Thermal emittance is an important property of roofing materials that describes how effectively a surface releases absorbed heat as infrared radiation. When evaluating a cool roof, thermal emittance works together with solar reflectance to determine how a roofing surface responds to the sun’s energy.
For commercial property owners, understanding both properties is important when comparing reflective roof coatings, membranes, and other energy-conscious roofing systems.
What Is Thermal Emittance?
Thermal emittance is the ability of a surface to emit absorbed thermal energy as infrared radiation. It is expressed as a value between 0 and 1.
A material with high thermal emittance can release absorbed heat efficiently, while a material with low thermal emittance retains more of that absorbed heat at the surface.
For example, a roofing surface with a thermal emittance of 0.90 has a relatively high ability to emit thermal radiation.
Thermal emittance is different from solar reflectance. Reflectance concerns how much incoming solar energy is reflected, while emittance concerns how effectively absorbed heat is released.
Why Does Solar Reflectance Matter?
Solar reflectance describes how much incoming sunlight a roof reflects.
A roof with high solar reflectance absorbs less solar energy than a roof with low solar reflectance, all else being equal. This can help reduce the temperature of the roof surface and potentially reduce heat transferred into the building.
This is why white and other highly reflective roof surfaces are commonly associated with cool roofing.
However, reflectance alone does not tell the entire story.
Why Is Thermal Emittance Important?
Even a reflective roof absorbs some solar energy. The absorbed energy can increase the roof’s temperature.
A surface with high thermal emittance can release much of that absorbed heat through infrared radiation. A low-emittance surface may release less of the absorbed heat in this way.
Therefore, two roofing products with similar solar reflectance can have different thermal behavior if their thermal emittance values differ.
This is particularly relevant when comparing roofing materials for buildings in hot, sunny climates.
Reflectance and Emittance Work Together
The simplest way to understand the relationship is:
Solar reflectance = how much sunlight the roof reflects.
Thermal emittance = how effectively the roof releases absorbed heat.
A cool roof generally benefits from a combination of high solar reflectance and high thermal emittance.
For example, a roofing surface that reflects a large percentage of sunlight while efficiently emitting the heat it does absorb can remain cooler than a surface with low reflectance and low emittance, depending on the specific materials and environmental conditions.
Is High Emittance Always Better?
Not necessarily in isolation.
Roof performance depends on the complete assembly and the building’s circumstances. Solar reflectance, thermal emittance, insulation, roof color, roof construction, climate, HVAC efficiency, building occupancy, and other factors all influence actual energy performance.
A property owner should therefore avoid selecting a roofing system solely because it has the highest thermal emittance or reflectance value.
The relevant product specifications should be considered together.
How Does This Affect Commercial Roofing Decisions?
Thermal emittance is especially relevant when owners are evaluating cool roofs, reflective roof coatings, and energy-efficient roofing systems.
For a commercial building in Florida or another hot climate, reducing roof surface heat can be an important consideration. However, the roofing system must also meet requirements for waterproofing, wind resistance, durability, drainage, compatibility, and expected service life.
A reflective coating with strong solar properties is not automatically the best solution for every existing roof. The underlying roof condition must be evaluated before restoration or coating is recommended.
How Should Property Owners Compare Products?
When reviewing roofing products, consider:
- Solar reflectance
- Thermal emittance
- Initial and aged performance, where available
- Roof assembly and insulation
- Climate and building use
- Product compatibility
- Warranty requirements
- Expected service life
- Maintenance requirements
Independent ratings and standardized product data can make comparisons more meaningful than relying solely on marketing claims.
Bottom Line
Thermal emittance measures how effectively a roofing surface releases absorbed heat, while solar reflectance measures how much sunlight the surface reflects. Both properties influence how a roof responds to solar radiation.
A high-reflectance roof can reduce the amount of solar energy absorbed, while high thermal emittance can help the roof release absorbed heat. Together, these characteristics are important considerations when evaluating cool roofing systems.
For commercial property owners, however, reflectance and emittance should be evaluated alongside the entire roofing system, existing roof condition, insulation, climate, drainage, wind requirements, and long-term maintenance needs.
Shieldline Roofing can help property owners evaluate commercial roof coatings, restoration systems, and replacement options based on both energy-performance considerations and the overall condition of the roof.
Related Questions
- What is the Cool Roof Rating Council and what does it certify?
- What ENERGY STAR criteria apply to roofing products?
- How do you calculate energy savings from a reflective roof in Florida?
- What is the Solar Reflectance Index (SRI) and why is it important for cool roofs?
- How is “cool roof” performance measured and verified?
- What is a whole-building energy model and how is the roof input?
- Is white EPDM a viable cool roof option in Florida?
