Unexpected Energy Rating Killers – Nathers


Have you ever designed a house that looked amazing, but didn’t perform as well as expected from a thermal performance point of view and you couldn’t work out why? Was your NatHERS assessor able to discuss with you why your home fell short? In this blog we discuss three design features that are lesser known energy rating killers.

1: Raked Ceilings –

Everyone loves the drama of raked ceilings, yet the extra space that the occupants get to enjoy is wreaking havoc in the roof space above the ceiling. Minimising the air gap between the ceiling and the roof reduces the R-Value in the ‘whole of roof’ construction. Heat transferring through the exterior roof and sarking has less distance to travel to reach the ceiling and a smaller volume of air in the roof space means the roof space temperature will rise and fall more rapidly. 


2: Operable glazing –

To make the most of prevailing winds in the area where the home will be constructed, make sure not to overlook the benefits of being able to open a window. Whilst fixed glazing will generally offer a better U-Value and SHGC than operable glazing, if you are struggling with the cooling load being too high, then using different types of glazing may help. The NatHERS software takes into account the amount a window can be opened to allow fresh air to enter the home. If there are large amounts of fixed glazing and the home is too hot, then specifying operable glazing on opposing sides of the home could greatly reduce the cooling load. This is due to the benefits of cross flow ventilation. Another area of consideration is any window type that would be reasonably expected to have safety restrictors, such as second storey windows. Windows with safety restrictors or without complying security screens can only be modelled at 10% operability unless otherwise specified.


3: Small rooms –

Small rooms such as mud rooms, butler’s pantries, tool rooms, cellars (if not underground) and in some cases airlocks must be modelled as separate ‘daytime’ zones according to the NatHERS technical notes. If your home has multiple small zones within the building envelope, often it will struggle in its energy rating. This is because the software views all of these individual zones as ‘high use’ daytime areas and rates them accordingly. Quite often when an assessment is performed on a house with multiple small rooms and the rating is less than what is desired, the problem can be traced back to the high heating or cooling loads needed for these small rooms. Because these small rooms generally don’t have many options like glazing to try and improve the rating, an assessor will need to look at other areas of the house to improve the overall rating. If the home is to be constructed in NSW other options to consider are where the entry to the small space is located. For example, a ‘tool room’ entered from a hallway must be zoned as a daytime zone, but if the entry is located in the garage, the tool room can then be modelled as an ‘unconditioned zone’. This is because in NSW the BASIX thermal comfort protocol takes precedence over the NatHERS technical notes and state that,

“All spaces are to be included in separate zones, except for spaces which do not have an operable window/door or skylight (e.g. bathroom, storeroom). These spaces must be included in the zone from which they are accessed.”


Conclusion –

So how are you going to prevent your next project from being sent to the energy rating morgue?

  • Ceilings – Limit raked ceilings to entertaining rooms (kitchen/living). Design the ceiling so that it is raked at a different angle to the roof.
  • Operable glazing – Position operable windows on opposite sides of the home to maximise cross flow ventilation
  • Small rooms – Limit the number of small rooms within a home, or talk to an experienced assessor who can advise ways to minimise the energy loss on these rooms, such as adding more thermal mass to the home.

Most importantly, engage a NatHERS assessor early in the design stage of a project to avoid the disappointment of having to make potentially costly changes to your design. An accredited assessor can provide detailed information and advice on how different building materials or design features will perform and if there are more thermally efficient alternatives available to suit both your budget and design aesthetics.