Pyrolytic coatings, typically made from indium tin-oxide, are melted into the glass while the glass is still molten. This durable “hard-coat” can be used in a single-glazing application such as a storm window. Smaller-scale window manufacturers often use this type because it can be handled more easily. Pyrolytic low-e coatings tend to tint the window more than sputtered silver coatings, and they also have a rougher surface that is harder to clean.
A recent development produces a sputtered, hard-coat low-e that is durable enough to be applied on the outside of the pane of glass that faces a room. Made of indium tin-oxide, like traditional hard-coat low-e, it isn’t melted into the glass, so the glass is much smoother, easier to clean, and less visible. This coating can be installed on the “warm side” (the face exposed to the room) in combination with a conventional sputtered soft-coat low-e within the IGU to achieve energy performance that was previously only available with triple glazing—a unit R-value as high as R-5 or a U-factor (the inverse of R-value) as low as 0.20.
Low-conductivity gas-fill. Different gases have different conductivity. Argon, a common gas, has significantly lower conductivity than air, so filling the inner spaces with argon improves the R-value significantly. Argon is an unreactive “noble” gas, which accounts for about 1% of air. Other noble gases, krypton and xenon, have even lower conductivities than argon, but because they are more rare, they are much more expensive. The impact of different low-conductivity gases on energy performance is shown in the “Window Gas Properties” table.
Most manufacturers assume that 1% of the gas leaks out per year, but Randi Ernst of FDR Design, a fabricator of gas-filling and testing equipment, has done informal testing showing that the leakage rate is closer to 0.6% per year. At that rate, an IGU that starts out with 95% argon will still have 79% of the argon after 30 years and 70% after 50 years—though as organic glazing seals deteriorate over decades, that leakage rate could increase. As the low-conductivity gas leaks out, the window R-value will drop over time.
Glazing spacer. As the performance of IGUs has improved, addressing heat loss at the edges of the IGUs and the window frames has become more important. After the welded-glass IGUs disappeared, the most common method for separating the glass layers was to use aluminum channel and butyl sealants. However, highly conductive aluminum causes significant thermal bridging, leading to condensation and cold interior temperatures at the edges of windows.
Most modern IGU manufacturers now use stainless steel spacers, which have two advantages over aluminum: it’s stronger, so the material can be thinner (less cross-sectional area for heat loss), and it is significantly less conductive. Other options include butyl rubber with a corrugated metal reinforcement strip and silicone glazing spacers, both of which have very low conductivity. Such spacers are often referred to as warm-edge spacers. The silicone Super Spacer from Quanex Building Products is the highest-performance glazing spacer available today.
Frame material. Early residential window sash and frames were made of wood, but polyvinylchloride (PVC) has increasingly gained market share to become more common than wood. In terms of energy performance, wood and vinyl are relatively similar, though if the hollow vinyl extrusions are filled with foam insulation the energy performance can exceed that of wood.
Metal windows—aluminum or steel—have lost popularity in residential applications due to their significant thermal conductivity. They remain common in commercial windows, due to their strength (particularly in large dimensions) and durability.
The highest-performance American-made windows have fiberglass frames. Fiberglass (usually a fiberglass-reinforced polyester plastic) has much lower expansion and contraction rates as it warms and cools than does PVC. This stability improves the window life and helps maintain air tightness. Like vinyl, fiberglass window frames are hollow and can be insulated with polyurethane.