A new take on wall construction

Zero Energy House

The construction of the walls (and roof) significantly influences the thermal performance of any building. In the case of the Zero Energy house, it not only has an effect on the comfort of the people who live in it, but also on its ability to achieve the goal of not using mechanical systems for heating or cooling. If heat were constantly escaping through the walls, the energy from the sun wouldn’t be enough to keep the house warm and more energy would have to be introduced into the building to maintain the target temperature range.

Most New Zealand homes need a heating system to keep the house warm. The insulation levels in the Zero Energy House mean that target temperatures (between 16-24℃ in bedrooms and 18-28℃ in living areas) can be maintained primarily by the energy from the sun, but also from day-to-day living (heat from occupants and electrical loads such as lighting and cooking).

Wall construction

Shay Brazier and Jo Woods, owners of the Zero Energy House, along with their design team sought to identify a practical, highly-insulated solution to wall construction that provided an alternative to the standard 90mm timber framing used in most New Zealand homes.

The insulation value (R value) of a wall is calculated using the R value of the insulation material and the percentage of framing in the wall, as timber is not as good at resisting the flow of heat as the insulation is. Standard 90mm framing with nogs has about 16% thermal bridging from the studs and nogs, meaning that heat can escape from the building through these points. The challenge for the design team of the Zero Energy House was to reduce the thermal bridging percentage and increase the R value of the walls.

Some of the options considered are shown below.

External battening, 90mm stud, no nogs.
140mm studs, 90 nogs, external 35mm insulated sarking board.
Internal battening, 90mm stud, 90mm nogs.
140mm studs, 90 nogs.

The solution they arrived at was to remove the nogs typically found between the studs and replace them with supporting battens that run along the inside of the wall. The same approach is taken in the skillion roof.

This method, which requires virtually no extra timber, achieved two key improvements over a standard framing approach:

  1. Reduced thermal bridging. By removing the nogs, uninterrupted insulation could be run vertically between the studs for the full height of each floor. With standard framing, panels of insulation are placed between each nog, with each nog acting as a thermal bridge. Taking out the nogs reduced this bridging effect, thus increasing the insulation properties of the walls.
  2. A second insulation layer. With supporting battens on the inside of the walls and ceiling, a space was created between the battens to run a second layer of insulation horizontally. This not only provided a thicker overall insulation layer, it also laid insulation over the studs - which reduced the thermal bridging effect even more.

An additional benefit of this framing method is that it created an internal wiring and plumbing cavity which allowed quicker installation of pipes and wires as they could be laid outside of the studs rather than having to be drilled through.

Insulation

It’s important to remember that the insulation requirements in the Building Code are a minimum specification that aims to prevent buildings from being unhealthy. People, however, are encouraged to go beyond those basic levels.

At the Zero Energy House, the reduction in thermal bridging and the use of double insulation layers enabled Shay and Jo to achieve a 50% increase in performance above Building Code. For the couple, investing in a little more insulation seemed worth it when considering the cost of heating a home for 25 years.

In addition to the efforts made during the design stage, Shay and Jo also made sure the quality of construction achieved very high standards. When installing the insulation, for example, even a 10mm gap can have a big impact on the performance of the building envelope, as heat will use this path to escape from the building. Also, segments need to be carefully cut to match the size of the frame, avoiding folds. With the help of friends and family, the couple installed the insulation product themselves over a few weekends. They chose an insulation product that was durable, while also being healthy to both install and live with. The batts they used had a 50-hyear warranty, were made with a high content of recycled materials and can also be recycled at the end of their life.

Showing the double layer of insulation, one of the keys to living without heating.
Completed double layers of insulation in upstairs rooms

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