Low-E coatings compared: soft coat vs hard coat
A low-E coating controls the heat that would otherwise radiate across the cavity, and there are two ways to make it: the soft coat, deposited in a vacuum and the higher performer, and the hard coat, fired onto the glass as it is made and the more durable. The soft coat has to live inside the sealed cavity; the hard coat can face the weather. The schedule picks the coat by where it goes and what it has to do, not by a generic low-E line.
What low-E does
Glass radiates heat across the cavity: the warm pane radiates to the cool pane, and that is a real part of the heat path. A low-E coating is a thin metallic layer that reflects that long-wave radiation back, which is what lowers the U-factor. The coating is almost invisible, and the difference between the two types is how it is made and where it can be placed.
The soft coat
The soft coat is sputtered onto the glass in a vacuum, layer by layer, which lets it be tuned for a high performance — a low U-factor, and it can be combined with solar control in the same multilayer stack. It is mechanically delicate, which is why it is called soft, so it is always placed on a cavity face inside a sealed IGU, never on an exposed surface. On the standard sealed unit it is the coat that delivers the best numbers.
The hard coat
The hard coat is applied during the float process, so it is fired in and hard: it can be handled, cut and even left exposed. It is slightly lower in performance than the soft coat, but it survives the applications the soft coat cannot — single glazing, exposed faces and the lean build. Where the unit is not a sealed IGU, or where the coating has to face the weather, the hard coat is the specification.
Which coat, where
- Soft coat for the sealed double and triple IGU — the standard insulated unit, where the coat sits on a cavity face.
- Soft coat combined with solar control where the U-factor and the solar heat gain both have to be low in one stack.
- Hard coat for single glazing, heritage and lean builds where the coat cannot be sealed in a cavity.
- Hard coat where a coating has to face the weather, on a single-glazed or exposed-laminated build.
Questions
Not by eye; both are essentially invisible. The data sheet names the coating and the face it sits on, which is the line the schedule reads.