What is the difference between soft coat and hard coat glass?

Both are low-E coatings, but they’re made differently and perform differently:

• Hard coat (pyrolytic) is applied to the glass at high temperature during manufacture, so the coating is fused into the surface and is very durable. It can be handled, toughened and even used as a single (monolithic) pane exposed to the air. Its thermal performance is good, but not as strong as soft coat.
• Soft coat (sputtered / off-line) is applied after manufacture in a vacuum chamber. It gives noticeably better thermal performance and a more neutral appearance, but the coating is more delicate, so it must sit inside the sealed unit where it’s protected.

In practice, soft-coat low-E is the standard choice for the insulating performance modern sealed units need, and it’s what SevenDay supplies as standard low-E, positioned on surface 3 inside the unit (see below). Hard coat is chosen where the coated surface needs to be exposed or handled — the classic example being self-cleaning glass.