Description

The first part of the Thermal bridging in external timber-framed walls project found that the percentage of timber framing in the walls of new houses was much higher than generally assumed. This suggests that designed R-values are not being achieved in practice.

Part 2 of the project, outlined in this Research Now, focused on the causes, impacts and potential solutions to the problem.

The result of this research:

1. This research found that standard 90 mm timber-frame construction complying with the Building Code produces new-build houses with actual whole-wall construction R-values that vary between R1.2–R1.4, even with R2.8 insulation installed. This highlights a sizeable performance gap between what is achieved in reality and the R-values calculated to meet Building Code compliance of R1.9/R2.0 in NZS 4218:2009.
   
2. The research found little unnecessary timber in wall framing – each piece is added for valid regulatory and practical reasons. Even one of New Zealand’s most experienced frame and truss detailers working on a simple design could not reduce the framing percentage below 25%. Optimising the framing percentage alone will not achieve actual entire wall R-values that match those of minimum construction R-values for wall areas in NZS 4218:2009. A number of homes already built or currently under construction have overcome the problem of thermal bridging by using a second layer of insulation. This creates a thermal break between the timber framing and the external environment as well as providing space to increase the thickness of the insulation.

Scope

This Research Now includes:

• Part 2 of the project
• Exploring drivers for higher framing percentages
• The impacts of high framing content and weak points
• Advanced framing and insulation solutions
• Two layers of insulation offer a range of benefits
• Conclusion