Timber framing and wall thermal performance

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The case for having a well-insulated house is now well established; there are proven health, energy efficiency, and financial benefits.  However, insulation effectiveness is compromised by thermal bridging - this is where heat escapes through materials which conduct heat more easily.  Timber is one of those materials. 

Timber framing has a significantly lower R-value than the bulk insulation materials typically used in walls of new houses. As framing content increases, wall R-value, as a whole, decreases accordingly.

Funded by the Building Research Levy, the Stage One of the Wall Project measured the extent of timber framing in 47 new builds in Auckland, Christchurch, Wellington and Hamilton.

Frame and Truss (F&T) manufacturers supply in excess of 90% of the framing to new residential builds in New Zealand.  The research approach utilised hard copies of F&T panel elevations and plan layouts on site to assist with data gathering.  The team also undertook a series of interviews with major suppliers to industry, cladding manufacturers, and representatives from F&T, which proved crucial to understanding how the design process worked and what drove design decisions.

 

The findings

The results show that the average percentage of timber framing compared to the area of the wall is above 34%. This is much higher than the 14 - 18% framing content generally assumed by both regulators and the industry when undertaking compliance calculations.

As  more timber framing in the wall means more limited space for insulation, this suggests that the overall thermal performance of the walls is lower than calculated.

Little additional framing was added on site  - the average additional site-added, full-depth framing is just 0.7%.  Builders and subtrades are not responsible for adding unnecessary framing.

Rather, the research highlighted evidence that increasing framing content is being driven by the requirements of various regulations, mostly in relation to structure and weathertightness (including cladding requirements).

 

 


  • 22-Sep-2020 (Report Wall/3)

    Measuring the extent of thermal bridging in external timber framed walls in New Zealand (PDF 3.4MB)

    Verney Ryan, Guy Penny, Jane Cuming, Ian Mayes, Graeme Baker

    This report shares the results and findings of a project to investigate the extent of thermal bridging in external timber-framed walls of new builds. The team investigated the percentage of framing in external walls in 47 newly constructed dwellings from Auckland, Christchurch, Wellington and Hamilton. Timber conducts heat, so added framing means more heat loss.

Research funded by: