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Most of the insulation industry talks about conductivity.
Very few talk about diffusivity.
That’s a problem.
Because when the sun hits a surface, what matters is not just how much heat a material conducts. What matters is how fast that heat moves through it and how much actually loads into the substrate in the first place.
Understand the difference is significant.
Roofs, tanks, containers, cladding. They all experience the same issue.
Solar radiation hits the surface.
The surface absorbs energy.
Heat loads into the substrate.
Internal temperatures climb.
Most traditional insulation systems focus on slowing heat once it has already entered the system. That’s where conductivity comes in.
But by the time you’re measuring conductivity through a 50mm batt or rigid board, the surface has already absorbed a significant thermal load.
Surface temperature drives everything.
If you don’t control the surface, you are managing symptoms, not the cause.
Thermal conductivity, measured in W/m·K, tells you how easily heat passes through a material.
Low conductivity means heat moves slowly from one side to the other.
That’s useful for bulk insulation inside cavities. Standards like ISO 8301 and ASTM C177 are built around this concept.
Reference:
ISO 8301 – Thermal insulation — Determination of steady-state thermal resistance and related properties
https://www.iso.org/standard/65769.html
Conductivity assumes steady-state conditions. Two sides. Temperature difference. Heat flow through thickness.
But solar heat gain is not steady-state. It’s dynamic. Intense. Directional. Radiative first, conductive second.
Which means conductivity alone does not explain what happens on a roof at 2 pm in January.
Thermal diffusivity measures how quickly a material responds to temperature change.
It combines three properties:
Formula:
α = k / (ρ × Cp)
Where:
k = conductivity
ρ = density
Cp = specific heat
Diffusivity is measured in mm²/s.
Low diffusivity means a material resists rapid temperature change. Heat does not penetrate quickly. The thermal wave slows down.
That is critical for surface management.
Reference:
ASTM E1461 – Thermal Diffusivity by the Flash Method
https://www.astm.org/e1461-13.html
Diffusivity explains why some materials feel stable under solar load while others spike dramatically in temperature.
And here’s the key.
You can have a low conductivity material with relatively high diffusivity if density and heat capacity allow rapid temperature response.
So the question becomes:
Are you slowing heat flow?
Or are you stopping rapid heat loading at the surface?
They are not the same thing.
Solar energy is not just “heat”. It’s radiation.
44% visible light
53% near-infrared
3% ultraviolet
If a surface absorbs infrared, it heats instantly. That energy then becomes conductive heat moving inward.
If you block radiation at the surface, you reduce what ever becomes conductive.
Reference:
U.S. Department of Energy – Cool Roofs
https://www.energy.gov/energysaver/cool-roofs
Traditional bulk insulation does nothing to stop radiation at the surface. It only slows heat after absorption.
So conductivity becomes a secondary defence.
Surface management is the first defence.
Real surface protection requires:
This is where thin ceramic insulation coatings behave differently to standard paints and bulk materials.
Instead of absorbing and then managing heat, they reduce initial load and slow thermal penetration.
Diffusivity becomes critical.
If the thermal wave cannot move quickly through the coating, the substrate temperature remains more stable.
That is not about thickness.
That is about physics.
Super Therm® is not designed around bulk conductivity like conventional insulation.
It is engineered around surface thermal behaviour.
At only 0.25 mm dry film thickness, it performs differently because of its multi-ceramic composition and very low thermal diffusivity.
Instead of absorbing solar radiation and letting conductivity manage the aftermath, it:
Independent testing and field performance results are documented here:
https://neotechcoatings.com/super-therm-testing-and-results/
The difference is fundamental.
Bulk insulation works inside cavities.
Super Therm works on the envelope.
Bulk insulation manages heat flow.
Super Therm manages heat load.
When you reduce load first, the job of conductivity becomes less critical.
That is why thin-film surface management can materially change internal temperature performance without adding thickness.
It would be wrong to say conductivity does not matter.
It absolutely does in wall systems, under slabs, and within traditional thermal modelling frameworks.
R-values are still relevant in cavity systems and cold climates where conductive loss dominates.
But in high solar load environments, particularly metal roofs, containers, tanks, and industrial assets, radiation is the dominant force.
And radiation is not controlled by conductivity.
It is controlled at the surface.
If you only optimise for conductivity, you are optimising the second stage of heat transfer.
If you optimise for diffusivity and reflectance, you are controlling the first stage.
That’s the distinction.
Thermal conductivity tells you how easily heat passes through a material.
Thermal diffusivity tells you how quickly heat penetrates and changes temperature.
For cavity insulation, conductivity is king.
For solar-exposed surfaces, diffusivity and radiation control determine performance.
The surface is where failure starts.
The surface is where protection must begin.
Super Therm® operates at that level.
It blocks the majority of incoming solar radiation before it becomes conductive heat.
It slows thermal wave penetration with low diffusivity ceramic structure.
It stabilises the building envelope rather than simply slowing heat once it has entered.
If you want to protect a surface, don’t just ask for the lowest conductivity.
Ask how the material behaves under real solar radiation.
Ask how fast heat moves through it.
Ask whether it blocks load or merely delays it.
That’s where the real performance difference sits.
ISO 8301 – Thermal insulation — Determination of steady-state thermal resistance
https://www.iso.org/standard/65769.html
ASTM E1461 – Standard Test Method for Thermal Diffusivity by the Flash Method
https://www.astm.org/e1461-13.html
U.S. Department of Energy – Cool Roofs
https://www.energy.gov/energysaver/cool-roofs
Super Therm Testing and Results
https://neotechcoatings.com/super-therm-testing-and-results/
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