New Part L timber frame guidance

With the changes to the Building Regulations Approved Documents Part L due for October, TRADA have produced two new electronic publications focusing on thermal performance.

These are: Timber frame: standard details for thermal performance. Volume 1: Mineral wool insulation and Volume 2: Rigid foam insulation. Autumn 2010 will see the publication of a further two volumes of ‘optimised' details enabling designers to select and combine details from either set to achieve regulatory requirements.

The Building Regulations in England and Wales impose increased performance requirements with the 2010 revision of Approved Document L: Conservation of fuel and power that takes effect in October. Compared to the 2006 Part L, the new rules demand a 25% improvement in thermal performance. TRADA Technology's Building Performance Service has identified 20 critical details that arise in timber frame construction, and has calculated the Ψ-Value in each instance using ‘standard' details previously published in TRADA's book and CD Timber Frame: Standard Details for Houses and Flats.

Designers following The Code for Sustainable Homes will find the thermal target for Code Level 3 matches the new 2010 Part L. Code Levels 4, 5 and 6 will be even harder to achieve. The 2010 Part L requires designers to analyse the entire fabric, with particular emphasis on cold bridging. The measure of heat loss via a cold bridge is the linear thermal transmittance (Ψ-value), measured in W/mK.

These new publications explain what cold bridging is, how to calculate the Ψ-value and how to demonstrate compliance when using the standard details with mineral wool and rigid foam insulation. Working out the Ψ-values needed to input to The Standard Assessment Procedure (SAP) can be taxing. Fortunately, software now exists to do the calculations. But even with the software, it can be a tedious process. SAP lists the situations in a typical building where cold bridging is likely to have a significant effect on overall thermal performance.