This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Reflective and low-e products: where they help, where they don’t and why

Many products claim to save energy by controlling radiation, including reflective paint additives, radiant barriers, reflective roof coatings and low-emissivity coatings on windows.  In an ideal world, the potential energy savings from controlling radiation losses and gains might be large.  In reality, many factors impact how effective these products are. 

First of all, heat is energy transferred from one body or system to another as a result of a difference in temperature. Heat always migrates from the hotter object to the cooler object, never the other way around.  Heat is transferred by three methods: conduction, convection and radiation.

Conduction requires the physical contact of two objects.  In the case of a wall, heat is conducted through the layers within the wall from the warmer to the cooler side.  Convection is heat transfer due to fluid or air flow.  A common example is when warm air rises (or cool air falls) on a wall’s inside surface, inducing air movement.   

Heat is transferred by radiation when surfaces exchange electromagnetic waves, such as light, infrared radiation, UV radiation or microwaves.  Radiation does not require any fluid media or contact, but does require an air gap or other transparent media between the surfaces exchanging radiation.  Radiation exchange occurs between two surfaces when one is warmer than the other and they are in “view” of each other; i.e., there is nothing physically between the two surfaces. 

Radiation is a significant component of heat transfer in buildings, in both heating and cooling even at typical temperatures and even in the absence of solar radiation. It is especially important for sun-exposed surfaces and where there are large temperature differences (e.g. radiant heating, refrigeration, industrial settings with warm surfaces, ice rinks, etc.)  Reflective (low-emissivity) products will generally be most effective in these applications.  In certain applications, however, a high-emissivity (non-reflective) surface will perform better.  Therefore, some understanding of these properties is helpful in evaluating a reflective product. 

Reflectivity and emissivity of a surface affect radiation heat transfer and how a reflective product performs.  The fraction of radiation arriving at a surface that is reflected by it is called its reflectivity.   Emissivity essentially is the surface’s tendency to emit radiation to other bodies.  Surfaces with high emissivity are also very absorptive—that is, they will readily absorb radiation striking them.  These properties may vary depending on the wavelength of radiation falling on the surface. For example, the surface may reflect much of the visible radiation (i.e., light) falling on it, but not much of the ultraviolet radiation or infrared radiation falling on it. 

Low emissivity will save energy whenever you want to reduce heat transfer.  Places you generally want to reduce heat transfer are:

• Between interior objects in a building (including people) and the interior surfaces of exterior walls – on both hot and cold days (i.e., whenever you are conditioning your space to counter the weather outside)
• Between the exterior surfaces of a building and its surroundings – on both hot and cold days (i.e., whenever you are countering the weather outside.)

Low-e coatings on windows save energy in most circumstances because they reduce heat transfer with the surroundings. As another example, a low-emissivity ceiling – such as unpainted aluminum or a reflective aluminum paint product – in an ice rink may have very good energy savings.  In this case, low emissivity (over all wavelengths) would reduce radiation heat transfer between the warmer ceiling and the cold surface of the ice.

On the other hand, a surface used as a radiant heater – such as a radiant floor or a radiator – is an example of where high emissivity is beneficial because we want to enhance heat transfer from the radiator.  Another example of where low-emissivity coatings will increase energy use is a building that requires cooling even on a cool day because, in this case, you generally want to enhance heat transfer (i.e., you are not countering the weather outside).  For example, low-e surfaces on windows or walls are bad in rooms with high internal gains, such as computer server or telephone switching rooms, because you generally want to get rid of heat at all times.  The only time low-e surfaces will help you in a server room is when it gets so cold outside that you have to start heating (i.e., when you start having to counter the weather outside).

Solar radiation is composed of visible and near-infrared radiation.  Far infrared-radiation is outside the solar band (both near- and far-IR are felt as heat).  Therefore, whenever we are trying to understand how a material behaves when exposed to solar radiation, its emissivity in each of these three bands is more important than its overall value summed over the entire spectrum.  For a “cool roof,” we want to reflect (i.e., not absorb) solar radiation — and whatever heat we do absorb we want to emit back to the surroundings.  Therefore, we want high reflectivity in the visible and near-infrared bands, but low reflectivity (i.e., high emissivity) in the far infrared bands. 

If you just look at a single number for emissivity you will miss the essential behavior of certain materials.  For example, both unpainted metal surfaces and white surfaces have high reflectivity when summed over all bands and so both reflect much of the solar radiation striking them.  But when you look at each of the three bands separately, you can see why unpainted metal roofs become hot in the sun but white surfaces stay cooler.  White roofs have high emissivity in the far-infrared band, while unpainted metal roofs have low emissivity in this band.