U-Value Requirements – Do you need 150mm cavities?

The updated Approved Document L: Conservation of fuel and power, Volume 1:Dwellings; 2021 edition incorporating 2023 amendments – for use in England which came into force on 15 June 2023, attempts to integrate u-value requirements for both new dwellings as well as existing dwellings.

It consolidates the previous Approved Document L1A: Conservation of fuel and power in new dwellings and Approved Document L1B: Conservation of fuel and power in existing dwellings into one document.

Emphasis is placed on SAP calculations and the notional dwelling for new dwellings and some extensions as well as stricter u-value requirements for extensions and refurbishments.

Chimneys and Fireplaces: Design Considerations

In the following sections we will look at what the notional dwelling and when SAP calculations are required, what the the specific u-value changes for both new dwellings, extensions and refurbishments, how these requirements can be met as a designer with both 100mm and 150mm cavities and other key considerations when designing junctions just as cold bridging.

In this post we will look at:

  • What is a notional dwelling
  • When are SAP calculations required
  • What are the specific u-value changes for new dwellings, extensions and refurbishments
  • Can the requirements be met with both 100mm and 150mm cavities
  • Other key considerations such as cold bridging

Let’s explore the specific u-value changes for both new dwellings, extensions and refurbishments, and how these requirements can be met as a designer with both 100mm and 150mm cavities…

Notional Dwelling and SAP Calculations

One of the key changes is the focus on how energy calculations are conducted. Older regulations advised on different u-value parameters depending on what type of calculation was being conducted.

The updated regulations state that all new dwellings must use the Government’s Standard Assessment Procedure (SAP)  for energy rating of dwellings which sets out the full properties of the notional dwelling.

This also applies to extensions if the total area of glazing to the extension is more that 25% of the floor area of the extension PLUS the total area of any windows and doors which no longer exist or are no longer exposed due to the extension.

The notional dwelling is the same size and shape as the actual dwelling and has standardised properties for fabric and services. A summary of these properties can be found in Table 1.1 within the regulations. The SAP calculation takes into account all the standard building elements such as wall and floor u-values, as well as the orientation and size of openings, specification of windows and door, heating systems, air leakage and renewable technologies where applicable. 

01 Table 1.1 Part L

Notional Dwelling Specification for New Dwelling from Approved Document Part L of the English Building Regulations

As designers, we are encouraged to use a fabric first approach by using the parameters in Table 1.1 to calculate the u-values of fabric elements such as walls, roofs, floors, etc. If these are used at an early stage in the design, the external envelope should meet the requirements when the official SAP calculations are conducted before and after construction. Some offices do these in-house or hire sustainability consultants to complete the calculation.

Sometimes minor changes to the design are required following the calculation and where a u-value can’t be met, the calculation can sometimes fall back to the limiting standards set out in Table 4.1, as the SAP calculation can compensate for this in other areas.

Update to BR Part L – New Dwellings

The updated BR Part L which came into force in June 2023 integrates all dwelling requirements into one document, consolidating tables and increasing thermal requirements. 

The tables below highlight the specific u-value requirements which have changed, the left hand side shows the up to date regulations and the right hand side shows the existing regulations.

02 U-Value Tables New Dwellings Table 1.1

Most of the changes to u-values in the  notional dwelling specification, Table 1.1 for new dwellings, do not actually apply to external walls. The requirements for an external wall have not varied from 0.18W/(m2⋅K). The new regulations have, however, gone into more detail of requirements for windows and doors based on the glazed area as well as increasing roof u-values requirements from 0.13W/(m2⋅K) to 0.11W/(m2⋅K).

03 U-Value Tables New Dwellings Table 4.1

Whilst there are a large number of changes to Table 4.1 “Limiting U-values for new fabric elements and air permeability in new dwellings”, this table should only be used as a last resort while conducting a SAP calculation and should not be used when designing build ups.

Update to BR Part L – Extension and Existing Dwellings

As part of the updates extensions and existing dwellings have been incorporated into the same regulations document as new dwellings. 

The new BR Part L requires SAP calculations to extensions if the total area of glazing to the extension is more that 25% of the floor area of the extension PLUS the total area of any windows and doors which no longer exist or are no longer exposed due to the extension.   

If this does not apply to your design, below are the updated U-value for both extensions and refurbishment of existing thermal elements to existing dwellings. The table on the left hand side highlights the updated u-values requirements and the table on the right hand side shows the previous values.

03 U-Value Tables New Dwellings Table 4.1

As we can see in the table above, new fabric elements in existing dwellings, such as an extension will require higher u-value for roofs, walls and floors. Requirements for a new wall have increased from 0.28W/(m2⋅K) to 0.18W/(m2⋅K), matching the u-value requirement for a new dwelling. This is a significant change. 

05 U-Value Tables Existing Dwellings Table 4.3

If conducting upgrades to existing elements of an existing dwellings, such as adding insulation to an existing external wall, only the requirements for roofs has increased. The new requirement of 0.16W/(m2⋅K) applies to all existing roofs, whether pitched with insulation at ceiling or rafter level or flat, if the existing elements do not meet the threshold of 0.35W/(m2⋅K).

How to meet the requirements – 150mm Cavities vs 100mm cavities and internal insulation

04 U-Value Tables Existing Dwellings Table 4.2

100mm cavity full-fill insulation with internal insulation and 150mm cavity full-fill insulation

Traditional masonry construction usually consists of a double skin wall with 100mm cavity, fully or partially filled with insulation. With the updated regulations, much of the advice out there has been centred on increasing cavities from 100mm to 150mm. However, for new building dwellings the requirements have not increased. For existing dwellings the requirements have increased to 0.18W/(m2⋅K).

In order to comply with the increasing thermal requirements a standard cavity wall construction may require upgrading depending on the insulation specification. 

Two simple ways of improving a masonry wall’s thermal performance is by:

  • Increasing the standard 100mm cavity within a masonry wall to 150mm with full fill cavity insulation or 
  • Building a masonry wall with a cavity of 100mm and adding internal insulation to the blockwork wall 


Pros of 150mm full fill cavity insulation

  • Ease of construction in comparison to 100mm cavity with internal insulation.
  • Depending on the insulation specified, it will meet Part L requirements. 


Cons of 150mm full fill cavity insulation

  • Thermal bridging can be higher at junctions and 
  • May still require internal insulation to solve cold bridging and to achieve desired U-value.
  • Wider foundations are not typical and will be harder and more costly to dig and build.
07 DL150 100mm full-fill insulation within a cavity masonry wall with internal insulation
DL150 – 100mm full-fill insulation within a cavity masonry wall with internal insulation
07 DL150 100mm full-fill insulation within a cavity masonry wall with internal insulation
DL156 – 150mm full-fill insulation within a cavity masonry wall

As u-value requirements are increased, and some councils or designers require better thermal performance than that stated in the building regulations, 150mm may become more commonplace and some of the disadvantages may disappear such as the issues with foundations. Cavities wider than 150mm may be even harder to construct due to the need of possible custom brick ties, possible thicker structural zone to support this, possible non-standard lintels and cavity barriers.

To help detail 150mm cavities, we have put together a set of details demonstrating a 150mm cavity construction, showing foundation through to window junction and roof. You can find the set here.

Cold Bridging and Psi Values

Another key area to understand is thermal bridging, commonly referred to as ‘cold bridging’. This phenomenon occurs when there’s a continuous path for heat flow across a temperature difference in an object that has differing thermal resistance. In buildings, these are areas where the insulation layer is interrupted, and heat can flow more readily, leading to localised cold spots.


09 Psi values taken from Undertanding Passivhaus A simple guide to Passivhaus detailing and design

Psi value illustrated calculations taken from Understanding Passivhaus showing Psi install values for different positionings on a window

Psi Values (Ψ-values) provide a metric to quantify this. A Psi value represents the linear thermal transmittance through a junction (the heat lost per unit length). Lower Psi values mean less heat is being lost through thermal bridges.

To ensure compliance with Part L and more holistic energy efficiency, architects must:

– Design to minimise repeating thermal bridges (e.g. at joist ends).

– Detail junctions to reduce the impact of non-repeating thermal bridges.

– Use thermal bridge assessment software or consult experts to calculate Ψ-values.

– Opt for insulating materials that inherently reduce the risk of cold bridging.


The recent updates to the England Building Regulations Part L are not just legal stipulations; they represent the future of construction. By understanding and implementing these requirements, architects and specifiers are not only adhering to the law but also pushing the envelope in sustainable construction. 

Energy-efficient buildings are the future, and understanding u-values, insulation, cold bridging and air leakage is fundamental in creating structures that stand the test of time and environmental scrutiny.

Get the Details

We have put together a set of details showing 150mm cavities to help pack in that extra insulation required to meet current u-values. 

We also have a huge amount of Passivhaus details where u-value requirements are higher than those in the Building Regulations as well as providing all of our details to the latest Part L requirements. 

The detail library provides a large range of different construction assemblies and build ups to provide varied options that can be adjusted for all sorts of different domestic and residential projects. If you are struggling to find the detail you are looking for, get in touch and we would be happy to guide you.

Links and Resources

The updated and existing Building Regulations Approved Document Part L can be found following this link.

There are a large amount of resources available to help with the above. Most insulation manufacturers / suppliers should be able to help with u-value calculations as well as condensation risk, location of dew / condensation points and VCL, DPM and DPC layers. Below is a short list of some of the most common insulation manufacturers:

Rockwool – https://www.rockwool.com/uk/resources-and-tools/tools/u-value-calculator/

Kingspan – https://www.kingspan.com/gb/en/campaigns/u-value-calculations/

Celotex – https://insulation-uk.com/technical-services/tools/online-u-value-calculator 

Knauf – https://www.knaufinsulation.co.uk/uvalue-calculator

Gutex – https://gutex.co.uk/constructions/

Termafleece – https://thermafleece.com/resources/application-notes

Hemspan – https://hemspan.com/


There are also a number of u-value calculators not associated with insulation manufacturer’s such as:






Extra care should always be taken with renovations and extensions, to maintain existing levels of air leakage or if air tightness is being improved, installation of VCLs. A useful document for this can be found here.


Written by Aida Rodriguez-Vega, architect and researcher. At the Detail Library, Aida keeps busy by carrying out technical research and drawing new details for the ever-growing library.

Links and Image Credits

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Architect credit: Studio MR [https://studiomr.co.uk/]

Photographer credit: Studio MR [https://studiomr.co.uk/]


Source URL: https://www.iqglassuk.com/products/flush-threshold-drain-for-doors/s62497/

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