Low-emittance (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from a warm pane of glass to a cooler pane. Coating a glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant amount of this radiant heat transfer, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light. Different types of low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.
In heating-dominated climates with a modest amount of cooling or climates where both heating and cooling are required, low-E coatings with high-, moderate- or low-solar-gains may result in similar annual energy costs depending on the house design and operation. While higher solar-gain glazings perform better in winter, lower solar-gain glazings perform better in summer. In cooling-dominated climates, the priority is to lower solar gains. Look at the energy use comparisons under Window Selectionto see how different glazings perform in particular locations.
Double-Glazed with High-Solar-Gain Low-E Glass
This figure illustrates the characteristics of a typical double-glazed window with a high-solar gain low-E glass with argon gas fill. These windows are designed to reduce heat loss but admit solar gain. High-solar-gain low-E glass products are best suited for buildings located in heating-dominated climates and are the product of choice for passive solar design projects.
High-solar-gain low-E glass is often made with pyrolytic low-E coatings, although sputtered high-solar-gain low-E is also available.
Double-Glazed with Moderate-Solar-Gain Low-E Glass
This figure illustrates the characteristics of a typical double-glazed window with a moderate-solar-gain low-E glass with argon gas fill. These windows are often referred to as spectrally selective low-E glass due to their ability to reduce solar heat gain while retaining high visible transmittance. Such coatings reduce heat loss and let in a reduced amount of solar gain making them suitable for climates with both heating and cooling concerns.
Moderate-solar-gain low-E glass is often made with sputtered low-E coatings, although pyrolytic moderate-solar-gain low-E is also available.
Double-Glazed with Low-Solar-Gain Low-E Glass
This figure illustrates the characteristics of a typical double-glazed window with a low-solar-gain low-E glass with argon gas fill. As with moderate-solar-gain low-E glass, these windows are often referred to as spectrally selective low-E glass due to their ability to reduce solar heat gain while retaining high visible transmittance. Compared to most tinted and reflective glazings, this low-E glass provides a higher level of visible light transmission for a given amount of solar heat reduction. Variants on low-solar-gain low-E coatings have also been developed which may appear slightly tinted. This type of low-E product reduces heat loss in winter and substantially reduces solar heat gain both in winter and in summer. Thus, low-solar-gain low-E glazings are ideal for buildings located in cooling-dominated climates.
Low-solar-gain low-E glass is typically made with sputtered low-E coatings consisting of either two or three layers of silver (also called double-silver or triple-silver low-E).