Most articles about glass talk about performance. Very few talk about what the glass actually looks like once it is sitting in your window.
We think that is the wrong way around. People notice appearance first and performance second. Two of our clients have asked us about visual details after their installation was finished, and in both cases what they were looking at was completely normal.
So this page is the version we would want to read before spending the money. It covers what you will see, why it is there, and the handful of things that would be worth a call.
The dots across the glass
Vacuum glass works by removing the air between two panes. Air carries heat, so taking it away is what gives the product its insulating value.
Remove the air and atmospheric pressure pushes the two panes together hard. To stop that, the glass is built with an array of tiny support pillars spaced evenly across the pane. They hold the two sheets apart and keep the gap intact.
You will see them. They look like a regular grid of very small dots, and they are more obvious in direct sunlight or when you are looking at the glass at an angle. Most people stop noticing them within a week or two, in the same way you stop noticing the frame of your glasses.
We show samples and videos of this during the quoting process on purpose. If you have not seen a pane in person and you would like to, ask us.
The line near the edge of the pane
Somewhere around 10mm in from the edge of the glass there is a component called the sorption getter. Its job is to absorb the small amount of gas that remains after manufacture, along with the gas that slowly releases from the glass itself over the years. Without it the vacuum would weaken and the insulating value would fall away.
It can look like a fine line, a faint band, or a strip of small regular marks, depending on the pane. It sits sealed inside the unit, between the two sheets, so it is not on the surface and it cannot be cleaned or rubbed off.
Whether you can see it after installation comes down to a few millimetres. The rebate in an original Australian timber sash window is usually only about 7 to 8mm deep, and the getter sits at roughly 10mm. The position, length and width of the getter are set during manufacture and vary from pane to pane and from batch to batch, so it is not in the same place on every unit.
The practical result is that one pane in a window can show a section of it while the pane beside it shows nothing at all. It also stands out more against freshly painted light timber, because the contrast is higher than it was before.
We cannot move it. It is inside a sealed unit. The only thing we can adjust is where the glass sits in the rebate, and there we have a couple of millimetres of play at most. Pushing a pane across to hide the getter on one side simply reveals it on the other, and glass cut large enough to bury it completely would not fit the opening.
We have written a full article on this, with photographs: What is the sorption getter in vacuum glass?
The seal around the perimeter
At the very outside edge of the pane, the two sheets of glass are fused directly to each other with a glass seal. That is what holds the vacuum in.
It is a thin dark border, usually hidden by the timber once the glass is installed. On shallow rebates a little of it can stay in view, for the same reason the getter can. How much depends on your frames, because when we retrofit we have to work within the character of the windows you already have.
The seal matters for more than appearance, and it is the reason for the next point.
Condensation behaves differently at the edges
The seal is solid material, and solid material conducts heat. So the outer few millimetres of a vacuum unit cannot hold the same insulating value as the middle of the pane.
That is normal for every insulated glass product, not just vacuum glass. It is also why performance figures are quoted at centre of glass.
In practice it means that if condensation ever forms on the inside of your windows on a very cold morning, it will appear in the corners and along the bottom edge first, because that is the coldest part of the assembly. It is not a sign that the glass has failed.
That one has its own article, because it is the question we are asked most often after a cold snap: Why condensation appears in the corners of new windows
Your windows are not new, and that is the point
Retrofitting vacuum glass into original timber sashes is not the same as buying a new window. We are working inside the frame you already have.
That means the finished result follows the character of your house. Old sashes are rarely perfectly square. Rebates vary in depth from window to window and sometimes along a single sash. Putty lines, sightlines and the amount of glass you can see will differ slightly between openings, because they differed before we arrived.
We work as sympathetically and as accurately as the original construction allows. What we will not do is cut into heritage joinery to force a uniform result.
A construction diagram showing how the glass sits within a timber sash is here: Double glazing for timber sash windows
What a bad retrofit looks like
Everything above describes a job done properly. It is worth knowing what the alternative looks like, because once you have seen it you cannot unsee it, and it is permanent.
Original timber sashes were designed around slim single glazing. A conventional insulated glass unit is thick and heavy by comparison. When someone tries to force one into a sash that was never built for it, there are only two ways out, and both of them show.
The first is that the unit sits proud of the timber. The rebate is not deep enough to take the thickness, so the glass stands out past the face of the sash. The putty line gets fatter, the sightline changes, and the window stops looking like the window it used to be.
The second is beads fixed over the outside, and that is the one to watch for. Rather than rebate the sash correctly, the installer applies timber beads over the face of the window to hold the oversized unit in place. It is faster and it needs less skill, and it permanently changes the external appearance of the house. On a heritage facade it is very hard to undo.
Neither of these is a glass problem. They are craftsmanship problems, and they come from putting the wrong product into the wrong window. Vacuum glass avoids them because it is only 3 to 4mm thicker than existing glass, close to the single glazing the sash was built for, so it goes back into the original rebate and gets glazed the traditional way.
The other consequences follow from the same root. Deeper routing takes out old growth timber. Heavier sashes need re-weighting and can stress old joinery, and some end up fixed shut because they will no longer balance.
There is more detail on all of that here: Retrofit double glazing for timber sash windows: can you keep the original frames?
What is not normal
Everything above is expected. These are not, and if you see any of them we want to hear about it:
- A chip, crack or scratch in the glass.
- Moisture or fogging that appears inside the unit, between the two panes, rather than on the room-facing surface.
- A visible change in the pane over time, such as new distortion or the appearance of the two sheets touching.
- A sash that no longer operates smoothly, or seals that lift or come away.
- Any part of the installation that looks different from the window next to it in a way this page does not explain.
The short version
You will see small dots across the pane, and you will stop noticing them. You may see a fine line or band near the edge of some panes and not others. On the coldest mornings you may see a little condensation in the corners. None of those are faults. They are what a high performance sealed unit looks like when it is fitted into a window that was built a century before the glass existed.
A bad retrofit is the opposite of all of that. It is visible from the street, it is permanent, and it has nothing to do with the glass itself.
If anything on your windows is not covered here, ask us. We would rather explain it now than have you wondering about it later.
