Double or triple glazing — when is it worth paying more
A double unit — two panes of glass with one cavity — was the standard for years. Today, in most cases, it no longer meets the Polish building regulations, which require a whole-window Uw of no more than 0.9 W/m²K.
A triple unit — three panes and two cavities — is the practical market standard precisely because it makes that requirement achievable. The extra cost of the third pane is therefore not a choice between good and better, but a condition for the window to be installed legally in a new building at all.
When replacing windows in older houses, where those rules do not apply directly, the decision is more flexible. But even there it is worth considering triple glazing to optimise bills for decades ahead.
Two numbers on the quote nobody asks about
Everyone asks about Uw. Almost nobody asks about LT and g — yet these decide whether a living room with a big south-facing window is a pleasant place in summer.
LT is light transmittance: what percentage of visible light passes through the unit. A normal triple unit has about 70–75 percent. Below that it gets visibly darker, especially on the north side.
g is the solar factor: how much solar energy gets inside. A standard triple unit has about 0.5 (also written as 53 percent). In winter that is free heat. In summer the same window turns the room into a greenhouse.
And here come the regulations: in summer the g of the window, calculated together with shading, must not exceed 0.35. The exceptions are north-facing windows and windows with an external shading element. So either solar-control glass, or an external roller shutter or blind. An internal curtain does far less.
When comparing quotes, check whether LT and g refer to the glass alone or to the whole window. The window value is always lower, because the frame lets through neither light nor sun.
Stopsol — the glass that reflects
Stopsol is AGC glass with a pyrolytic reflective coating: a layer of metal oxides fired onto the hot pane on the float line, fused into the glass. That is why it can be toughened, laminated and bent. Soft coatings do not allow that freedom.
The effect on the façade is a mirror. By day you cannot see inside, because it is brighter outdoors. At night, with the lights on, it works the other way and the room is visible from the garden.
AGC's figures for a 6-16-4 double unit with the coating in position 2 and iplus low-e glass: Stopsol Classic Grey has LT 16 percent, g 17 percent, light reflection 10 percent. Classic Bronze: LT 19, g 19. For comparison, the light Supersilver Clear variant has LT 54 and g 42 percent.
So Classic Grey lets in roughly one sixth of the light ordinary glass does. That is much darker than people imagine from photos. We offer Classic Grey and Classic Bronze; values for a specific triple unit are given in the quote, because AGC's catalogue figures refer to a different configuration.
Antisol is not a coating
Quotes often say "Antisol coating". That is wrong. Antisol is body-tinted glass: metal oxides are added to the molten glass at the glassworks. The whole thickness of the pane is coloured; there is no layer on the surface.
That changes how it works. Stopsol reflects radiation, Antisol absorbs it. The pane heats up and gives part of the heat back inside, part outside. Reflection is low, so there is no mirror effect from outside — just a dark pane.
Absorption has one more consequence: the heated centre of the pane behaves differently from its shaded edge in the frame. With large formats and partial shading, the glass manufacturer sometimes requires toughening precisely because of thermal stress.
The colour depends on thickness: the same glass in a 4 mm pane is lighter than in 6 mm. That is why the catalogue light transmittance of tinted glass is a wide range, from about 32 to 72 percent. The name Antisol itself is a trade name — the same glass is called Parsol, Optifloat Tint or Planibel Coloured by other makers. We offer brown and graphite variants.
Cool-Lite — solar control without losing the light
Stopsol and Antisol share one problem: to get g down to 0.2 you have to give up most of the light. Selective coatings such as Saint-Gobain's Cool-Lite do it differently. They let visible light through and reflect infrared. The measure is selectivity: LT divided by g. Ordinary glass is around 1.4, Cool-Lite SKN reaches 2, Xtreme exceeds 2.3.
Saint-Gobain's catalogue figures for a double unit: SKN 176 II — LT 68 percent, g 0.34; SKN 154 II — LT 50 percent, g 0.26. For comparison, Stopsol Classic Grey at g 0.17 gives LT 16 percent. So SKN 154 lets in three times more light at a g higher by less than a tenth.
Xtreme is the higher tier: triple silver instead of double. Xtreme 70/33 II — LT 70 percent, g 0.30, selectivity 2.33. It lets in as much light as ordinary triple glazing and half as much sun. The name tells you what to expect: the first number is LT, the second is g in percent.
Turning this into an order
There is no single number for "solar-control glass". There is a number for this specific glass, in this unit, in this window. That is why in the quote we state LT and g for exactly the configuration you choose in the configurator — and tell you when ordinary glass with external shading will work out better.