize what it really is. In physical terms what is radiant heat? Wonderfully, thermographic cameras help us “see” what “radiant heat” looks like. While thermographic images are a representation of what can not be seen, they look warm an understandable.

Radiant Heat lies just outside of the visible light we see in the Infrared band

For those of you who do not know, Thermography is the use of thermograms to study heat distribution in structures or regions, for example in detecting tumors or in our case understanding radiant floor heating. The wonderful thing about Thermographic cameras is they give us images and readings of the Infrared part of the electromagnetic spectrum, almost all of which is just beyond our range of seeing visual light. So, in a sense these cameras extend our senses and understanding of radiant heating.

Whether it is hydronic warm water radiant floor heating or electric radiant floor heating, a radiant floor heating system emits almost all of its electromagnetic energy as infrared (IR) radiation, specifically in the long-wavelength IR range (around 8–15 microns). Very little to no energy is emitted in other electromagnetic frequencies that aren’t picked up by an infrared camera — especially if the camera is designed for building diagnostics (which usually covers the key thermal IR spectrum). So, you’re already “seeing” a visualrepresentation of the vast majority of radiated energy.

Infrared cameras come in many shapes and sizes These are from FLIR.

Radiant heat energy is transmitted at a frequency of the electromagnetic spectrum that we can’t see but can feel and usually cant hear makes for its hidden silent comfort. But by using an infrared camera we are seeing a good visulization of the radiant energy we can’t see. They also have many uses for the contractor.

Courtesy of Infrared Thermal Imaging Services (above 3 images)

This image was used to find a leak but thermography can help find where you can drill in a radiant floor. They are also usefull in analyzing leaky and under insulated areas in a thermal envelop of a home.

Courtesy of Infrared Thermal Imaging Services

I have A FLIR Thermal Imaging Camara which I use to compare radiant heating prototype products. This camera, when set correctly and verified by temperature sensing thermistors is a very useful tool. FLIR and other brands have proprietary software where you can import thermal images and edit them. The most usefull tool in this software allows you to make a rectangle of an area and it will compute the high, low and average temperature of the rectangle. If that area is a correctly chosen area of a radiant floor heating board product it can be very useful in initial product protype selections and comparisons.

The above image shows a box I drew using FLIR Ignite sofware. The box contains a cross section of 2 runs of radiant tubing in a radiant floor system covered by 1/4″ of Hardy Backerboard and 3/4″ of wood. The box has been formatted to show the high low and computed average surface temperature in the box. The left tube is next to a colder outside wall. In this case the high was 85.6 °F, the low 80 °F and the computed average surface temperature was 83.6 °F.

This can become even more useful when compared to the radiant heating Nomograph found in the ASHRAE Handbook when you know the water temperature and R-Value of the flooring goods and AUST (Average Unheated Surface Temperature).

The first section of this blog shows some examples thermal images that map radiant floor heating systems using a thermal imaging camera. For Many of you this first section may be the bulk of what you find interesting. The Part 2 that follows is a geeky dialog I had with CHAT GPT about thermograpy and radiant floors. I have only slightly edited it:

Q: Is much radiant energy outside the Infrared?:

A hot water radiant floor heating system emits almost all of its electromagnetic energy (heat as infrared (IR) radiation, specifically in the long-wavelength IR range (around 8–15 microns). Very little to no energy is emitted in other electromagnetic frequencies that aren’t picked up by an infrared camera — especially if the camera is designed for building diagnostics (which usually covers the key thermal IR spectrum). So, you’re already seeing the vast majority of radiated energy.

Most building-grade IR cameras (like FLIR or Testo models) detect radiation in the 8–14 micron range. This is perfect for detecting radiant floor heating systems. You’re essentially capturing nearly all radiated thermal energy aimed at its field of view.

The radiated energy spectrum comes from the top surface’s temperature and emissivity, not from the heat source (e.g., not directly from the water pipes) underneath.

Q: What do the floor coverings affect?:

Emissivity (How Well the Surface Radiates)

Different materials have different emissivities (effectiveness in emitting IR radiation).
High-emissivity materials (like matte tile or wood): Radiate well; your IR camera sees accurate thermal patterns.
Low-emissivity materials (like polished stone, aluminum foil): Reflect IR and can mislead your camera — you might be seeing reflections instead of true temperature readings.

some portion of the IR energy is scattered/emitted in directions other than toward the camera, but this is not energy the camera is “missing” in error — it’s just not in its field of view, it is just expected behavior based on physics.

The percentage of total emitted radiant energy that doesn’t go toward the camera depends on your viewing angle and coverage, but under typical conditions, it is far more than 10% — closer to 80–90% of the energy radiates in other directions.

Let me explain:

Q: Why So Much Energy Goes Elsewhere?

🔸 1. Radiant Heat is Emitted in All Directions (Hemisphere)

A surface radiates over a hemisphere (180°).
The FLIR camera sees only a narrow cone (e.g., 25–45° field of view depending on lens).

Q: If so much of the radiant energy is being missed, how is the reading relevant or close to the standard radiant floor heating Nomograph contained in the ASHRAE Hanbook? The answer is the camera is quite accurately reading the floor temperature in the area of its view. The AHRAE Nomograph relates Floor Tempertures to Room temperatures with adjustments for AUST (Average Unheated Surface Temperature).

Some portion of the IR energy is scattered/emitted in directions other than toward the camera, but this is not energy the camera is “missing” in error — it’s just not in its field of view. The FLIR camera captures the radiance in the direction it’s pointed, and this is expected behavior based on physics.

Let me explain:

Why Does So Much Energy Goes Elsewhere?

🔸 1. Radiant Heat is Emitted in All Directions (Hemisphere)

A surface radiates over a hemisphere (180°).
The FLIR camera sees only a narrow cone (e.g., 25–45° field of view depending on lens).

2. The hemispherically scattered energy is heating walls and surfaces and contributes to AUST.

Quick Breakdown:

FLIR camera can give you a radiometric average based on infrared emissions.
ASHRAE nomograph requires a true average surface temperature of the radiant floor to estimate heat transfer and resulting comfort levels, with corrections based on AUST.

Q: How well do these match up then?

They can correlate well—**but with caveats:

FLIR Average = Emissivity-Weighted Average
FLIR calculates temperature from infrared radiation, which is affected by:
- Surface emissivity (wood, tile, carpet vary)
- Angle of view (cosine losses outside normal incidence)
- Reflections (from walls, furniture, windows, etc.)
- To a lessor extent by humidity and ambient temperature
- You’ve set the correct emissivity in the FLIR,
- You’re measuring at a perpendicular angle (or correcting for angle),
- You’ve minimized or corrected for reflected IR,

…the “average” it shows could be biased.

ASHRAE Nomograph Assumes an Area-Averaged, Conductively Driven Surface Temp
ASHRAE’s nomograph expects an accurate spatial average of the floor surface that represents the actual conductive-coupled radiant emission—not just the IR signature. For example:
- A cold rug or air leak nearby could skew the FLIR average downwards more than it affects heat emission.

Q: What Are Best Practices for Alignment:

🔧 Emissivity Calibration: Make sure your FLIR emissivity settings match the actual floor material (e.g., wood ≈ 0.9, tile ≈ 0.85–0.95).
📏 Multiple Angles & Stitching: Take FLIR images from multiple locations and stitch or average them to approximate full-area values.
🌡️ Compare with Thermistor Probes: If possible, use spot-sensors or contact thermistors in a few zones to calibrate your FLIR reading.
Use Corrected FLIR Average for ASHRAE Input: After adjustment, the FLIR average floor temperature can be reasonably close to the ASHRAE value—typically within ±1–2°C, if the above factors are well-controlled.

CHAT GPT CONCLUSION:

The FLIR-derived average floor temperature can be used in conjunction with the ASHRAE radiant floor nomograph, but it needs to be interpreted with care. Proper emissivity settings, angle of capture, and a sense of how IR scattering affects the reading will help you use it effectively. When those are well managed, your FLIR data becomes a strong proxy for what ASHRAE’s model needs.

THE AUTHORS CONCLUSION:

In tests we have done we added very accurate flow meters, water temperature guages and radiant surface temperature thermistors and mostly found good correlation to the results of the Thermogarphic cameras, but the other equipment is an essential way to check accuracy.

ABOUT THE AUTHOR

Hoyt Corbett

Partner, Warm Brothers Inc.

Hoyt has been involved with hydronic heating since the early 1990’s and developed and sold a national brand of radiant floor heating that was acquired by Uponor. Subsequently, Hoyt was Associate Director of the Radiant Panel Association, the industry trade group.

He’s produced extensive technical writing on radiant floor heating, writing several major national hydronic vendor’s radiant heating design and installation manuals as well as founding and publishing The Radiant Flooring Guide.