Low emissivity and high reflectivity are two important properties of building insulation materials, especially aluminum foil laminates and radiant barriers. Although often discussed together, they describe different aspects of heat transfer. Understanding the difference helps architects, contractors, and material buyers evaluate insulation performance, compare product specifications, and select the right materials for roofing and building envelope applications.
In building insulation, “low emissivity” and “high reflectivity” are often mentioned together, especially in technical literature for aluminum foil composites, reflective roofing membranes, radiant barriers, and Low-E building materials. Although they may appear to mean the same thing, they describe two different directions of radiant heat transfer: high reflectivity emphasizes reflecting incoming radiation, while low emissivity emphasizes a surface’s limited ability to emit thermal radiation.
Judging a material only by its shiny, foil-like appearance can easily lead to confusion between these two properties. The thermal performance of a building envelope depends on more than whether a material looks reflective. It also depends on whether the surface interacts with shortwave solar radiation or longwave thermal radiation exchanged between roofs, walls, equipment, and indoor objects. Equally important are the presence of an effective air space, surface dust accumulation, and correct installation orientation.
Low Emissivity Measures How a Surface Emits Heat
After a roof is heated by the sun, heat passes through the roof assembly by conduction. The underside then emits infrared radiation toward cooler surfaces, such as the attic floor and ductwork. A radiant barrier installed beneath the roof targets this radiative heat transfer across the air space.
Emissivity compares the thermal radiation emitted by a material’s surface with that emitted by a blackbody at the same temperature. Its value ranges from 0 to 1. At the same temperature, a surface with lower emissivity emits less thermal radiation. Low emissivity therefore does not necessarily mean that a material has a lower temperature; it means that the surface releases heat less readily through radiation.
This also explains why “low-emissivity insulation membranes” and “high-emittance cool roofs” can both make sense. The former reduce radiant heat exchange within a building assembly, while the latter help release heat already absorbed by the roof to the outdoor environment.
High Reflectivity: What Type of Radiation Is Being Reflected?
Solar reflectance is the proportion of incoming solar energy reflected by a surface. It is mainly used to evaluate roofing materials directly exposed to sunlight. Longwave infrared reflectance, by contrast, concerns the thermal radiation emitted by heated surfaces such as roofs and walls. These are not the same property.
According to Lawrence Berkeley National Laboratory, solar radiation falls primarily within the wavelength range of 0.3–2.5 micrometers, while thermal radiation from building surfaces falls primarily within 4–80 micrometers. Because these wavelength ranges differ, solar reflectance and thermal emittance can vary independently. A material that reflects sunlight well does not necessarily have low thermal emittance.
For this reason, exposed cool roofs generally benefit from a combination of high solar reflectance and high thermal emittance: the former reduces heat absorption, while the latter helps release heat. Simply stating that “high reflectivity means low emissivity” confuses properties measured in two different wavelength ranges.
The Key Differences
| Comparison |
High Reflectivity |
Low Emissivity |
| Direction of radiation |
Radiation arriving at the material’s surface |
Thermal radiation emitted by the material’s surface |
| Key question |
How much incoming radiation can the surface reflect? |
How much thermal radiation does the surface emit? |
| Typical purpose |
Reduce solar energy absorbed by the roof and limit surface heating |
Reduce longwave radiant heat exchange beneath roofs, in attics, and within wall cavities |
| Common metric |
Solar Reflectance (SR) |
Thermal Emittance (TE) or Emissivity (ε) |
| Preferred value |
Higher is better for the intended reflective function |
Lower is better for limiting radiant heat exchange |
| Typical materials |
White coatings, light-colored roofing, reflective roofing membranes, and metallized surfaces |
Aluminum foil, foil composite membranes, low-emissivity coatings, and Low-E coated materials |
| Dependence on an air space |
Reflecting sunlight from an exterior roof surface does not necessarily depend on an adjacent air space |
Yes; a low-emissivity surface must face an air space to function effectively as a radiant barrier |
| Main type of radiation addressed |
Shortwave solar radiation |
Longwave thermal radiation exchanged between building surfaces |
In simple terms: high reflectivity means less heat absorbed; low emissivity means less heat emitted.
Why Are Low Emissivity and High Reflectivity Often Associated with Aluminum Foil?
For a surface that does not transmit radiation, Kirchhoff’s law states that emissivity equals absorptivity under the same wavelength, direction, temperature, and other relevant conditions. Since absorptivity and reflectivity add up to 1 for such a surface, low emissivity corresponds to high reflectivity under those same conditions.
However, this relationship cannot be applied across different wavelength ranges. In particular, solar reflectance cannot be used directly to calculate thermal emittance.
This is the principle behind aluminum foil radiant barriers: they reduce the absorption of longwave infrared radiation while also limiting their own thermal radiation. However, “reflecting most radiant heat” does not mean “blocking the same percentage of total heat transfer through a building,” because building heat transfer also involves conduction and convection.
Installation Location Matters
For materials used on the exterior surface of a roof, the key properties to evaluate are solar reflectance and thermal emittance. For an aluminum foil radiant barrier installed beneath a roof, the relevant property is the emissivity of the surface facing the air space.
The U.S. Department of Energy states that a reflective surface must face an air space to effectively reduce radiant heat transfer, and that dust accumulation can reduce its performance.
If aluminum foil is tightly sandwiched between two solid materials without an adjacent air space, it should not be expected to provide the same radiant-barrier effect as it would when installed facing an air space. Aluminum foil itself does not reduce conductive heat transfer in the same way as conventional insulation.
When reviewing product specifications, the most useful questions are not simply “How high is the reflectivity?” but:
- Which wavelength range does the reflectivity value refer to?
- Which side of the material was tested?
- Will that side face an air space in the actual building assembly?
Low emissivity and high reflectivity only have clear engineering significance when the measured properties are matched to the material’s installation location and function.
