Special coatings can be applied to a float glass surface to make it reflective to short wave radiation from the sun and/or long wave radiation from heat inside or outside the building. These coatings are known by a variety of terms, but there are two main types:
- Pyrolytic coatings: in this process, semi-conducted metal oxides are directly applied to the glass during float glass production, while the glass is still hot, in the annealing lehr. These coatings are called on-line or hard coatings, and are relatively less harmful to the environment. The best feature of this product is its durability; it can be easily handled like a standard square of glass. It can also be easily cut, heat strengthened or toughened.
- Sputtered coatings: in this process, one or more coats of metal oxide are applied under a vacuum to finished glass. The coatings applied by this technique are soft and require protection from the external environment; they are therefore applied on the inner side of glass panes. Their low resistance makes them better off when used in a double glazing system. Cost-wise, this glass is relatively expensive.
Traditional reflective glass has a mirror-like appearance and reflects and absorbs a major proportion of the sun’s direct short wave solar radiation. The degree of reflectivity is dependent on the type of coating and the orientation of the glass. The use of reflective glass is more popular in commercial glazing as it provides superior solar control performance to clear or tinted glass products, and thus improves the energy efficiency of the building.
Low Emissivity (Low E) coatings are traditionally clear and are designed to reflect long wave radiation. They are available in both pyrolytic and sputtered coatings and the performance varies. Some modern reflective glass products have Low E coatings to reflect long wave radiation as well as the sun’s short wave radiation.