Thermal emittance is an important property that shows how well a surface can release the heat it has absorbed as thermal energy. It is useful for understanding the energy efficiency and heat-control performance of roofs, walls, and other building surfaces. Along with solar reflectance, thermal emittance helps explain how much sunlight a material absorbs and how effectively it can get rid of that heat.
In simple words, a surface with high solar reflectance sends more sunlight back instead of absorbing it, while a surface with high thermal emittance releases the heat it has absorbed more easily. Together, these properties affect the surface temperature, the need for cooling, and the overall energy performance of a building.
But how is thermal emittance measured? This is where testing standards such as ASTM C1371 are useful. This standard provides a recognised way to measure the thermal emittance of materials using portable emissometers. It helps manufacturers and building professionals check and understand how well a surface performs.
In this guide, we will explain what thermal emittance means, how solar reflectance and thermal emittance work together, how ASTM C1371 measures thermal emittance, and why these properties are important for energy-efficient building materials and cool roofing systems. Understanding these basics can help you make better choices when selecting materials that reduce heat absorption and improve indoor comfort.
What Is Thermal Emittance?
When sunlight falls on a roof or any other outdoor surface, the surface does not remain at the same temperature. It absorbs some of the solar energy, which makes it heat up. Thermal emittance explains how well that surface can release the heat it has absorbed as thermal radiation.
Thermal emittance is usually shown as a value between 0 and 1. A higher value means the surface can release absorbed heat more easily, while a lower value means it is more likely to hold on to that heat.
This property is especially important when considering roofing materials. A roof stays exposed to sunlight for long periods, particularly during summer. If it absorbs a lot of heat and cannot release it properly, the heat can eventually pass through the roofing system and lead to higher indoor temperatures.
However, thermal emittance should not be considered by itself. Solar reflectance is also important because it shows how much sunlight a surface can reflect before that energy gets absorbed.
In simple terms:
Solar reflectance = How much sunlight is reflected
Thermal emittance = How effectively absorbed heat is released
Understanding both properties provides a clearer idea of how a surface will perform in hot conditions.
What Is ASTM C1371?
Knowing the thermal emittance of a material is useful only if that property can be measured in a consistent way. This is where ASTM C1371 becomes important.
ASTM C1371 is a standard test method used to measure the emittance of materials near room temperature with a portable emissometer. In simple terms, it provides a recognised method for checking how effectively a material’s surface releases thermal energy.
A portable emissometer is placed against the material being tested. The equipment checks how well the surface emits thermal radiation, which helps determine its thermal emittance value.
The standard is useful for manufacturers and professionals who need dependable information about the thermal properties of building materials. For example, a roofing manufacturer may use thermal emittance data together with solar reflectance data to understand and explain how well a roofing product can manage heat.
Why Is ASTM C1371 Useful?
A standardised testing method makes it easier to compare materials using consistent measurements. It can be particularly relevant when evaluating:
- Roofing materials
- Coatings and roof coatings
- Cool roofing products
- Building envelope materials
- Energy-efficient surfaces
It is worth remembering that ASTM C1371 measures thermal emittance; it does not, by itself, determine the complete energy performance of a roof or building. Other factors, including solar reflectance, insulation, climate, roof design, and ventilation, also influence actual performance.
Why Does High Thermal Emittance Matter?
A surface with high thermal emittance can release the heat it absorbs more effectively. This can be helpful for roofs and other surfaces that receive direct sunlight for many hours during the day.
Think about a roof on a hot summer afternoon. Sunlight falls on the roof, and some of that energy is reflected while some of it is absorbed. The absorbed energy increases the roof’s temperature. A surface with high thermal emittance can then release more of that absorbed heat through thermal radiation.
This can help reduce the amount of heat that stays in the surface.
For buildings, managing this heat can be useful. If less heat moves from the roof into the building, the cooling system may not need to work as hard. The actual impact depends on several factors, including the local climate, insulation, building design, air conditioning system, and the roofing system itself.
This is why high thermal emittance is often discussed when talking about cool roofs and energy-efficient roofing systems.
However, there is an important point to remember. A material can have high thermal emittance and still become very hot if it absorbs a large amount of sunlight. This is why solar reflectance and thermal emittance are usually considered together.
Solar Reflectance vs Thermal Emittance
The terms solar reflectance vs thermal emittance can sometimes be confusing because both are related to how a surface handles heat. They are actually describing two different stages of the process.
Solar reflectance tells us how much of the incoming solar radiation is reflected away by a surface. A surface with high solar reflectance reflects more sunlight and therefore has less solar energy available to absorb.
Thermal emittance, on the other hand, tells us how effectively the surface can release the heat it has already absorbed.
| Property | Solar Reflectance | Thermal Emittance |
| What does it describe? | Reflection of sunlight | Release of absorbed heat |
| Main energy involved | Incoming solar energy | Thermal energy |
| Higher value generally means | More sunlight reflected | More absorbed heat emitted |
| Main benefit | Reduces solar heat absorption | Helps release retained heat |
A simple example makes the difference easier to understand.
Imagine two roofs exposed to the same sunlight. Roof A reflects a large portion of the sunlight, so it absorbs less energy in the first place. Roof B may absorb more energy, but if it has high thermal emittance, it can release the absorbed heat more effectively.
The ideal combination depends on the material, application, and climate, but looking at both properties provides a much clearer picture than considering either one alone.
How Do Surfaces Release Absorbed Heat?
Once a surface absorbs heat, that energy has to go somewhere. There are several ways heat can move away from a surface, with thermal radiation, conduction, and convection being the main heat-transfer mechanisms.
Thermal Radiation
Thermal radiation is directly related to thermal emittance. A warm surface releases energy in the form of electromagnetic radiation, much of which is in the infrared portion of the spectrum.
A surface with higher thermal emittance is generally more effective at emitting this thermal energy. Unlike conduction, thermal radiation does not require direct physical contact between objects.
Conduction
Conduction occurs when heat moves through a material. For example, heat absorbed by the outer surface of a roof can travel through roofing layers toward the building below.
The amount of heat that eventually reaches the interior depends on factors such as the roofing assembly, insulation, material thickness, and temperature difference.
Convection
Convection involves heat transfer between a surface and moving air. As a roof becomes warmer, the air immediately around it can also heat up. Air movement can carry some of this heat away from the surface.
These three processes can happen at the same time. The balance between them depends on the material, surface temperature, surrounding conditions, wind, and other environmental factors.
Why These Properties Matter for Cool Roofing
A roof’s ability to manage solar heat depends on more than its colour or appearance. Solar reflectance and thermal emittance provide useful information about how the surface responds when exposed to sunlight.
A roof with good solar reflectance can reflect a significant portion of incoming sunlight, reducing the amount of energy it absorbs. If the surface also has suitable thermal emittance, it can release absorbed heat more effectively.
This combination is one reason these properties are commonly considered when developing and evaluating cool roofing systems.
Understanding thermal emittance, solar reflectance, and ASTM C1371 can therefore help building owners, architects, contractors, and manufacturers make better-informed decisions about roofing and other heat-management materials. Rather than looking at one property in isolation, considering how these characteristics work together provides a more realistic understanding of a material’s thermal behaviour.
