GROUND FLOOR AND FLAT ROOF U-VALUES IN PART L 2021

When compared to Part L 2013’s requirements, Part L 2021 features lower notional dwelling U-values and tighter limiting U-values.

The values changed for all thermal elements, including ground floors and flat roofs, which are the two main uses for extruded polystyrene insulation (XPS).

Effective from June 2022, Part L 2021 applies in England and is the focus of this blog post. However, changes to Part L in Wales and Section 6 in Scotland have also introduced improvements, with the similar aim of reducing the energy use and carbon emissions associated with the built environment.

What is the notional specification in Part L 2021?

<p”>The overall aim of Part L 2021 is to deliver a 31% reduction in carbon emissions from dwellings, and a 27% reduction in carbon emissions from buildings other than dwellings, compared to Part L 2013. It is an uplift towards the Future Homes and Future Buildings Standards, both expected in 2025.

As has been the case since 2006, dwellings are assessed against a notional dwelling, which has a specification as defined in Approved Document L1. A range of factors are taken into account alongside building fabric performance, including airtightness, ventilation, heating, hot water and renewable technology. The table below summarises the U-values only.

Notional specification U-values (W/m²K) for main thermal elements of dwellings
Thermal elementPart L 2013Part L 2021
Ground floor0.130.13
External wall0.180.18
Flat roof0.130.11
Pitched roof0.130.11

While it might appear that relatively little has changed in terms of thermal element U-values, it’s worth bearing in mind that the actual U-values that a project will need to achieve depends on the specification of all those other factors we described above.

Ground floors often represent an excellent opportunity to include more insulation without significantly impacting the structural design of the building, so balanced and economical Part L 2021 specifications may feature ground floor U-values lower than 0.13 W/m²K, with roofs likely to need to be at or around the 0.11 W/m²K in the notional specification.

For buildings other than dwellings, whole building assessments are still required. However, the notional building specification does not feature in Part L 2021. Instead, Approved Document L2 lists limiting values that thermal elements must achieve.

U-values (W/m²K) for main thermal elements in buildings other than dwellings
Thermal elementPart L 2013 notional specificationPart L 2021 limiting values
Ground floor0.220.18
External wall0.260.26
Flat roof0.180.18
Pitched roof0.180.16

What are limiting U-values in Part L 2021?

The notional dwelling/building approach offers designers and specifiers flexibility in achieving a compliant specification. However, to ensure that building fabric performance is not compromised as part of that flexibility, the regulations include limiting or ‘worst-case’ U-values that thermal elements cannot exceed.

Limiting U-values (W/m²K) for main thermal elements of dwellings

Thermal element

Part L 2013

Part L 2021

Ground floor

0.25

0.18

External wall

0.30

0.26

Flat roof

0.20

0.16

Pitched roof

0.20

0.16

As we have seen, in Part L 2021 the limiting values for buildings other than dwellings now serve as the main guide to thermal element performance, as well as being the worst-case values that should be achieved.

Limiting U-values (W/m²K) for main thermal elements of buildings other than dwellings

Thermal element

Part L 2013

Part L 2021

Ground floor

0.25

0.18

External wall

0.35

0.26

Flat roof

0.25

0.18

Pitched roof

0.25

0.16

As part of our extensive technical services, Polyfoam XPS provides U-value calculations for any thermal element featuring our insulation products, including ground floors and flat roofs. Contact us with details of your build-ups, or to discuss how extruded polystyrene can help you to achieve your project U-values.

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