U-values and Part L: what the numbers mean in practice
Lower is better, but a great U-value on a badly sealed opening still loses heat. What to compare, what the regulations ask for, and where the performance really goes.
- A U-value measures heat loss through the whole unit, frame included. Lower is better.
- Part L sets the minimum performance for replacement windows and doors in dwellings.
- A good frame fitted badly performs worse than a modest frame fitted well.
A U-value measures how much heat passes through a material for each degree of temperature difference, so lower is better. It is the number quoted on every window brochure, and it is only part of the story.
The units, briefly
U-values are given in watts per square metre per kelvin, written W/m2K. Glass, frame and spacer all contribute, and the figure that matters is the one for the whole unit rather than the centre of the glass alone.
A single pane sits around 5.6. A good double glazed unit is between 1.0 and 1.4. Triple glazing can reach 0.8 or lower, which is genuinely useful on a north elevation and much less useful on a wall that faces the sun.
What Part L asks for
In England, replacement windows and doors in existing dwellings normally need to meet a maximum U-value of about 1.4 W/m2K, with slightly different figures for some elements. New extensions are assessed differently again, because the whole building has to demonstrate compliance.
The future homes standard tightens these figures further, so specifying better than the minimum now avoids an unpleasant surprise at resale.
Where the numbers go wrong
A frame with an excellent U-value still leaks heat if the perimeter is not insulated and sealed. Closing the cavity, insulating the reveal and taping the airtight layer at the junction usually does more for the room than upgrading from double to triple glazing.
Ask the installer to show you how the junction with the wall will be finished. It is the part of the job you cannot see once it is done, and it is the part that decides how the room feels in January.
Written by the Minimal Frame Projects UK team. If you would like advice on a specific opening, send us the dimensions and a few photographs and we will come back with a specification.
