The short answer
Bifold doors must meet the maximum U-value set by Building Regulations Part L for the conservation of fuel and power. A U-value measures how much heat passes through the door — lower is better. Replacement doors in existing homes face a maximum U-value limit, and doors fitted as part of new work or an extension face the standard for new building. The exact figure depends on the version of Part L in force and whether the work is a replacement or new build, so the right benchmark is always the current Approved Document L. A door's U-value depends on the glazing (double or triple, low-E coatings, gas fill, warm-edge spacers) and the frame material. Reputable bifold systems quote a tested whole-door U-value so you can confirm compliance.
Passing Part L comes down to the door's U-value — its rate of heat loss. Here is what the figure means, how the requirement varies, and how to make sure the bifolds you choose comply.
U-value essentials
- RegulationPart L (conservation of fuel/power)
- Lower U-valueBetter insulation
- Replacement vs newDifferent limits apply
- Key driversGlazing and frame material
- What to checkTested whole-door U-value
What a U-value actually measures
A U-value is the standard measure of how readily heat passes through a building element — in this case a door. It is expressed in watts per square metre per degree of temperature difference (W/m²K), and the lower the number, the better the insulation: less heat escapes from inside to out. A solid, well-insulated wall has a very low U-value; a single pane of glass has a high one. Doors sit in between, and modern glazed doors have improved enormously as glazing technology has advanced.
Building Regulations Part L — the part dealing with the conservation of fuel and power — uses the U-value as the gateway test for whether a door is thermally efficient enough. The Regulations set a maximum permitted U-value, so the door's figure must be at or below that limit to comply. For glazed doors like bifolds, the relevant figure is the whole-door U-value, which accounts for the glass, the frame and the way they perform together, rather than the glass alone. This is the number to look for on a manufacturer's specification, because a door can have excellent glazing but a poorer frame, and the combined figure is what counts for compliance.
Replacement versus new-build requirements
An important distinction is whether your bifolds are a replacement in an existing home or part of new work such as an extension:
- Replacement doors in an existing dwelling must meet the maximum U-value that Part L sets for replacement glazing. The aim is to ensure that when you swap doors, the new ones are properly efficient rather than worse than current standards.
- New and extension work is held to the standard for new building elements, which is part of the overall energy performance of the new structure.
Because the figures are set by the Approved Document and have been tightened over successive updates as energy standards rise, the only reliable approach is to check the current version of Approved Document L for the exact maximum that applies to your situation. Your installer should specify doors that meet the relevant limit, and where work is self-certified through FENSA or CERTASS, confirming the U-value is part of demonstrating compliance.
What determines a bifold door's U-value
Several design choices drive the final whole-door U-value, and understanding them helps you specify a compliant, efficient set:
- Glazing units: double glazing is standard; triple glazing can lower the U-value further. Low-emissivity (low-E) coatings reflect heat back inside, and an inert gas fill (such as argon) between panes reduces conduction.
- Warm-edge spacers: the spacer bar around the glazing unit can be a thermal weak point; insulated warm-edge spacers improve the figure.
- Frame material: aluminium frames need a thermal break to insulate them, since bare metal conducts heat readily; uPVC and timber frames have their own thermal characteristics. The frame's performance feeds into the whole-door figure.
- Overall design and sealing: good seals and a well-engineered profile reduce heat loss around the perimeter.
The practical upshot is to look at the manufacturer's tested whole-door U-value on the specification sheet and confirm it meets the Part L limit for your project. Because bifolds are large areas of glazing, choosing a thermally efficient system makes a real difference to comfort and running costs, not just to passing the Regulations.
U-values, WER ratings and the whole picture
When comparing bifold doors you may meet two related but different measures: the U-value and the Window Energy Rating (WER). The U-value, as covered above, is the rate of heat loss — lower is better — and it is the figure most directly tied to the Part L limit for doors. The WER is a banded rating (similar in style to the A-to-G labels on appliances) that combines heat loss with solar gain and air leakage into a single grade. For largely glazed doors, manufacturers often quote the U-value as the primary compliance figure, but a good WER band is a useful shorthand for overall efficiency.
It is also worth keeping the U-value in perspective alongside the rest of the installation. A door can have an excellent whole-door U-value on paper, yet underperform in practice if it is poorly fitted — gaps around the frame, compressed or missing insulation in the reveal, or worn seals all let heat escape regardless of the glazing specification. This is one reason proper installation and self-certification matter: the U-value is the designed performance, but the fitted performance depends on the workmanship sealing the door into the opening. When specifying bifolds, the sensible approach is to confirm the tested whole-door U-value meets the current Part L limit, look at the WER band as a cross-check, and then make sure the doors are installed and sealed properly so the real-world performance matches the figures on the specification sheet. Efficient glazing and careful fitting together are what actually keep the heat in.
When you receive a quotation, ask the supplier to state the tested whole-door U-value in writing and confirm it meets the current Part L limit for your type of work. A reputable installer will supply this readily, along with the WER band if available. Keeping that specification with your paperwork, alongside the FENSA or building control certificate, documents both that the doors were efficient and that the installation was certified — useful evidence of a properly specified, compliant set when you come to sell.
Frequently asked questions
What does U-value mean for bifold doors?
A U-value measures how much heat passes through the door, in W/m²K. A lower number means better insulation and less heat loss. Building Regulations Part L sets a maximum U-value that the door must meet, using the whole-door figure that accounts for both glazing and frame.
Is the U-value limit different for replacement and new bifold doors?
Yes. Replacement doors in an existing home must meet the maximum U-value Part L sets for replacement glazing, while doors in new work or an extension are held to the new-build standard. Always check the current Approved Document L for the exact figure that applies.
How do I make bifold doors more energy efficient?
Choose a lower whole-door U-value: double or triple glazing with low-emissivity coatings, an argon gas fill, warm-edge spacers, and a frame with a proper thermal break. Because bifolds are a large glazed area, an efficient system improves comfort and running costs as well as meeting Part L.
Sources & further reading
- GOV.UK — Approved Document L (conservation of fuel and power)
- FENSA — Energy ratings and U-values
- Energy Saving Trust — Windows and doors
Figures on this page are typical UK ranges drawn from published sources and depend on your specific opening and material. They are guidance, not a quotation.