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Dormer energy label and energy performance

A dormer is a chance to improve the energy performance of your roof. Done well, it can raise your energy label. Done poorly, it creates a cold spot.
5 min leestijd·Onafhankelijke informatie

Short answer

A dormer affects the energy label through insulation, glazing and airtightness. High-performance insulation and low-U-value windows help; thermal bridges and draughts hurt.

Impact on energy label

The energy label is based on the whole building envelope. A dormer is part of the roof, so its quality matters. A well-built dormer can compensate for some weaknesses elsewhere; a badly built one can drag the whole roof down.

Performance targets

  • Insulation — aim for an overall roof insulation level that matches or exceeds local building regulations.
  • Windows — U-value of 1.0 W/m²K or lower is a good target for new dormers.
  • Airtightness — a draught-free dormer is more comfortable and uses less energy.

How to improve

Insulation and glazing are the big levers, but airtightness and ventilation are also important. Without controlled ventilation, a very airtight dormer can trap moisture.

The full checklist

Checklist

Energy performance checklist

  1. 01Insulation meets or exceeds local building regulations.
  2. 02Window U-value is 1.0 W/m²K or lower if possible.
  3. 03Thermal bridges are minimised at the junction.
  4. 04Vapour barrier is sealed and intact.
  5. 05Ventilation is planned for the room behind the dormer.
  6. 06Airtightness is tested or visibly checked.
  7. 07Energy certificate is updated after major work if required.

FAQ

FAQ

Veelgestelde vragen

01Does a dormer affect the energy label?
Yes. A well-insulated dormer with good glazing and no thermal bridges can improve the energy performance of the roof. A poorly insulated one can make it worse.
02What U-value should dormer glazing have?
Lower is better. For new dormers, aim for a window U-value of around 1.0 W/m²K or lower with triple glazing or high-performance double glazing.
03Can a dormer help me reach a better energy label?
It can help, especially if the old roof was poorly insulated. The improvement depends on the overall insulation, glazing, airtightness and ventilation of the house.
04Do I need an energy performance certificate for a dormer?
Not always. If you are selling or renting, you may need an updated energy certificate. Rules vary by country and local authority.
Conclusie
A dormer is not just extra space — it is a new surface in your thermal envelope. Choose high-performance insulation and glazing, and the energy label will usually look after itself.