What target U-values should you use for the walls, roof, floor and windows of your building?
Unfortunately, there is no single set of U-values that every building should use. Your targets need to meet the relevant Building Regulations, while also being appropriate for the building, its energy and sustainability goals, and the level of performance you are trying to achieve.
In this article, we compare a few well known UK U-value benchmarks, including Part L, LETI and AECB CarbonLite, and explain how you can use them to select suitable targets for your project.
What is a target U-value?
A U-value measures how easily heat passes through a building element, such as a wall, roof, floor or window. The lower the U-value, the less heat is transferred through it.
A target U-value is simply the U-value that the design is aiming to achieve for that specific element.
These values are important when designing a building and when carrying out digital simulations to test how the building is likely to perform. They can affect Part L compliance, which forms part of the Building Regulations, as well as heating and cooling demand, overheating risk, energy consumption and the size of the heating and cooling systems required.
U-values can also affect the thickness of walls, roofs and floors, the amount of insulation required and construction costs. Using more material can also increase embodied carbon, which is the carbon associated with producing and constructing the materials used in the building.
This is why the lowest possible U-value is not automatically the right answer. The aim is to select fabric performance that works well for the building as a whole.
UK target U-value benchmarks
The table below compares some of the most widely recognised UK regulatory and industry benchmarks.
All U-values are in W/m²K
| Source | Wall | Roof | Floor | Windows | Doors |
| Part L Vol. 1: New dwellings | 0.26 | 0.16 | 0.18 | 1.60 | 1.60 |
| Part L Vol. 1: New/replacement elements in existing dwellings | 0.18 | 0.15 | 0.18 | 1.40 | 1.40 |
| Part L Vol. 1: Renovated existing elements | 0.30* | 0.16 | 0.25 | 1.40** | 1.40** |
| Part L Vol. 2: New/replacement elements | 0.26 | 0.16 pitched / 0.18 flat | 0.18 | 1.60 | 1.60 |
| Part L Vol. 2: Renovated existing elements | 0.30* | 0.16–0.18 | 0.25 | 1.60** | 1.60** |
| LETI: Commercial Offices | 0.12–0.15 | 0.10–0.12 | 0.10–0.12 | 1.0 triple / 1.2 double | 1.20 |
| LETI: Best Practice Retrofit | 0.18 | 0.12 | 0.15–0.18 | 1.00 | 0.80 |
| LETI: Exemplar Retrofit | 0.15 | 0.12 | 0.15 | 0.80 | 0.80 |
| AECB CarbonLite: Component Method guidance | EnerPHit criteria*** | EnerPHit criteria | EnerPHit criteria | <1.20 | <1.20 |
* Part L gives 0.55 W/m²K where the improvement is cavity wall insulation only.
** Replacement windows and doors are subject to the relevant Part L limiting values.
*** AECB gives <0.40 W/m²K for internal wall insulation and refers to EnerPHit criteria for other opaque elements.
Download Target U-Value Table below.
Part L: the regulatory starting point
Part L is part of the Building Regulations in England and covers the conservation of fuel and power. The u-values specified in Part L are regulatory limits, not best-practice targets.
They are therefore a useful starting point when selecting U-values, but a project may need or choose to perform considerably better.
LETI: best-practice benchmarks
LETI, the Low Energy Transformation Initiative, is a voluntary network of built-environment professionals producing guidance to help the UK move towards net zero carbon.
LETI’s Climate Emergency Design Guide provides indicative high-performance fabric targets for building types including commercial offices.
Its separate Climate Emergency Retrofit Guide provides best-practice and exemplar retrofit targets for homes. These are voluntary benchmarks rather than regulatory requirements.
AECB CarbonLite: a low-energy building standard
AECB, the Association for Environment Conscious Building, is a UK organisation focused on low-energy and environmentally responsible buildings.
CarbonLite is its voluntary low-energy building standard. Unlike LETI’s simple fabric benchmark tables, CarbonLite is primarily concerned with whole-building performance, including space-heating demand, airtightness and ventilation as well as fabric performance.
Its Component Method guidance also refers to EnerPHit criteria for a number of building elements rather than providing one universal U-value for every construction.
What target U-values should you actually use?
Start with the Part L requirements, because the proposed fabric needs to comply with the Building Regulations that apply to the project. From there, the question is how much further the design should go.
A useful approach is to compare a few different levels of fabric performance, for example a Part L-level specification, an intermediate option, and a more ambitious LETI-based specification. The right choice will depend on the project, because lower U-values can affect more than just heat loss as explained above.
To work through these issues, feasibility studies supported by energy modelling can be used to test different U-value options and simulate how each would affect the building before the specification is fixed. This helps show which option gives the best overall balance of compliance, energy performance, comfort, carbon and cost for the project, rather than simply choosing the lowest U-value possible.
Need help selecting U-values?
If you need any help selecting appropriate U-values for your project, or have any questions or comments about the information on this page, please feel free to contact us.