Low-E glass: what it is and why it matters

No. They are different technologies solving different problems. Low-E deals with thermal insulation (the cold); asymmetric acoustic glass deals with noise. A single pane can carry both at once, but they are distinct layers of the solution.

Low-E on its own already improves insulation considerably. Argon adds a further gain at low cost, which is why we fit it as standard in double glazing.

Not in any noticeable way. The coating is transparent to the naked eye; the difference is in how it reflects infrared heat, not in the visible light entering the room.

There are two manufacturing processes. Hard-coat (pyrolytic) is applied at high temperature during manufacture of the float glass itself, becomes chemically bonded to the surface, is tougher and more resistant to handling, but has a higher emissivity (insulates less). Soft-coat (cathodic deposition, or "sputtered") is applied afterwards, in several ultra-thin layers, has a much lower emissivity (insulates more) but is more sensitive, which is why it is always fitted facing into the gas cavity of the double glazing, never exposed. Most high-performance Low-E glass used today in efficient windows is soft-coat.

Faces are numbered from outside in: face 1 (exterior), faces 2 and 3 (the two facing the gas cavity), face 4 (interior). Soft-coat Low-E almost always goes on face 3, protected inside the sealed cavity, on the side closest to the inside of the home, to reflect interior heat back inward in winter.

No: they are two different measurements, and confusing them is a common mistake. Uw measures heat loss by conduction (cold getting in or out). The solar factor (g or g-value, measured to EN 410) measures the fraction of solar energy that passes through the glass and heats the room. It matters mainly in summer, to avoid overheating on facades with heavy sun exposure. A good Low-E glass can have a low Uw (insulates well against cold) while also having a g-value matched to the facade's orientation, so it doesn't let in too much heat in summer.

Yes. With double glazing, Low-E and argon, our windows sit comfortably below the maximum Uw required for the winter climate zone of Lisbon and Setúbal (2.80 W/m²·°C, zone I1, under Decree-Law 101-D/2020 and Ordinance 138-I/2021). The same ordinance also sets maximum solar factor limits by summer climate zone (V1/V2/V3), stricter in the hotter zones. We match the glass to your facade's orientation and exposure within those limits.

The coating itself, protected inside the sealed cavity, barely degrades. The real risk is failure of the perimeter seal on the double-glazed unit, if the seal fails, moisture gets in and the unit fogs up between the panes (not repairable, only replaceable). It's a low risk with quality manufacturing, but not zero, hence the factory warranty on the glass unit, separate from the frame warranty.

Not necessarily. On a north-facing facade, the goal is to maximise thermal insulation (low Uw) with no real overheating concern. On a south or west-facing facade with heavy sun exposure. It can pay off to combine Low-E with solar control (a lower g-value), so the room doesn't turn into a greenhouse in summer. We specify this per facade, not as one single glass for the whole house.

It insulates more: a well-specified triple pane can push Ug even lower, but it is heavier (needs reinforced hardware and frame), more expensive, and the marginal gain over a good double Low-E is smaller in a climate like Lisbon and Setúbal than in a harsher winter climate. For most homes in our service area, double Low-E with argon already meets the required Uw comfortably; triple rarely pays for itself here.

No. Since it is protected inside the sealed cavity of the double glazing, the Low-E coating never comes into direct contact with cleaning products, you clean the outer face of the glass normally, like any window.