England & Wales Building Regulations: Part L: Conservation of fuel and power - Party wall thermal bypass (Version 1)

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Approved Document L (ADL) 2010 is scheduled to come into effect in England and Wales in October 2010. There has been a number of changes throughout the document, ranging from improved backstop U-values, a requirement to increase the amount of air tightness testing and an overall improvement in the dwelling emission rate. All of these changes or improvements represent a significant increase from the 2006 requirements.

Prior to the 2006 edition of Part L, the thermal performance of a party wall was assumed to be 'perfect' (with a U-Value of 0.0 W/m²K) based on the fact that both sides are within the heated envelope of a building so there should be no temperate differential from one side to another.

One of the key changes to ADL2010 is that party walls must now be assigned a U-Value based on certain criteria, and that this U-Value is used in SAP calculations to determine a building's CO2 emissions.

This Construction Briefing discusses why the changes to Part L were necessary and what potential consequences for timber frame construction could be.


  • Background
  • The requirement 
    • Table 1 - Approved Document L 2010 U-values for party walls
  • Effective edge sealing
  • Cavity insulation

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