Yet More Basics....

The third big factor in DGU thermal performance is the coating (if any) on the glass. Put simply, thin coatings act like a see-through mirror, letting visible light through but reflecting selected long-wave radiation (heat) back into the room (or out again if you live in Dubai and want to keep the heat OUT).

There are two main classes of coating: Hard-coat and soft coat. Hard coat films are 'baked on' during the process. The hard-coat film is less efficient in keeping heat in than soft-coat, and also suffers from a reputation of leaving the glass with a dirty appearance in certain lighting conditions. It has a major advantage for small manufacturers in that it requires no special handling techniques in the construction of sealed units. So it's often pushed, not because its the best, but because its cheap and easy to use. Buyer beware! (It IS more effective at GATHERING heat from outside - the so-called solar gain. In the UK winter this is hardly likely to be a reason to use it, and in the summer it's likely to be a real pain.)

Soft coat films are becoming very sophisticated. A still-developing technology, they are capable of being 'tuned' to maximise their performance. Early issues with tinting and colour changes upon toughening are a thing of the past, and generally I consider them far superior to the hard coat films. As much as anything because I hate arguments, and will not take the risk of supplying my customers with what appears to be 'dirty glass' even if it isn't.

Soft coat films require special handing during the manufacture of DGUs, so they tend to be more expensive and are rarely available from the back-street fabricator. However, used in conjunction with warm-edge spacer separating the two sheets of glass, it is difficult to conceive a more efficient and cost-effective way of providing insulating glass.

Next I'll discuss the fine tuning with gas-filling: Another hornet's nest.