From single or double glazing to HR++?
Heat transfer is much greater through single glazing than through standard double glazing. This means that upgrading from single glazing to HR++ offers a greater improvement. HR+ and HR++ are themselves types of double glazing, so compare them with standard double glazing, not with double glazing as a whole category.
Two example scenarios; U = 5,8 from RVO, U = 2,8 and 1,1 from Energiehuis Slim Wonen
| Replacement | Indicative U-values | Difference in heat transmission |
|---|---|---|
| Single glazing → HR++ | 5,8 → 1,1 W/m²K | 4,7 W/m²K less |
| Standard double glazing → HR++ | 2,8 → 1,1 W/m²K | 1,7 W/m²K less |
Worked example: replacing 10 m² of standard double glazing
This is a simplified heat loss calculation, not a prediction of your gas consumption or energy bill. We use the method and assumptions of Energiehuis Slim Wonen.
- Glazed area: 10 m².
- U-value of old glazing: 2,8; new HR++ glazing: 1,1 W/m²K.
- Indoor temperature: 20 °C; average outdoor temperature: 6,6 °C.
- Heating period: 213 days, theoretically 24 hours a day.
The calculation step by step
10 × (2,8 − 1,1) × (20 − 6,6) × 213 × 24 ÷ 1.000 = approximately 1.165 kWh less theoretical heat loss through the glass.
The factor of 1.000 converts watt-hours to kilowatt-hours. The result relates solely to heat transfer through the glass under the chosen constant assumptions. It is not a measured annual energy saving.
Why this is not the same as your savings
In practice, you do not heat every room constantly to 20 °C. Sunlight, curtains, lower overnight temperature settings, ventilation and changing outdoor temperatures affect heating demand. Window frame quality and draught-proofing also matter. A percentage improvement in the glazing therefore does not translate into the same percentage reduction in the total energy bill.
We have deliberately not converted this example into euros or cubic metres of gas. Doing so would require, among other things, actual heating consumption, system performance and a specified energy price. A gas-based example would also be inappropriate for a heat pump or district heating.
What will you notice besides energy savings?
A warmer glass surface reduces cold downdraughts and radiant cooling near the window. Draughts through gaps must be addressed separately. Ensure sufficient ventilation after replacement; a more airtight home is no reason to close vents.
Can a subsidy reduce the investment?
HR++ or triple glazing may qualify for ISDE subject to conditions. The subsidy amount depends on the glazing type, area, year of installation and any combination with another measure. The window frame itself is not a standalone ISDE insulation measure. Read more on Subsidised glazing.
Advice for your own windows
Have the existing glazing and frames assessed and discuss which rooms you heat. We advise on suitable insulating glazing and subsidy options. An estimate tailored to your home is more useful than a general annual savings figure.
Frequently asked questions
How much can you save with HR++ glazing?
The savings depend on your existing glass, glazed area, heating habits and heating system. Under the same conditions, replacing single glazing delivers a greater improvement than replacing standard double glazing. A heat loss calculation shows the effect of a lower U-value, but does not directly represent your annual savings in gas or euros.
Is 1,165 kWh also my annual energy saving?
No. This is the theoretical difference in heat loss for 10 m² of glazing at constant example temperatures, with heating running 24 hours a day. Actual savings depend on the home, heating system and usage. This result has therefore not been converted into gas consumption or euros.
Can I install HR++ glazing in my existing window frames?
This is often possible if the window frames have sufficient space and load-bearing capacity and are in good condition. An on-site assessment will determine whether replacing only the glazing is a sound option.