Roughly a fifth of the heat lost from an average home goes out through the windows. That makes sense: glass is thin, and an old frame rarely closes properly any more. New windows with insulating glass tackle that in three ways.
1. The glass
The biggest gain is in the glass, simply because it is the largest surface. The U-value tells you how much heat leaks through per square metre:
- Single glazing: about 5.8 W/m²K
- Old double glazing: about 2.8 W/m²K
- HR++ glazing: about 1.1 W/m²K
- Triple glazing: 0.7 W/m²K or lower
The jump from single to HR++ is therefore enormous. The step from HR++ to triple is relatively smaller, but with large areas of glass it certainly counts.
2. The profile
A frame with five or six air chambers and a thermally broken build-up prevents a cold bridge. You notice that in two things above all: condensation no longer forms on the frame itself, and the cold downdraught by the window disappears, that sense of a draught running past your legs even though there is no gap to be found.
3. The draughts
This is the one most often underestimated. An old frame that has moved no longer seals all round, and the sealant around it is hard and cracked after twenty years. New windows with double seals all round, sealed airtight inside and out, take those draughts away. In many homes that delivers more immediate comfort than the U-value of the glass.
What does it deliver?
The honest answer: that depends heavily on your starting point. If you are coming from single glazing in poorly closing timber frames, the difference is large. If you already have reasonable double glazing, it is more modest. What we see in practice is that people mention comfort above all: no more cold spots by the window, less noise from outside, and a living room that stays warm in the evening.
Want a real calculation rather than an average? Ask an energy adviser for a bespoke assessment, or use our product selector for a first sense of direction.
Getting the most out of it
- Do it in one go. New windows in a house with an uninsulated roof only solve part of the problem. Combine measures where you can.
- Do not forget ventilation. An airtight home needs fresh air. Trickle vents in the frame or a tilt-and-turn setting are part of the package. Without them you get damp and mould problems.
- Think about the sunny side. On south and west elevations, solar control glass can limit overheating in summer, so you need less cooling.
- Combine for double subsidy. Two insulation measures within 24 months double the ISDE amount per m².
What does the subsidy come to?
If you replace the windows as an owner-occupier, the ISDE scheme run by RVO returns a fixed amount per square metre of glass: € 25 per m² for HR++ glazing and € 111 per m² for triple glazing in new frames. Take a second insulation measure within 24 months. Roof or floor insulation, for instance. And those amounts double. You need at least 3 m² of glass and RVO subsidises a maximum of 45 m².
We prepare the application and supply all the details; you submit it yourself to RVO using DigiD. On our subsidy page you can work out in a few clicks what it comes to in your situation.
Curious what it would come to for you?
Request a no-obligation quote or book a free measuring visit. Our own surveyor comes round, takes the measurements and advises you on the spot, you are under no obligation.