October 20, 2026 · 5 min read
Low-E glass: how low emissivity works and which one suits Monterrey
What a low-emissivity coating does, the difference between thermal and solar-control low-E, and how to read light transmission, solar factor and selectivity.
A low-E glass looks almost the same as a clear one. The difference is a coating of metal oxides and silver thinner than a thousandth of a millimetre, applied to one of its faces, which changes completely how the glass handles heat. Chosen well, it is the single most effective upgrade you can make to a window.
What emissivity is
Every surface radiates heat as infrared. Emissivity measures how much: it runs from 0, a surface that radiates nothing, to 1, one that radiates as much as possible.
Uncoated glass has an emissivity of 0.89, close to the maximum. That means once it warms up, it hands that heat on by radiation to whatever is colder. A low-emissivity coating cuts the figure drastically: hard, pyrolytic coatings sit around 0.15 to 0.2, and the Saint-Gobain soft coatings we work with are between 0.01 and 0.04.
What it does to light and heat
Sunlight carries three things: visible light, near infrared, which is direct solar heat, and some ultraviolet. A good solar-control low-E coating lets most of the visible light through and rejects much of the infrared.
Then there is the effect it is named for. The outer pane absorbs sunlight and warms up; if the face looking into the cavity emits very little, that heat no longer crosses to the inside.
What happens inside a double-glazed unit
- Visible light
- Solar heat (near infrared)
- Heat radiated by the outer pane
Two different families
This is where most of the confusion lies: "low-E" is used for two products that do almost opposite things.
Thermal low-E. Ranges such as Planitherm. They let in plenty of light and solar heat, but stop indoor heat from escaping. They are designed for heating climates, and the coating goes on face 3, the inner pane's cavity face.
Solar-control, or selective, low-E. Ranges such as Cool-Lite SKN and Cool-Lite Xtreme. They let in much of the light but hold back solar heat. The coating goes on face 2, the outer pane's cavity face, so the heat is rejected before it gets in.
In Monterrey, which needs cooling for most of the year, the second is almost always the right choice.
Faces are numbered from outside in. In double glazing, face 1 looks at the street, faces 2 and 3 look into the cavity, and face 4 is the one you see from the room.
How to read a data sheet
Three values tell you almost everything:
- LT, light transmission. What percentage of visible light gets through. More means a brighter room.
- g, solar factor. What fraction of all solar energy ends up inside, counting what passes straight through and what the glass absorbs and re-radiates. Less means less heat. US data sheets call it SHGC; it measures the same thing with a different calculation method, so the figures are close but not identical.
- Selectivity, LT/g. How much light you get per unit of solar heat. Above 2 counts as highly selective.
This is how Saint-Gobain's solar-control coatings compare, all in 6 + 16 + 4 mm double glazing:
| Coating | LT | g | Selectivity | External reflection |
|---|---|---|---|---|
| Cool-Lite SKN 176 | 70 % | 0.37 | 1.89 | 13 % |
| Cool-Lite SKN 165 | 61 % | 0.34 | 1.79 | 16 % |
| Cool-Lite SKN 154 | 50 % | 0.27 | 1.85 | 18 % |
| Cool-Lite Xtreme 70/33 | 70 % | 0.33 | 2.12 | 11 % |
| Cool-Lite Xtreme 60/28 | 60 % | 0.28 | 2.14 | 14 % |
| Cool-Lite Xtreme 50/22 II | 47 % | 0.21 | 2.24 | 16 % |
The useful comparison is along a matching g. SKN 154 and Xtreme 60/28 let in almost the same solar heat, 0.27 and 0.28, but the Xtreme lets through ten points more light. That is its triple silver layer at work. In the Xtreme range, the numbers in the name are roughly its LT and g.
External reflection matters on a façade: it decides how mirror-like the building looks from the street. Modern selective coatings keep it low.
What to watch for
- Soft coatings live inside double glazing. They cannot take weather or direct cleaning, so they are protected in the cavity. The coating is removed around the perimeter so the sealant bonds to the glass.
- Tempering. Some coatings cannot be tempered once applied. Saint-Gobain marks the versions that can with "II", such as SKN 154 II.
- Do not mix versions on one façade. The manufacturer itself warns that SKN 154 and SKN 154 II can look slightly different in transmission, and recommends checking a sample if both will be used together.
And the U-value
The coating changes insulation too. In our EN 673 calculation, a clear 6 / 16 / 4 double glazed unit has a Ug of 2.82 W/m²K; with a low-E coating and argon in the cavity it drops to 1.06. We go through it step by step in double glazing, U-value and SHGC.
How we approach it
We work with the Saint-Gobain range and assemble the double-glazed units at our plant. To choose a coating we need three things: the orientation of each façade, how much daylight you want inside, and how the building should look from the street. With that we will propose one or two options with their data-sheet values.
Sources
- Saint-Gobain, Glass for Façade 2018, pp. 42 and 44: Cool-Lite LT, g and reflection in double glazing, to EN 410 and EN 673.
- Scottish Government, Tables of U-values and thermal conductivity: uncoated glass emissivity and hard-coating ranges.
- Ug values: VECA calculation to EN 673.