Glass is most of a window, and it is the line of a quote where the biggest differences hide. Two quotes that both say “double glazing” can describe very different glass. This guide shows how to read what is written (4/16/4, Low-E, argon, Ug) and what to ask when something is missing.
The comparison between double and triple glazing is in double or triple glazing, and Uw as a whole in thermal insulation. Here you get what is inside a double-glazed unit and how to compare one.
How to read the specification: 4/16/4
The numbers are thicknesses in millimetres. In 4/16/4 there is a 4 mm pane, a 16 mm gap and another 4 mm pane. The gap (the cavity) is the sealed space between the panes, not a pane. It is normally read from outside to inside, but each manufacturer writes it in its own way: ask which pane is the outer one.
Quotes also add abbreviations. One manufacturer we checked uses “A” for argon (4/16A/4) and “BE” for low emissivity (a Portuguese abbreviation you may see). Others use other codes: ask. On a Portuguese quote you will also see câmara (gap), fator solar (solar factor), espaçador (spacer) and laminado (laminated).
| On the quote | Means | Ask |
|---|---|---|
| 4 (the first) | A 4 mm pane, normally the outer one | Whether the inner pane is a different thickness |
| 16 | A 16 mm gap | Whether it is air or argon |
| A, árgon | Argon gas instead of air | Whether it is written down |
| BE, Low-E | A transparent coating on one face | Which type and which name |
| 44.2 | Laminated: two 4 mm panes and a film | Which pane and what for |
| Ug | Heat through the centre of the glass | And what is the window’s Uw |
What the Low-E coating is
Low-E stands for “low emissivity”. It is a transparent coating on one face of the glass that reflects heat instead of letting it through. In winter it helps keep indoor heat inside. In summer it helps stop outdoor heat radiating in.
Silver-based coatings are fragile, so they sit on the gap side, protected. The faces are counted from outside in: face 1 is the outer one, and faces 2 and 3 face the gap. Which face gets the coating is decided by the glass manufacturer. What matters to you is which type of coating is on the quote.
Keeping the cold out or the sun out
There are two families. An insulating coating stops indoor heat escaping and lets most of the sun through. A solar-control coating also insulates, but lets less sun in.
The number that measures this is the solar factor, or g-value: the share of the sun’s energy that gets inside, from 0 to 1 (or 0 to 100 %). The lower it is, the less heat comes in. In one manufacturer’s example, one glass lets through about 43 % of the sun’s energy and another about 29 %. According to Wikipedia, below 0.5 is already called solar control.
A low g has a cost: in winter the sun that comes in is free heat, and glass that blocks it gives it up. So it depends on orientation. South and west faces get more sun, and a lower g, together with outside shutters, may be worth it. North faces get little sun and an insulating coating usually does the job. Ask for the glass’s commercial name and its g, not just “Low-E”.
Argon and the width of the gap
Argon conducts less heat than air, about two thirds as much, according to Wikipedia. It works as a complement to the Low-E coating. Catalogue values from one manufacturer, for double glazing:
| Build-up | Approximate Ug (W/m²K) |
|---|---|
| No coating, air gap | 2.6 to 2.7 |
| Low-E, air gap | about 1.4 |
| Low-E, argon gap | down to 1.1 |
Your glass may give another figure, so ask for the data sheet of the glass you will receive. Argon is also not a proven noise barrier.
A wider gap is not always better. The manufacturers we checked recommend around 16 mm. Above about 18 mm the air starts to circulate inside the gap and insulation gets worse again. Gaps of 6 or 8 mm, common in older glass, are below the recommended range: a 4/6/4 insulates less than a 4/16/4.
The spacer: the strip between the panes
The spacer keeps the two panes apart and closes the gap, with a drying agent inside. An aluminium spacer conducts heat along the edge, which means heat loss and condensation in the corners. A warm-edge spacer, made of plastic or another material that conducts less, reduces this. The specification does not say which one is used, so ask.
Ug is not Uw
Ug measures the glass, at its centre. Uw measures the whole window: glass, frame and the edge where the spacer sits. If the quote only gives Ug, it shows a more favourable number than the window you will actually get.
One window manufacturer gives this example: with the same frame, a glass with Ug 1.1 gives Uw 1.3 with an aluminium spacer and 1.2 with a warm-edge PVC spacer. The window usually comes out worse than the glass, and the spacer changes the gap.
Ask in writing for Ug, Uf (the frame) and Uw, for the window you will receive. The CLASSE+ label also rates the complete window, not only the glass.
Laminated glass and safety glass
Laminated glass is two or more panes bonded with a plastic film (PVB). If it breaks, the pieces stay stuck to the film. The numbering follows the panes: 44 is two 4 mm panes. The “.1” or “.2” refers to the thickness of the film and the safety level.
It is used for safety, in glass doors and low windows, or for noise, with an acoustic film (see sound insulation). We could not confirm a general Portuguese rule on where safety glass is compulsory. Ask the installer whether, in that window, the glass is safety glass and why.
Water between the panes
Drops or mist between the panes mean the unit’s seal has failed: the drying agent is saturated and moisture got in. It cannot be wiped away from inside. The usual fix is replacing the glass unit, not the window. See window condensation.
Before you buy, ask in writing for the glass warranty: how long it lasts, from which date it counts and whether it covers only the glass or the labour too. Keep the invoice. See warranties and certificates.
Comparing the glass in two quotes
Put the quotes side by side and fill in, line by line: the build-up and which pane is outer; the gas (air or argon); the Low-E coating, with type and name; the g and the Ug; the spacer; the laminated glass, which pane and why; the Uw of the complete window; the glass warranty.
Blank lines do not mean the glass is worse, they mean you cannot compare. Ask them to complete it; the quote checker shows what is missing.
The price of the glass is driven by thickness, a solar-control coating, laminated glass, the number of panes and the size. For figures see PVC window prices.
Sources
- Guardian Sun: Como ler um orçamento de janelas que inclua os detalhes dos vidros: 4/16/4 notation, abbreviations, laminated glass and what to ask (in Portuguese).
- Novoperfil: Como pode o vidro ajudar na qualidade da eficiência da janela?: 16 mm gap, argon, Low-E, solar factor of 43 % and 29 % (in Portuguese).
- Guardian Sun: Vidro baixo-emissivo: Ug values with air and with argon (in Portuguese).
- Neuffer: U-Values for Windows: definitions of Ug, Uf and Uw and the two-spacer example.
- Wikipedia: Insulated glazing, Solar heat gain coefficient and Low emissivity: argon, gap, spacer, seal failure, g-value, Low-E coating.
What to do next
- Measure your windows: how to measure windows.
- Read the rest of a quote: how to read a window quote.
- Get an idea of the cost with the calculator.
- Already have the Ug, Uf and ψ? Combine them in the Uw calculator to see the Uw of the whole window.
- Want to know the Portaria’s limit on sun? See the window solar factor.