Glossary
U-values and the insulation London building work needs
Heat transfer coefficient. Lower = better insulation.
Heat transfer coefficient. Lower = better insulation.
A U-value is the rate of heat loss through a building element, in watts per square metre per degree kelvin (W/m²K). Lower is better. Approved Document L sets maximum U-values for new thermal elements in an extension: 0.18 for walls and floors, 0.16 for a pitched roof, and 1.4 for windows and doors.
An uninsulated solid 215 mm brick wall is around 2.1 W/m²K. An unfilled 1930s cavity wall is about 1.5. A filled cavity is roughly 0.5. A modern extension wall with 100 mm of PIR in the cavity reaches 0.18. Single glazing is about 5.0; standard double glazing about 1.6; a good modern double-glazed unit 1.2 or better.
That range is why a Victorian terrace loses so much through its walls, and why wall insulation is the biggest single fabric improvement available — and also the most disruptive.
New thermal elements in an extension must meet the new-build standards. Existing elements being renovated have less demanding threshold values — 0.30 W/m²K for a wall, 0.18 for a pitched roof at ceiling level — and even those can be relaxed where the measure is not technically or functionally feasible or the payback exceeds fifteen years.
By summing the thermal resistances of every layer in the build-up, including internal and external surface resistances, and taking the reciprocal. Repeating thermal bridges such as timber studs or joists are accounted for using the combined method in BS EN ISO 6946, which is why 100 mm of insulation between studs does not deliver the figure the insulation datasheet suggests.
How u-value plays out in Harrow: London Borough of Harrow (Harrow Council) maintains 29 conservation areas, the Harrow on the Hill Conservation Area and the Pinner Village Conservation Area among them, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Anyone dealing with u-value at a Harrow address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where u-value still qualifies.
| Unit | W/m²K — lower is better |
|---|---|
| New wall in an extension | 0.18 |
| Pitched roof at ceiling level | 0.16 |
| Floor | 0.18 |
| Window or door | 1.4 |
| Existing wall on renovation | 0.30 threshold value |
0.18 W/m²K as a new thermal element.
Only when renovating them, and feasibility and payback tests can relax the standard.
Repeating thermal bridges through studs or joists reduce the effective performance.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
