Guide
Insulation requirements for commercial and residential buildings
Quick-reference guide to insulation U-value requirements by building element, building type, and work type under Part L of the Building Regulations 2010. Covers wall, roof, and floor insulation options, fire safety considerations for external insulation, and PAS 2030/2035 retrofit standards.
Insulation requirements under Part L
This reference covers insulation U-value requirements for walls, roofs, and floors under Approved Document L of the Building Regulations 2010. Requirements differ depending on the building type (domestic or non-domestic), the type of work (new build, extension, renovation, or replacement of a thermal element), and the building element being insulated.
U-values measure how much heat passes through a building element. A lower U-value means better thermal performance. All U-values below are maximum permitted values for England.
U-value requirements by building element
Non-domestic buildings (Part L Volume 2, 2021 edition)
Domestic buildings (Part L Volume 1, 2021 edition)
Requirements by work type
New builds and extensions
New buildings and extensions must meet the full U-value targets listed above. Actual design U-values may need to be better (lower) than the limiting values to meet the overall target CO2 emission rate for the building. Extensions must meet the same U-value standards as a new build.
Renovation and replacement of thermal elements
When you renovate a thermal element (wall, roof, or floor) beyond specified thresholds, you must upgrade the insulation to current Part L standards under Regulation 23.
Wall insulation options
Choosing between cavity fill, EWI, and IWI
The best approach depends on your existing wall construction, exposure to driving rain, planning constraints, and budget:
- Cavity wall insulation is the most cost-effective option for existing cavity walls. It is minimally disruptive and typically takes one day. Not suitable for walls with existing moisture problems or in areas of severe exposure without appropriate product certification.
- External wall insulation (EWI) wraps the building in insulation and a protective render or cladding system. It avoids internal disruption and eliminates thermal bridges, but changes the external appearance. Planning permission may be needed in conservation areas. Fire safety requirements under Part B and the Building Safety Act 2022 apply to external insulation on buildings over 11 metres.
- Internal wall insulation (IWI) adds insulated dry-lining to the inside of external walls. It does not alter the external appearance (important for listed buildings) but reduces room sizes, requires redecorating, and can create condensation risks if not properly detailed.
Roof insulation
Floor insulation
Fire safety considerations for external insulation
External wall insulation systems on buildings above 11 metres must meet Building Regulations Part B fire safety requirements. Following the Grenfell Tower fire, the Building Safety Act 2022 introduced additional obligations:
- External wall fire review is required for buildings over 11 metres tall
- Combustible materials (including certain insulation types) are banned on the external walls of buildings over 18 metres under the Building (Amendment) Regulations 2018
- Products must achieve a European classification of A2-s1,d0 or better (Class A1 or A2) for buildings over 18 metres
- For buildings between 11 and 18 metres, a fire risk assessment of external wall systems is still required
Even for buildings below 11 metres, designers and installers should consider fire performance when specifying external insulation materials.
PAS 2030/2035 retrofit standards
If insulation work is funded through a government scheme (such as ECO4 or the Great British Insulation Scheme), it must comply with PAS 2030:2023 (installation standard) and PAS 2035:2023 (retrofit framework). Key requirements include:
- A retrofit assessment by a qualified retrofit assessor before work begins
- A retrofit coordinator overseeing the project
- Work carried out by a TrustMark-registered installer certified to PAS 2030
- A medium-term improvement plan to avoid unintended consequences (such as condensation or overheating)
Even where PAS 2030/2035 is not mandatory, following the framework is considered best practice for retrofit insulation work.