first post
this is my first post to the forum, and it is long - sorry...
There are two primary types of window coatings available - pyrolitic or hard coat - and soft or sputter coat.
Softcoats are multi-layered coatings consisting of several different metallic oxides with silver as the primary heat-blocking ingredient. Softcoats are applied in vacuum chambers to finished glass.
Hardcoats or pyrolitic coatings are applied to the glass while the glass is still semi-molten in the tin bath portion of the float process. Basically, the bottom side of the glass will have a coating of tin from floating in the bath (as does all float glass), but the topside will also have the layer of (primarily) tin oxide - which is the LowE coating.
So in one sense, glass with a pyrolitic coating has two "tin-sides", but one - the LowE side - of them is much thicker than the other, although only one coat affects energy performance.
Often, the pyrolitic coating is applied to the #3 surface of an IG unit if the unit is intended for use in a heating-dominated climate - but not always since there are also solar-reflective pyrolitic LowE coatings that are applied to the #1 or #2 surface of an IG unit to reflect solar heat gain in cooling dominated climates.
Sputter LowE coatings are also applied to the air-side of the glass, primarily because sputter coats adhere better to the air-side, but also it is possible for the slight metallic (tin) layer on the tin-side to affect the performance of the coating.
A sputter coat is applied in multiple layers, it may be about 7 to 11 layers, depending on the coating. People really don't realize, or can appreciate, how thin a typical sputter LowE coating is. Sputter coat folks measure the thickness of each metal oxide layer, as well as the finished coating, in angstroms, or by how many atoms thick the coating is. A typical softcoat LowE coating is generally less than 1000 atoms thick.
Sputter coats are generally applied to surface #2 of an IG unit, although they can also be applied to surface #3. Consider a typical IG unit - two lites of glass separated by a spacer.
The exterior of the exterior (as you said) is surface #1. The interior of that lite is surface #2. The other lite then has surface #3 and #4 - with #4 being the surface that is actually exposed inside the home.
In a heating dominated climate, there are two reasons for placing the LowE coating on the #3 surface of the IGU. First is to allow for solar heat gain in the winter and second to block the transference of the heat inside your home to the outside.
Which brings us to High Solar Heat Gain (or HSHG) coatings and Low Solar Heat Gain (or LSHG) coatings…
What does that mean? Well, all LowE coatings are designed to block far - or longwave - infrared energy. This is the range that includes typical household-produced heat. This is also the frequency range of heat that is produced when the sun warms an object – the heat you feel when you touch the sidewalk on a hot, sunny summer day. While direct solar energy is shortwave IR, the heat released by a sun-warmed object is longwave IR…and hopefully that made sense.
A typical hardcoat or single-silver layer softcoat works in this application since both types of coatings block the far infrared energy - thus keeping winter heat indoors - but neither is designed to be effective at blocking shortwave infrared - thus "allowing" solar heat access to the home - winter or summer.
Placing a high solar gain coating on surface #3 maximizes the level of solar heat gain thru the IG unit which can be an advantage in winter and can also be a disadvantage in summer.
A Low Solar Heat Gain product, on the other hand, is designed to block both near and far infrared energy. It will keep heat - including direct solar gain – from passing thru the window in both summer and winter.
These coatings are placed on surface #2 to maximize effectiveness against direct solar gain by blocking solar heat before it can pass into the airspace in the IG unit – and into the home.