What the numbers on glass mean

Ug: how much heat escapes

How much heat passes through one square metre of glass for every degree of difference between inside and outside. Example: with a Ug of 1.1, at 20 °C inside and 0 °C outside, 20 × 1.1 = 22 W passes through every square metre. The lower the number, the less heat escapes.

Ug depends on three things: the coating, the spacer width (the gap between the panes) and the gas fill. It is calculated to EN 673.

What does a difference of 0.3 mean? 0.3 W less per square metre for every degree of difference. On a house with 10 m² of glass, at 20 °C inside and 0 °C outside, that is 0.3 × 10 × 20 = 60 W, like an old light bulb burning day and night: 1.4 kWh a day.

The most common misconception: triple glazing is always better. It is not. When the gap is too narrow, heat passes more quickly through the thin layer of gas, so triple glazing with spacers under 8 mm insulates worse than double glazing with a Sun coating.

Calculated to EN 673: 4 mm panes, 90% argon. Double glazing with a Sun coating, triple glazing with two low-E coatings.

g: how much solar heat gets in

The share of solar energy that ends up inside the room: what passes through the glass, plus what the glass absorbs and then gives off to the inside. Determined to EN 410.

In winter, a high g is a gain: free heating. In summer, it is a cost, because the air conditioning has to remove that heat. Large areas of glazing facing south, east and west therefore call for a solar control coating.

For comparison:
IG 4+16+Low-E 4 [24 mm] 64%
IG Low-E 4+16+4+16+Low-E 4 [44 mm] 53%
IG Sun4+16+4 [24 mm] 35%

Values to EN 410. Triple glazing with low-E lets in less heat than double glazing, but also less light. A Sun coating nearly halves the heat and takes only a little of the light. Dark glass stops the heat, but stops even more of the light.

Ψ: warm edge

The heat that escapes along the edge of the glass, through the spacer. Ψ is not a property of the glass alone; it applies to glass installed in a particular frame, so the frame is always stated alongside the figure.

For the whole window, Uw is calculated, and it takes in the Ug of the glass, the Uf of the frame and the Ψ of the edge. On smaller windows, the edge accounts for a large share of the heat loss.

A warm edge also means a warmer glass edge on the room side. At −10 °C outside and 20 °C inside, the edge of a double-glazed unit in a PVC window reaches 8.6 °C with an aluminium spacer and 11.3 °C with TGI. At 50% room humidity, condensation forms below 9.3 °C: with aluminium the edge mists up, with TGI it does not.

We work with both aluminium and TGI warm edge spacers.

For comparison:
Double glazing, PVC window: aluminium 0.068, TGI 0.040
Triple glazing, PVC window: aluminium 0.069, TGI 0.038

Argon

The gas in the cavity between the panes. It conducts heat less well than air, so with the same coating it gives a lower Ug. The fill is usually around 90%.

It works best in a cavity of 15 to 18 mm. It does nothing for sound; for sound, what helps is thicker glass, different pane thicknesses and acoustic interlayer.

Over time, argon slowly escapes through the edge seal. EN 1279-3 allows a loss of up to 1% a year. The effect on Ug is very small: when the fill drops from 90 to 80%, the Ug of IG 4+16+Low-E 4 [24 mm] rises from 1.07 to about 1.10. That is why the edge seal matters so much in an insulating glass unit.

For comparison:
IG 4+16+Low-E 4 [24 mm]: with air Ug 1.4, with argon 1.1

LT: how much light gets through

The share of daylight that passes through the glass. Every pane and every coating takes a little of the light.

For comparison:
IG 4+16+Low-E 4 [24 mm] 82%
IG Low-E 4+16+4+16+Low-E 4 [44 mm] 74%
IG Sun4+16+4 [24 mm] 70.5%
IG Stopsol Bronza 4+16+Low-E 4 [24 mm] 25%
IG Parsol Dark Grey 6+16+Low-E 4 [26 mm] 8%

Ra: do colours look natural?

How much the colours of objects change when you look at them through the glass. 100 means no difference; above 90 is very good, above 80 is good.

For comparison:
IG 4+16+Low-E 4 [24 mm]: 98
IG Sun4+16+4 [24 mm]: 95
IG Stopsol Bronza 4+16+Low-E 4 [24 mm]: 87
IG Parsol Dark Grey 6+16+Low-E 4 [26 mm]: 85

Selectivity

How much light you get per unit of heat. A selectivity of 2 means twice as much light as heat.

For a long time, 2 was thought to be the upper limit. Triple-silver coatings now exceed it.

Below 1 means the glass stops more light than heat. Dark glass is like that.

Selectivity is LT divided by g, using the same values as the chart in the g card.

Rv: how much the glass reflects

The share of light reflected by the glass. External Rv tells you how the building looks by day; internal Rv how much you see yourself in the glass in the evening.

Solar control coatings do not have to be reflective. Some have the same reflectance as ordinary glass; others are reflective, for the look of the façade.

Watch the internal reflectance: some glass reflects less on the outside but more than 25% on the inside. In the evening, with the lights on in the room, you see yourself in it as in a mirror.

For comparison:
IG 4+16+Low-E 4 [24 mm]: outside 12.4%, inside 12.6%
IG Sun4+16+4 [24 mm]: outside 13.6%, inside 15.5%
IG Stopsol Bronza 4+16+Low-E 4 [24 mm]: outside 15%, inside 32%
IG Parsol Dark Grey 6+16+Low-E 4 [26 mm]: outside 4%, inside 9%

UV transmittance

How much ultraviolet radiation gets through. UV fades fabrics, pictures, wooden floors and furniture.

Ordinary double glazing stops more than half of the UV. Laminated glass stops almost all of it: even with two interlayers, as in 3.3.2, only about 0.5% gets through.

Where solar energy goes

Solar energy falling on the glass splits three ways: part passes through (τe), part is reflected (ρe) and part is absorbed by the glass (αe). The total is always 100%.

g is not the same as τe. g also includes the share of absorbed heat that the glass gives off into the room.

Example, IG Sun4+16+4 [24 mm]: 33.4% passes through, 45.3% is reflected, 21.4% is absorbed by the glass. g is 34.9%, slightly more than 33.4%, because the glass gives off part of the absorbed heat to the inside.

Which surface the coating is on

Glass surfaces are numbered from outside to inside: a double-glazed unit has surfaces 1–4, a triple-glazed unit 1–6. The coating always faces the cavity, where it is protected.

A solar control coating goes on surface 2, on the outer pane, to reflect solar heat before it gets in. Low-E goes on surface 3 in double glazing, and on surfaces 2 and 5 in triple glazing. When solar control and low-E are combined in triple glazing, the solar control coating is on 2 and the low-E on 5.

Absorptance and thermal stress breakage

How much solar energy the glass retains, heating up as it does so. Darker and solar control glass absorbs more.

When part of the pane is in sun and part in shade (a soffit, column, tree or the building opposite), the glass heats unevenly and can crack. Ordinary glass can take a difference of about 40 °C across the same pane. The crack starts at the edge, at right angles. Toughened glass copes with such a difference without any problem.

So: keep radiators at least 30 cm from the glass, and on site do not leave stacks of coated units in the sun; cover them with an opaque sheet.

For comparison:
IG 4+16+Low-E 4 [24 mm]: 14%
IG Sun4+16+4 [24 mm]: 21.4%
IG Stopsol Bronza 4+16+Low-E 4 [24 mm]: 59%
IG Parsol Dark Grey 6+16+Low-E 4 [26 mm]: 90%

Thermal breakage: the part of the pane in the sun heats up and expands, while the shaded part and the edge in the frame stay cold. Once the difference exceeds about 40 °C, ordinary glass cracks from the edge. The crack starts at right angles to the edge, runs straight for 2–5 cm, and then branches.

Rw: how much the glass reduces sound

A number that tells you how much the glass cuts down sound. The higher it is, the quieter it is inside. It is determined by laboratory measurement to EN ISO 717-1. The figures in our quotes and on our website are calculated from such measurements, not measured on your glass.

Decibels do not add up like ordinary numbers: two 80 dB sources give 83 dB, not 160. The ear hears a 10 dB difference as half as loud.

Example: next to a road it is 69 dB outside. IG 4+16+Low-E 4 [24 mm] (Rw 30) leaves about 39 dB inside.

Triple glazing on its own helps only a little, from 30 to 32 dB. Laminated glass and different pane thicknesses help more, and an acoustic interlayer (SR) adds a few more decibels.

For comparison:
IG 4+16+Low-E 4 [24 mm]: 30 dB
IG Low-E 4+16+4+16+Low-E 4 [44 mm]: 32 dB
IG 6+16+LamiLowE 3.3.2 [28.76 mm]: 37 dB
IG LamiLowE 3.3.2+16TGI+4+16TGI+LamiLowe 4.4.2 SR [51.52 mm]: 47 dB
IG Lami Sun 4.4.2+16TGI+4+16TGI+LamiLowe 4.4.2 SR [53.52 mm]: 48 dB

C and Ctr: which kind of noise

Rw comes with two adaptation terms in brackets, for example for IG Sun4+16+4 [24 mm]: Rw (C; Ctr) = 30 (−1; −4).

C applies to speech, music, children and fast-moving traffic. Ctr applies to urban traffic, lorries and slow trains, in other words low frequencies, where glass is at its weakest.

Next to a busy road, look at Rw + Ctr: in this example, 30 − 4 = 26 dB.

EN 12600: body impact

A pendulum test that simulates a person running into the glass. The pendulum is raised to one of three heights and released onto the glass, and the classification tells you what the glass withstood and how it broke.

The classification works like a ranking in a competition: 1 is first place. 1(B)1 is better than 2(B)2.

Example: 4.4.2 and 4.4.6. Both are two 4 mm panes. 4.4.2 has two interlayers (0.76 mm), 4.4.6 has six (2.28 mm). Under EN 12600, both are 1(B)1: they withstand the impact from the greatest height, and when they break the pieces stay on the interlayer with no opening. Against body impact, they protect equally.

The difference only shows up for break-ins, under EN 356: 4.4.2 is P2A and 4.4.6 is P5A.

Heights and classifications to EN 12600 (GlassTime 7.4.4.1). Laminated glass with a single interlayer, such as 4.4.1, only achieves 2(B)2. We make laminated glass with at least two interlayers.

EN 356: resistance to break-ins

EN 356 tests the resistance of glass to break-ins, that is, to manual attack. It has two separate tests, and the glass is classified on one of them.

Test A, ball drop (P1A–P5A): simulates a stone or heavy object thrown at the glass. A 4.11 kg steel ball is dropped on the same spot: P1A from 1.5 m three times, P2A 3 m, P3A 6 m, P4A 9 m three times, P5A 9 m nine times.

Test B, axe (P6B–P8B): simulates a burglar with tools. A machine strikes with a 2 kg axe until a 40 × 40 cm opening is made: P6B 30 blows, P7B 51, P8B 70. This is the territory of thick laminates with many interlayers.

The class is determined by the number of interlayers, not by the glass thickness. Here, a higher number is better, the opposite of EN 12600.

Example: 4.4.4. The customer is given both classifications: EN 12600 1(B)1 and EN 356 P4A. The first tells you how the glass protects against body impact, the second how well it resists a break-in.

For comparison:
Laminated 4.4.2: 1(B)1, P2A
Laminated 4.4.4: 1(B)1, P4A
Laminated 4.4.6: 1(B)1, P5A

Testing to EN 356 (GlassTime 7.4.4.2). The classes apply to factory-made laminated glass with a class tested by the manufacturer.

Condensation on the outside of the glass

On spring and autumn mornings, triple glazing can mist up on the outside. This is not a fault but proof that the glass insulates well: heat from the house does not warm the outer pane, so it cools below the dew point.

Condensation between the panes is a different matter. It means the seal has failed and the unit needs replacing.

Pillowing or dished units

The gas in an insulating glass unit is sealed in at the temperature and pressure of the factory. When it is warmer or colder outside, or the air pressure changes, the gas expands or contracts and the panes bow slightly outwards or inwards. Reflections in the glass then look distorted.

This is not a fault but a sign that the edge is well sealed. It is most noticeable on large units, triple glazing and wide cavities.