What does a U-value tell you?
A U-value describes heat transmission through a building element, such as a wall or roof. It is expressed in watts per square metre per kelvin: W/(m²·K). Lower values mean less heat transmitted through the same area under the same temperature difference.
The value belongs to the complete construction, rather than a single insulation product. The insulation, structural members, other layers and repeating thermal bridges all influence the calculated result.
A simple illustration
| U-value | Calculated heat flow |
|---|---|
| 0.30 W/(m²·K) | 600 W |
| 0.20 W/(m²·K) | 400 W |
| 0.15 W/(m²·K) | 300 W |
| 0.10 W/(m²·K) | 200 W |
This follows heat flow = U × area × temperature difference. It excludes additional junction losses, air leakage, changing weather and other building elements. It does not predict annual bills or whole-home energy savings.
The whole-building picture
A low wall U-value is useful, but it is only part of an efficient home. Roofs, floors, windows, doors, thermal bridges, airtightness and ventilation all need to work together. Solar gains, summer shading and the heating system also influence comfort and energy use.
Airtightness limits unintended air movement through the building fabric. Ventilation provides fresh air in a planned way. Improving one without considering the other is not a complete design strategy.
Why installation matters
Gaps, compressed insulation, discontinuous layers and poorly connected junctions can compromise the intended performance. Drawings should define a continuous insulation layer and a clear air-barrier line, with buildable details at openings and changes in geometry.
Before work is concealed, the project team should arrange appropriate checks and records. Any required testing should be planned into the programme rather than treated as a final surprise.
The role of vapour control
A vapour control layer limits moisture movement by diffusion; an air barrier limits air leakage. One product may serve both purposes, but its suitability and position must follow the project’s moisture strategy and the manufacturer’s requirements.
What if you are aiming for Passivhaus?
Passivhaus is a whole-building approach with energy and comfort criteria, detailed modelling, rigorous quality assurance and testing. It cannot be established by specifying a particular wall U-value alone.
If certification is part of your brief, involve a suitably experienced designer and certifier early. TSK can discuss the installation scope with that team; the work shown on this website is not presented as evidence of certified Passivhaus projects.
What should your project brief include?
- The intended thermal performance of each building element.
- Calculated and coordinated wall, roof and floor build-ups.
- A continuous air-barrier strategy with junction and penetration details.
- A moisture and ventilation strategy appropriate to the construction.
- Clear installation responsibilities and a plan for protecting completed work.
- Inspection, evidence, testing and handover requirements.
Further reading
Official guidance on energy performance requirements in England, including links to applicable editions.
An introduction to whole-building principles, performance and certification.
Reviewed 1 November 2026.