Surface emissivity is the amount of infrared energy emitted by a specific material. It is often expressed as a ratio or percentage of “1” with the “1.0” value being perfect “black body”. A material that emitted heat energy at half that rate, like polyester film, would have a rating of .50 or 50%. Objects like aluminum or copper, with low surface emissivity (low-e) do not transmit heat well because most of the infrared energy is reflected instead of absorbed.
In the insulation industry, surface emissivity is a major factor in the effectiveness of the product, yet one often overlooked and overshadowed by thermal conductivity or “k values”. Both are important in that they address different methods of heat transfer (infrared radiation vs. conduction) but in some applications -- like attic insulation in warm climates, shipping and receiving goods at extreme temperatures, and industrial insulation for equipment operating at high temperatures -- infrared radiation plays a bigger role than conduction, and low surface emissivity is key.
Surface Emissivity in Home Insulation
Fiberglass home insulation reduces thermal conductivity by trapping air in the pockets between fibers (unmoving air has surprisingly low thermal conductivity). An inch of fiberglass or cellulose insulation has an R-value of 2.2 to 3.8, depending on the density and installation method. Building codes require R-38 to R-60 depending on the climate, which means 10 to 27 inches of insulation. Because this type of insulation typically has a surface emissivity of over 0.75 (or 75%), the addition of a low emissivity surface may be needed to make it more efficient.
Placing a layer of low-emissivity material (like aluminum or copper) in the attic above the insulation acts as a “radiant barrier” and decreases the amount of infrared radiant heat that reaches the attic insulation below. Depending on the level of existing insulation, the low emissivity metal can reduce the total heat flux through the ceiling by as much as 50%. It is especially effective in warm climates where a large portion of the home’s energy usage goes toward cooling.
Reducing the surface emissivity of the insulation saves so much energy, it is a requirement for ENERGY STAR Qualified Homes in certain circumstances. Learn more at ENERGY STAR.
Surface Emissivity in Shipping and Packaging
Shipping costs are impacted by the weight of an object being shipped and its volume, making options like fiberglass or thick foam less than optimal for many shipping solutions. Foam works by reducing conductivity, but has a high emissivity of about 0.6 (or 60%) so it takes several inches to insulate well. In addition to foam taking up a lot of space for the shipper, it is difficult to recycle for the end user.
A series of low emissivity metalized films inside the box retains the same R-value as if it were insulated with foam inserts. Because these films have a surface emissivity of less than 0.9 (or 90%), they are able to insulate well while taking up very little space. When shipping a temperature sensitive item, like frozen foods, the item will stay cold as long as if it were insulated with the much bulkier foam, with the added advantages of packing flat, weighing less, and being 100% recyclable.
Surface Emissivity in Industrial Insulation
Industrial insulation is designed to address two possible problems: insulating against high temperature differences and deadening sound. While low emissivity surfaces offer little in the way of sound deadening, they excel at reducing heat transfer.
Insulation for cryogenic, for example, uses vacuum to eliminate convection paired with multiple layers of a low-e material. These aluminum, copper, or gold layers are separated by small air spaces. This type of multi-layer insulation may contain as many as 60 layers per inch, and with each layer of metal emitting as little as 2-3% of insulation, radiation is virtually eliminated as well.
In hot applications, like insulating hot pipes or storage tanks, conduction and radiation are addressed together. This addresses worker safety, fire protection, and equipment efficiency. The standard product chosen for this type of equipment is an insulating jacket with fiberglass on the inside and low emissivity aluminum on the outside. These types of jackets can withstand temperature up to 550 degrees F, with the fiberglass addressing conductive heat but having high surface emissivity (over 75%), and the low-e foil having high conductivity but low surface emissivity (about 3%), and with both the fiberglass and the aluminum foil being highly flame retardant.