Technoform recently promoted their Thermal Isolator Clip in conjunction with Nu-wall’s aluminium cladding and rail system at the Passive House Hui in Wellington. The Nu-Wall cladding system and Technoform Thermal Isolator utilise aluminium with 80% recycled content and thermal breaks to; reduce the carbon footprint, eliminate thermal bridging, and deliver a low maintenance and non-combustible facade system.
A large focus of the Hui was how to deliver Passive House, warm, dry, healthy homes at scale. The trouble with medium density housing in New Zealand is the extensive thermal bridging caused by high timber content framing. The result of which is reduced insulation fitted within the wall, and multiple thermal bridges between the exterior and interior of the building.
When completing H1 calculations only thermal bridging caused in clear walls is taken into consideration, with no consideration to internal corners, external corners, inter-storey junctions, window penetrations, slab edge etc.
In these assumptions, timber only accounts for 14 to 16% of thermal bridging in the wall. However, in a recent study by Beacon Pathway in conjunction with BRANZ it was found that the average house had 34% timber content with some as high as 57%.
What does this mean for the wall R value?
If a typical timber stud wall spaced at 600mm centres with 16% thermal bridging has an R value of around 2.4 m2K/W, when we adjust the timber to wall ratio to 34% we actually only achieve an R value of 1.4 m2K/W. This is a reduction of 42% and is below the schedule method requirements in H1 which is 1.9 m2K/W.
This means we are significantly underestimating the amount of energy that is being lost through walls which will mean considerably more energy (power) required to keep homes warm, dry and healthy.
What is the solution?
Insulation needs to be used where it works best: on the outside. Externally insulated facades work by placing fit-for-purpose mineral wool insulation on the outside of the structure. As the insulation runs continuous on the outside, all the thermal bridges caused by timber framing are covered.
The difficulty with continuous insulation is connecting the cladding rail system back to the structure whilst minimising thermal bridges from any form of fixing and maintaining a weathertight seal. The Technoform Thermal Isolator Clip in conjunction with the Nu-wall cladding system is a solution to this problem. The isolator clip is a thermally broken, non-continuous fixing system that is laid out on the structure in a grid pattern. Insulation is then installed around the clips with the rail and cladding being installed afterwards.
Detailed 3D thermal analysis was completed by Jason Quinn of Sustainable Engineering Ltd. to determine the thermal bridging caused by the Technoform Thermal Isolator Clip. This analysis allows us to determine the effective R value of walls using different depths of insulation and the appropriate isolator clips.
The R values calculated below were determined with isolator clips set out on a 600 x 600mm grid with no insulation installed within the framing cavity:
- 50mm external insulation: 1.90 m2K/W
- 75mm external insulation: 2.60 m2K/W
- 100mm external insulation: 3.25 m2K/W
The R values calculated below were determined with isolator clips set out on a 600 x 600mm grid with 90mm insulation installed within the framing cavity:
- 50mm external + 90mm internal insulation: 3.00 m2K/W
- 75mm external + 90mm internal insulation: 3.45 m2K/W
- 100mm external + 90mm internal insulation: 4.14 m2K/W
What is clear from the results is that very high R values can be achieved with 90mm stud framing when using external insulation without the need to go to 140mm framing or an internal service cavity.