“U-Factor” or “U-value” measures the rate of heat transfer and refers to how well a window prevents heat loss (the opposite of R-value which refers to insulating value). The values are usually between .20 and 1.20, and the lower the U-value the lower the amount of heat that transfers and therefore the greater a window's insulating value. Windows with lower U-value also have less of tendency form condensation. U-factor is affected by the airflow around the window and the emissivity of the glass. Emissivity is the ability of a product to absorb certain types of energy (specifically infrared) and radiate that energy through itself and out of a room. A product with high emissivity, such as one pane of clear glass, will transfer over 84 percent of the infrared energy from a warm room outside to the cold air. The lower the conductivity and emissivity of the glass, the lower the rate of heat loss and the lower the U-factor. It is now possible to obtain windows with low emissivity (low-e) coatings on the glass.

Low-e coatings. According to the Department of Energy, windows manufactured with Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%. Keep in mind that not all low-e coatings are alike. A Low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The Low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain so it is important to choose a window based on your climate zone. In the northern climate of NYC the low-e coating should be applied to the inside pane of glass which will keep heat inside the house; in southern climates the low-E coating should be applied to the outside pane of glass to reduce solar heat gain. A Low-E coating can also reduce a window's visible transmittance unless you use one that is spectrally selective which means it can filter 40%-70% of the heat while still allowing a full amount of visible light to transmit through the pane.

Window manufacturers apply Low-E coatings in either soft or hard coats. Soft Low-E coatings degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life. Therefore, manufacturers carefully apply them in insulated multiple-pane windows. Hard Low-E coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. The energy performance of hard-coat, Low-E films is slightly poorer than that of soft-coat films.

Gas fills. Windows are now readily available with an injection of argon or krypton between the panes -- inert gases (do not react readily with other substances) that have a higher resistance to heat flow than air and, consequently, help reduce the U-factor. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. Krypton is more expensive but has a better thermal performance. Gas fills increase the insulating properties, thus lowering the U-factor, of windows.