Energy-Efficient Glazing for Windows

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All parts of a window are important when you consider energy efficiency, but the glazing comes first. About 75% of a window's surface area is glazed, with the sash and frame making up the other 25%, so it is important to make the right choices for the glazing.

Low-e coatings

Low-emissivity glass is made by applying a coat of clear silver or tin oxide to the glass. The metallic coating blocks long-wavelength light (such as infrared, which is the heat in sunlight) while allowing short-wavelength, visible light to pass through unaffected. The practical result is that low-e windows can block up to 90% of the heat carried by sunlight at the cost of some visible light. You will also lose all the solar gain for your home's internal thermal mass.

Two types of low-e coating are possible: soft and hard. The soft coat is more common and consists of silver oxide and anti-reflection coatings pasted on the inside of one of the two panes of glass. Since the coating is applied in layers, it can be repeated to produce a supercoating that improves the window's R-value from around 2 (for a one-half-inch air space) to 3.45. A normal coating weighs in at around R-3.2.

A hard coat uses tin oxide and is incorporated as a layer in the glass at the manufacturing stage. It is a lot more durable than a soft coat, but does not provide as much heat retention/resistance.

Low-e windows add about one R unit to the window's insulation value – an effect which can also be achieved with triple glazing. The difference is about 15% better than standard double glazing.

An alternative to low-e glass is polyester films, which are treated with a low-e coating and suspended in the air gap in double glazing or hung as individual sheets on normal windows.

Single and double glazing

To be honest, it is unusual to find single-glazed windows for sale these days. Virtually all windows are manufactured with energy efficiency in mind, which means they are almost all double glazed. Double glazing uses two panes of glass with an air space between them: it is not the glass that helps make the setup more efficient, but the reduced heat transfer in the air trapped between the panes.

Air and inert gases

Most modern manufacturers use an inert gas, such as argon, in the air space in double-glazed windows. Inert gases are even less efficient at transferring heat than air, so these windows add a whole R unit to the insulation value. The noble gas krypton is even better than argon at resisting heat transfer, but it's a lot more expensive so is generally only used in the highest-performance windows.

The down-side to filling double-glazed windows with inert or noble gases is that they leak out over time. While this does no harm to people living in the house, it does reduce the insulation efficiency of the windows. Thankfully, only about 10% of the gas leaks out over a 20-year period, so the loss isn't too big and the windows are