Low-emissivity (Low-E) glass has a thin coating, often of metal, on the glass within its airspace that reflects thermal radiation or inhibits its emission reducing heat transfer through the glass. A basic low-e coating allows solar radiation to pass through into a room. Thus, the coating helps to reduce heat loss but allows the room to be warmed by direct sunshine. The low-e coating is usually on surface #3; if solar control is required then the coated surface is moved to surface #2 to reflect or absorb solar radiation. The change in location of the coating does not affect the insulating properties of the IGU, only the percentage of solar heat gain. Further solar radiation control can be added through the use of tinted glass and/or metallic coatings[7]. Low-e glass reflects the radiation rather than absorbing it improving performance compared to the glass in a simple greenhouse. Its effect can be noticed by an increase in temperature of the inside glass surface and the reduction of condensation that would normally form on the unit because of a change in the dew point.
There are two types of low-e coatings available, "hard-coat" and "soft-coat". Hard-coat glass is manufactured by applying molten tin to the glass surface as the glass sheets are being manufactured. The tin bonds to the surface of the glass and forms a relatively thick coating. Hard-coat glass is considered a medium performance coating since the emissivity is greater compared to the soft-coat product. The advantage of hard-coat glass is that it does not require special handling in the IGU assembly process to maintain the surface's coating integrity and does not scratch easily. It does require that the glass surface in contact with the spacer be abraded to improve adhesion of the sealant. Soft-coat glass uses vacuum deposition to apply a thin metallic coating to the glass surface as an additional manufacturing step. The coating is fragile compared to hard coat glass, requiring special handling and storage for both the manufacturing process and IGU fabrication. Choosing a soft-coat glass over a hard-coat glass improves thermal performance of the IGU by about 13%.[8] Most low-emissivity glass sold for IGU manufacturing is of the hard-coat type.[citation needed]
