Two words come up at nearly every site visit, and they decide more about what we can fit than anything else in the house. If your home was built or had its windows replaced any time since roughly the mid nineties, this applies to you.
What it is
Low-E stands for low emissivity. It is a microscopically thin metallic coating applied to one of the inner glass surfaces inside a sealed double glazed unit.
Its job is to handle radiant heat. In winter it reflects heat from inside the house back into the room instead of letting it escape through the window. In summer it does some of the reverse. It is one of the genuinely good developments in residential glazing and it is the reason a modern window outperforms an old one even when both are double glazed.
It is also nearly invisible, which is the whole design goal and the reason so many owners have no idea it is there.
How to tell whether you have it
There is a test you can do tonight and it takes about ten seconds.
Wait until dark, turn the room lights down, and hold a flame or a phone torch a few inches from the glass at a slight angle. You will see a row of reflections, one for each glass surface. Ordinary double glazing gives you reflections that are all the same colour.
If one of those reflections is a noticeably different shade from the rest, usually a little warmer or more copper, you are looking at the Low-E coating.
It is a decent field test and it is what a lot of people in this trade use. It is not a substitute for the assessment we do before quoting, because it tells you the coating exists without telling you which type it is or which surface it is on. Both of those matter.
Why it changes what can go on your windows
Here is the part that gets glossed over by anyone quoting you over the phone.
A sealed glass unit is engineered to absorb a certain amount of solar energy and shed it safely. Adding a covering to the surface changes that balance. Put an absorbing product on a pane that already has a Low-E coating doing similar work and you can push the unit past what it was built for.
Two things go wrong when that happens.
Thermal stress cracking. The centre of the pane gets hot while the edges, shaded by the frame, stay cooler. Glass expands unevenly, stress concentrates at an edge, and eventually the pane cracks. It typically starts at the perimeter and runs inward, and it can happen weeks or months after the work rather than on the day.
Early seal failure. The extra heat load ages the seal around the unit faster. The unit eventually loses its gas fill and admits moisture, and you get the permanent haze between the panes that means the window needs replacing. A unit that had fifteen good years left gives you five.
Neither is common when the product is matched properly. Both are entirely avoidable, and both are expensive in a way that dwarfs the cost of the original job.
What a proper assessment looks like
Before we quote anything, someone stands at your glass and establishes four things: whether there is a coating, which surface it sits on, whether the unit is annealed or tempered, and the condition of the existing seals.
That last one matters more than people expect. We are not going to bond a covering to a unit whose seal has already failed, because the window is on its way out regardless and the work would be money spent on a pane you will be replacing. If we find that, we tell you, and it costs you nothing because you have not paid for anything yet.
We then quote only product rated for the glass in front of us. If nothing suitable exists for a particular window, we say so and that window comes off the quote.
The edges, and why we leave them alone
One detail worth knowing, because it separates a careful install from a cheap one.
On interior work we apply the covering to the glass surface and leave the perimeter edges unsealed. The sealed unit stays free to breathe as it was designed to. Sealing the edges shut looks tidier for about a year and is a good way to interfere with how the unit manages moisture and pressure, which is exactly the mechanism most window warranties are written around.
Ask anyone quoting you whether they seal the edge perimeter on interior work. The answer tells you a lot.
The exception is genuine exterior applications, which do need a sealed edge to survive weather, and which are a different specification with a different price and a seasonal install window.
What to do with this
You do not need to become an expert. You need to ask one question of anyone who wants to work on your windows: how are you going to establish what glass I have before you quote?
If the answer involves seeing it, good. If you are given a price over the phone for a house nobody has visited, you are being quoted by someone who is guessing with a sealed unit that costs considerably more to replace than the job is worth.
