Energy-Efficient Windows near you

Approved Document L Volume 1 sets limiting U-values for new fabric elements in existing dwellings in Table 4.2, and windows appear there with two ways of meeting the standard: a maximum U-value of 1.4 W/(m²·K), or a Window Energy Rating of Band B as a minimum. Doors are split by how much glass they carry. A door with more than 60 per cent of its internal face glazed must meet 1.4 W/(m²·K) or a Doorset Energy Rating of Band C minimum; other doors must meet 1.4 W/(m²·K) or a Doorset Energy Rating of Band B minimum. Rooflights sit separately at 2.2 W/(m²·K).

The two routes are genuinely alternative, and they measure different things: a U-value counts heat leaving, while an energy rating nets solar gain off against it, which is why a window can pass on one route and not the other. The document notes that the methods for calculating Window Energy Rating and Doorset Energy Rating are set out in the Glass and Glazing Federation's Glazing Manual Data Sheet 2.3. It adds that building control bodies may accept a Window Energy Rating or Doorset Energy Rating as evidence of compliance where it comes from a certification scheme providing a quality assured process and a supporting audit trail from calculating the performance of the window through to the window being installed.

One footnote deserves more attention than it gets. Table 4.2 notes that if other performance requirements — it names wind load, safety, security or acoustic attenuation — require thicker glass to be used, an equivalent window unit with standard 6mm glazing should be shown to meet the required standard. So specifying acoustic glass for a window on a main road, or heavier security glazing on a vulnerable elevation, does not force you to fail the energy standard. The comparison is made against an equivalent unit with standard glazing.

Britain's housing stock is old enough that a blanket standard would be unworkable, and Approved Document L says so. Paragraph 4.10 provides that if windows or fully glazed external pedestrian doors cannot meet the requirements of Table 4.2 because of the need to maintain the character of the building, one of two things should be met instead: the fittings should not exceed a centre pane U-value of 1.2 W/(m²·K), or single glazing should be supplemented with low-emissivity secondary glazing.

Read that carefully, because the two limbs point in opposite directions. The first is a slim-profile route, where a very thin sealed unit in a traditional frame is judged on the centre of the pane rather than on the whole window, which is how heritage glazing can pass at all. The second sanctions secondary glazing outright: keeping the historic window and adding a low-emissivity inner layer is not a compromise the document tolerates, it is a compliance route it names.

That matters for anyone advised that a listed building must simply lose its windows. Listed building consent is a separate regime from the Building Regulations and is often the binding constraint, and where consent will not permit replacement, paragraph 4.10 gives the energy side a workable answer. Get the conservation officer's view and the building control route settled together, rather than letting a glazing salesman decide both.

Replacing like for like and creating something new are treated differently, and the rule that catches people sits in paragraph 10.5. If a window is enlarged or a new one is created, either the area of windows, roof windows, rooflights and doors should not exceed 25 per cent of the total floor area of the dwelling, or, if it does exceed 25 per cent, compensating measures should be taken to improve the energy efficiency of the dwelling.

This is a whole-house calculation, not a room one, and it is cumulative. A house that has already gained bifolds across the back, a large rooflight over a kitchen and a glazed side return can be close to the threshold before anyone thinks to measure. It does not prohibit more glass — compensating measures are an accepted answer — but it converts what looked like a window order into a conversation with building control about insulation or heating elsewhere in the house.

It also explains an odd asymmetry homeowners notice. Swapping an existing window for a better one of the same size engages only Table 4.2. Making the opening bigger engages Table 4.2, paragraph 10.5, and the structural requirements for the new opening. If a quotation for a larger window says nothing about any of this, that is a sign the installer is treating a new opening as a straight replacement.

The zero rate of VAT on energy-saving materials is the scheme people most often assume covers windows. It does not. HMRC's VAT Notice 708/6 lists the qualifying items — insulation, draught stripping, controls for central heating and hot water systems, solar panels, wind and water turbines, ground, air and water source heat pumps, micro combined heat and power units, wood-fuelled boilers, and, from February 2024, batteries for storing energy converted from electricity and smart diverters.

Glazing is then excluded in terms. The notice states that the installation of all other energy-efficient products, naming secondary or double glazing and low-emissivity glass among them, is standard-rated. So a quotation for energy-efficient windows carries VAT at the standard rate, while a quotation for loft or cavity insulation on the same house does not. The zero rate runs from 1 May 2023 to 31 March 2027, after which the qualifying items revert to the reduced rate of 5 per cent — worth knowing if you are sequencing insulation and glazing work.

Draught stripping is the one adjacent item on the qualifying list, which is a useful reminder of where the cheap wins are. Sealing an existing window well costs a fraction of replacing it and sits inside the zero rate; replacing the window does not. On a period house where paragraph 4.10 already sanctions secondary glazing, the combination of draught stripping and secondary glazing is frequently both the cheaper and the more consent-friendly route.

GOV.UK currently publishes two generations of Approved Document L side by side, and it is easy to quote the wrong one. The edition in force is the 2021 edition incorporating 2023 amendments, which came into effect on 15 June 2022 and contains the Table 4.2 figures above. Alongside it sits a 2026 edition, published as guidance for buildings subject to the 2026 standards, which is not yet in force.

For anyone buying windows today, the practical consequence is that the standard your installer must meet is the 2021 edition, and a quotation citing the 2026 document is either forward-looking or confused. It is worth asking which the figures come from, particularly where an installer is presenting a product as exceeding requirements.

It also affects how long a specification stays current. GOV.UK sets out transitional arrangements in the usual form — the changes did not apply where a building notice or initial notice had been given, or full plans deposited, before a cut-off date, provided the work started within a further period. If your project is likely to straddle a change, settle in writing with the installer which edition the job is being designed and certified against.