Building Regulations Part L: Insulation and Glazing Requirements

Part L of Schedule 1 to the Building Regulations 2010 is the conservation of fuel and power requirement. It applies to buildings, or extensions of such buildings, or work to or in connection with a building where that building is a roofed construction having walls and uses energy to condition the indoor climate¹. For a householder, that means the regulation bites when a thermal element is built, renovated or replaced, and when glazing is swapped out as part of building work.

The headline numbers for new dwellings are limiting U-values of 0.18 W/(m²·K) for walls in dwelling houses, 0.13 W/(m²·K) for roofs and 0.15 W/(m²·K) for floors, with windows and roof windows at 1.4 W/(m²·K) and doors at 1.6 W/(m²·K)². These are worst acceptable figures, not targets: a dwelling still has to satisfy the wider compliance metrics, and the limiting values simply cap how poor any single element may be.

For existing dwellings the limits are looser but still binding. New fabric elements in an existing dwelling are held to 0.18 W/(m²·K) for walls, 0.15 W/(m²·K) for roofs and 0.18 W/(m²·K) for floors, while a wall receiving internal or external insulation has a threshold of 0.70 W/(m²·K) and an improved value of 0.30 W/(m²·K)¹. The gap between the threshold and the improved figure is where most retrofit arguments happen.

What Part L covers, and what it does not

Part L is titled conservation of fuel and power, and the Welsh Government has moved to amend that title to recognise that the requirements also reduce greenhouse gas emissions⁴. The scope is broad but bounded. It applies to buildings, or extensions of such buildings except those of Class 7 type, or work to or in connection with such a building, where the building is a roofed construction having walls and uses energy to condition the indoor climate¹.

The regulation is, in general, technology and fuel neutral⁵. It does not name a heating appliance or ban a fuel. What it does is set the fabric and energy performance the dwelling must reach, and leave the route to the designer. That neutrality is worth stating plainly because it is often misread: the constraint is on the building's performance, not on the boiler's badge.

Approved Document L is the guidance that shows one way to comply. The approved documents set out what, in ordinary circumstances, may be accepted as one way to comply with the Building Regulations¹. It is not the only route, and it is not the regulation itself, but in practice it is the route almost every dwelling project follows.

The 2025 Welsh review consulted on changes to Part L including consequential changes to Part F on ventilation, Part O on overheating and associated approved document guidance⁶. In Northern Ireland, a 2023 discussion document covered Part F, Part K on ventilation, overheating risk in dwellings and electric vehicle infrastructure at parking spaces⁷. The direction of travel across the administrations is towards tighter fabric and a closer coupling of insulation with ventilation and overheating.

U-values explained: what W/m²K means and why lower is better

A U-value is the rate at which heat passes through one square metre of an element for each degree of temperature difference across it, expressed in W/(m²·K). Lower is better, because less heat escapes for the same temperature gap. The figure covers the whole assembly, not just the insulating layer: for a window it includes the frame, and the approved document requires the value to be calculated using the actual size and configuration of the window in line with BS EN ISO 10077-1 and BS EN ISO 10077-2².

That calculation standard matters in practice. A window's U-value is not a fixed property of the glass unit; it depends on the frame, the spacer, the size and the proportion of frame to glass. A large fixed pane and a small opening casement built from the same components will not produce the same number.

The R-value is the reciprocal idea: the thermal resistance of a material or layer, where higher is better. A well insulated element therefore has a low U-value and a high R-value. The two are not competing measures, they are two directions of looking at the same property.

Some elements carry their own limits that are easy to overlook. Party walls have a worst acceptable U-value of 0.20 W/(m²·K) in both new and existing dwellings, and walls to flats 0.21 W/(m²·K)². Under the Energy Company Obligation, solid underfloor insulation should achieve a U-value of at least 0.25 W/m²K⁸. These are separate regimes, but they show how widely the same unit is used to set a threshold.

U-value limits for new dwellings

The limiting U-values for new dwellings are set out in a table that caps each element. For dwelling houses the wall limit is 0.18 W/(m²·K); for flats it is 0.21 W/(m²·K)². Roofs, including dormer roof parts, are limited to 0.13 W/(m²·K), and floors to 0.15 W/(m²·K)². Windows and roof windows, including curtain walling, are limited to 1.4 W/(m²·K), and doors including glazed doors to 1.6 W/(m²·K)².

The 2021 edition of the same approved document, as amended in 2023, gives a different set of figures for new dwellings: walls 0.26 W/(m²·K), all roof types 0.16 W/(m²·K), floors 0.18 W/(m²·K), windows 1.6 W/(m²·K) and doors 1.6 W/(m²·K)⁹. Where a project is being assessed, the edition that applies to it governs.

Meeting the limiting values is necessary but not sufficient. A newly constructed dwelling must be shown to meet regulations 26, 26A, 26B and 26C by producing calculations demonstrating the target primary energy rate, the target emission rate, the target fabric performance values and a minimum energy efficiency rating¹. The limiting U-values are the floor of acceptable performance; the targets are the ceiling the design has to reach.

Insulation and glazing standards for existing homes, extensions and renovations

Existing dwellings are governed by a different table, and the trigger is the work rather than the building. New fabric elements in an existing dwelling are limited to 0.18 W/(m²·K) for walls, 0.15 W/(m²·K) for roofs and 0.18 W/(m²·K) for floors¹. The 2021 edition as amended gives the same wall and floor figures and the same roof figure of 0.15 W/(m²·K)⁹.

Renovation is where the rules become conditional. Where a thermal element is being renovated, the guidance requires the thermal insulation of the element to be upgraded to a reasonable standard¹⁰. For external walls, the regulations would normally apply and the thermal insulation would normally have to be improved¹¹. For an external wall renovation, the thermal insulation of the wall would have to meet the standards required by building regulations approved documents¹².

The threshold and improved values for a wall receiving internal or external insulation are 0.70 W/(m²·K) and 0.30 W/(m²·K) respectively¹. The 2020 Welsh guidance for buildings essentially domestic in character gives a renovated wall with external or internal insulation a limiting value of 0.70 W/m²K, and a cavity-insulated wall the same 0.70 W/m²K¹⁰. The pattern is consistent: a poor existing wall is not required to become a new-build wall, but it must be brought up to a defined standard.

There is a limit to how far this can be pushed. A lesser thermal performance may be appropriate if meeting the standard would reduce the internal floor area of the bounded room by more than 5%, provided the wall achieves the best practicable thermal performance². The same 5% test appears in the 2020 Welsh guidance as a test of functional and technical feasibility¹⁰. This is the provision that keeps a narrow Victorian room from being rendered unusable by internal insulation.

Windows and doors: maximum U-values and glazing area limits

Replacement windows and doors must not exceed the maximum U-value given in the approved document, and the figure covers the frame as well as the glass¹⁵. In new dwellings the window and roof window limit is 1.4 W/(m²·K) and the door limit is 1.6 W/(m²·K)². In existing dwellings, doors with more than 60% of the internal face glazed are limited to 1.4 W/(m²·K) or a Doorset Energy Rating of Band C minimum⁹. Other doors are limited to 1.8 W/m²K under the 2020 Welsh guidance¹⁰.

The distinction between replacing a window and replacing glazing is decisive. Replacing just the glazing or door leaf while retaining an existing frame is not building work and so does not have to meet the energy efficiency requirements¹⁰. Replacing the whole unit is building work, and the new unit must comply.

Glazing area is capped as well as glazing performance. In Scotland, the area of windows, doors and roof lights within an extension should be limited to 25% of the floor area of the extension plus the area of any existing openings built over¹⁷. The Northern Ireland Technical Booklet F1 prescribes a maximum limit of openings of no greater than 25% of the total floor area for dwellings, with windows, roof windows, glazed rooflights, curtain walling and pedestrian doors at a maximum area-weighted U-value of 1.40 W/m²K and no worse than 3.00 W/m²K at any point¹⁹. For buildings other than dwellings, there is no limit on the extent of glazing that can be provided due to the variety of building types¹⁹.

Air tightness and pressure testing

Air permeability is capped at 8.0 m³/(h·m²) at 50Pa in new dwellings, with an alternative expression of 1.57 m³/(h·m²) at 4Pa². The 2021 edition as amended gives the same 8.0 m³/(h·m²) at 50Pa figure in its Table 3.1⁹.

Achieving the cap is not the same as achieving the best. Air-tightness levels of 5 to 7 m³/h.m² at 50Pa or better are readily achievable in dwellings constructed to the 2007 and 2010 Accredited Construction Details (Scotland)¹⁷. That is a Scottish figure and it describes what is achievable rather than what is required.

Testing is expected. Air-tightness testing should be carried out on new dwellings to demonstrate that air infiltration rates deliver both the stated design level and that the proposed ventilation strategy remains appropriate¹⁷. Air tightness testing should be undertaken to verify as-built air infiltration rates¹⁷. There is an exception: where a default design value of 15 m³/h.m² at 50Pa is stated in demonstrating compliance under Standard 6.1, testing need not be carried out²⁰.

For existing buildings, a default value of 10 m³/h.m² at 50Pa can be assumed, or testing of the extension carried out as identification¹⁷. That default is the practical figure for a retrofit where no test is commissioned.

Tightening a building has a consequence. Lower air infiltration rates, of less than 5 m³/h.m² at 50Pa, may give rise to problems with internal air quality and condensation unless addressed through planned ventilation¹⁷. The approved document also requires that the building fabric be constructed so that the insulation is reasonably continuous across newly built elements, following the applicable guidance in Appendix F².

How compliance is assessed: SAP, the notional dwelling and Approved Document L

Compliance for dwellings is demonstrated through the Standard Assessment Procedure. The calculations must use the UK Government's Standard Assessment Procedure for energy rating of dwellings, version 10¹. The full notional dwelling specification used in the Standard Assessment Procedure is given in Appendix R of SAP version 10, summarised in Appendix E¹. The notional dwelling is a reference design of the same size and shape as the proposed dwelling, and the proposed design is measured against it.

The assessment models help to deliver government policy initiatives including Building Regulation compliance checks²¹. For Part L 2021, the residential methodology is SAP 10.2, producing a Dwelling CO2 Emissions Rate²². The newer edition moves to SAP version 10 as the required calculation basis¹.

Approved Document L Volume 1 applies to dwellings only and contains current and future guidance covering the energy efficiency requirements as set out in Part L of Schedule 1 to the Building Regulations and in a number of specific building regulations²³. Volume 2 applies to non-domestic parts of mixed-use buildings and rooms for residential purposes². In a building containing more than one dwelling, heated common areas follow Volume 2, while unheated common areas should meet the minimum standards in Section 4 of Volume 1³.

Guidance for newly constructed dwellings is given in Sections 1 to 14 of the approved document³. Guidance for existing dwellings is given in Sections 3, 4, 5, 10, 11 and 12, covering extensions and work on existing dwellings¹. A householder looking up a specific situation needs the right section as well as the right volume.

Ventilation and Part F: the companion requirement

Part L and Part F are read together. The Welsh 2025 review consulted on Part L including changes to Part F on ventilation, Part O on overheating and associated approved document guidance⁶. The Northern Ireland discussion document covered Part F, Part K on ventilation, overheating risk in dwellings and electric vehicle infrastructure⁷.

The reason for the coupling is physical. As air permeability falls, the incidental ventilation that used to keep a dwelling fresh disappears, and planned ventilation has to replace it. The Scottish guidance is explicit that rates below 5 m³/h.m² at 50Pa may give rise to problems with internal air quality and condensation unless addressed through planned ventilation¹⁷.

In Northern Ireland, the Technical Booklet F1 sets the window and door limits alongside the glazing area cap, treating the envelope and the ventilation strategy as one package¹⁹. The same logic runs through the other administrations even where the documents are separate.

The practical consequence for a householder is that an insulation or glazing upgrade is not a standalone fabric decision. Where a home is being made tighter, the ventilation provision has to be assessed at the same time. The Part F ventilation requirements page sets out what that involves, and home ventilation systems covers the equipment side.

England and Wales: different volumes, editions and effective dates

England and Wales share the Building Regulations 2010 but not the approved document editions or their dates. The 2021 edition of Approved Document L Volume 1 for dwellings, incorporating 2023 amendments, applies in England⁹. The 2026 edition of Approved Document L Volume 1 for dwellings takes effect in Wales on 4 March 2027, with transitional arrangements for notices submitted before that date and work started before 4 March 2028².

The Welsh 2026 edition is the tighter document. Its new dwelling limits are 0.18 W/(m²·K) for walls in dwelling houses, 0.13 W/(m²·K) for roofs and 0.15 W/(m²·K) for floors, against 0.26, 0.16 and 0.18 W/(m²·K) in the 2021 edition². The window limit tightens from 1.6 to 1.4 W/(m²·K)⁹.

The Welsh amendments relating to Part L of Schedule 1 to the Building Regulations 2010 come into force on 4 March 2027⁴. The listed volume 1 dwellings and volume 2 buildings other than dwellings documents take effect on the same date²⁴. Approved Document F volume 1 dwellings also takes effect on 4 March 2027²⁵.

The implementation history is long. Part L was implemented in 2002, again in 2010 and again in 2013²⁶. The 2013 changes were set out in an impact assessment with ISBN 9781409839842, applying to England²⁷. The 2026 Welsh edition is the next step in that sequence.

Scotland and Northern Ireland run their own regimes. Scotland's building standards are set out in the Technical Handbook, with the 2022 domestic edition giving the air-tightness and glazing figures cited above¹⁷. Northern Ireland's Technical Booklet F1 sets the window and door limits and the glazing area cap¹⁹. The Scottish building standards and Welsh building regulations pages cover those regimes in more detail.

What Part L means for a household upgrading insulation, glazing or heating

The regulation applies to work, not to occupancy. It bites when a thermal element is built, renovated or replaced, or when glazing is replaced as part of building work. A household that is not doing notifiable work is not caught by it, and a household that is doing such work is held to the limits that apply to the element in question.

The limits are worst acceptable values, and the compliance route is wider than any single figure. A new dwelling must meet the target primary energy rate, target emission rate, target fabric performance values and a minimum energy efficiency rating, not merely the limiting U-values¹. An existing dwelling being extended or renovated is held to the existing-dwellings table and to the reasonable-standard test for renovated thermal elements¹⁰.

For energy independence, Part L is a fabric rule rather than a supply rule. It reduces the heat a dwelling needs, which reduces what has to be bought from a supplier or generated on site. It does not make a home independent of the grid, and it does not require or forbid any particular heating technology, since it is in general technology and fuel neutral⁵. What it does is set a floor under how leaky and how poorly insulated a dwelling may be after work is done.

The dependence that remains is on the compliance route and on the assessment. A dwelling's performance is demonstrated through SAP calculations and, for new dwellings, through air-tightness testing¹. Those are carried out by others, and the household relies on their accuracy. Where a home is made tighter, the ventilation strategy has to keep pace, or the gain in heat retention is paid for in air quality and condensation¹⁷.

For the wider picture, the U-values and R-values page explains the units in more depth, replacement window rules covers building control and certification for glazing work, and airtightness and air leakage deals with the testing regime. The pillar guide sets the whole subject in context.