Part L U-values: What are they?

Part L U-values

Part L U-values: What are they?

Part L of the Building Regulations contains ‘limiting U-values’ for insulating fabric elements in new dwellings (excerpt below). These are sometimes mistakenly referred to as ‘minimum U-values' however this term is not quite accurate.  

Whilst these indicate a minimum thermal performance requirement for each element, they are actually maximum U-values (i.e. heat transfer through the element should not exceed this value).  

It’s important to note that matching the limiting U-values throughout a build will not result in compliance overall. 

Limiting U-values for new fabric elements in new dwellings 

Element type 

Maximum U-value W/(m2K) 

All roof types 






Party wall 


Source: Approved Document L. View the full Limiting U-values table here. 

The updated regulations also include a ‘notional dwelling’ - a recipe showing an example route to compliance for a typical home. The U-values in the notional dwelling are significantly lower than those in the table above, but that doesn’t mean you need to rigidly copy them, in order to meet the regulations. 

Compliance can be achieved in a much smarter and more cost-effective way.  


Let’s look at an example… 

Three-bedroom semi-detached house, two wet rooms, gas combi boiler 

The Part L notional dwelling suggests a wall U-value of 0.18 W/(m2K) but you could also achieve compliance with improvements to floor and roof insulation and the air tightness of the building, alongside low-cost wastewater heat recovery.  

Using a simple ventilation system would mean including some rooftop solar PV, or alternatively this could be avoided by using upgraded mechanical ventilation with heat recovery.  


Find the best route for you 

There are lots of ways to comply with Part L – it’s about finding an approach which works for your project.  

Here are three more example routes you could take, depending on your priorities: 

  1. Prioritising flexibility 
  2. Prioritising simplicity 
  3. Prioritising technology 

Again, in all three examples, the loft insulation goes above and beyond the U-values outlined in the notional dwelling. Loft insulation is a relatively cost-effective and easy way to increase the overall thermal efficiency of a home, allowing you the freedom to choose cavity insulation and heating technology that best suit your requirements and budget. 


Your next steps 

To find your best solution, undertake calculations using the latest SAP methodology and determine the ideal whole-house recipe for your specific build. 

For further advice, speak to our Technical Support Team. 

Use Knauf Insulation’s online U-value calculator Use Knauf Insulation's online Psi-value calculator