Planitherm glass: €25 more for insulation, but less light

€25 for better insulation, but not free when it comes to light

A quote for new windows raises the question almost nobody asks: paying the surcharge for a low-emissivity glass such as Planitherm costs around €25 per unit, according to the advice given to someone replacing their glazing. The sales pitch is twofold: in winter it keeps the heating in, and in summer it blocks the heat from outside. The problem appears when you start looking at the colour.

The low-emissivity coating is a thin metallic film applied to the inner face of the glass, in contact with the air gap. It works, but it is not invisible. Anyone who has seen trains or shop windows with tinted glass describes a bluish or smoky tone which, with many panes together, turns a sunny day into a misty scene. The question is not whether it insulates. It is how much that insulation costs in light quality.

What the numbers say about heat transmittance

The figures being discussed speak for themselves. A double glazing unit with a 6 mm gap goes from a K factor of 2.9 with two Planilux panes to 2.2 with one Planitherm pane. With an 8 mm gap, from 2.8 to 1.9. And with 12 mm, from 2.6 to 1.5. The wider the gap, the greater the improvement. The jump is real, although the range depends on the assembly.

The catch comes with transparency. Ordinary 6 mm float glass does not exceed 91% transparency; extra-clear glass approaches 98%. And a configuration with a low-emissivity coating can drop to 82%, or even lower. There lies the trap: insulation is bought with light. The difference between a neutral glass and a coated one is noticeable up close, although human visual memory is poor and most people cannot tell with the panes side by side.

The frame matters more than the glass

The most uncomfortable consensus on the matter is that the glass is the last piece of the puzzle. Good glass with a mediocre frame, a careless installation or a poorly fitted shutter box saves nothing. The investment only pays off if the whole window is airtight. On small surfaces, the extra cost is hardly ever recovered; on large sliding balcony doors, the calculation changes.

Some go further and argue that an air gap that is too wide is counterproductive: beyond a certain thickness, the air circulates inside and convection eats up the thermal resistance gained. The alternative would be to replace the air with argon or insert an intermediate film that prevents that movement. The technical debate remains open.

The sun that is lost in winter

The experience of those who already have the coating installed adds a cost that does not appear in the brochure: in winter, the sun barely heats through the glass. In a cold house, that free contribution is missed. The advice given is geographic: in the south, fit it on east- and south-facing windows; in other areas, it may not be worth it.

The alternative for those who prioritise light lies in extra-clear panes such as Diamant, with transparency above 93%, or configurations with a neutral-looking coating. The problem is finding them: the distribution network of some brands is limited and the installer usually offers whatever they have at hand.

In the end, the decision comes down to a question no catalogue answers: how much natural light are you willing to give up for a few euros less on the bill?