From May 2024, Victorian Councils require a 7 star rating house for a NatHERS Certificate. Needless to say, many architects and builders have concerns about the additional NatHERS star rating requirement. They know already how hard it is to build a luxury house with high raked ceilings and glazing, and to obtain a 6 star rating for it. So what is the solution here? Are we still able to build a house with a reasonable cost associated with this additional star? Here, we propose solutions to how you can adapt to these changes cost-effectively.

Orient yourself!

Orientation is a major part of this adaptation. You need to design your house in a way to decrease both heating and cooling demand consequently. Therefore, you need to design in a way that the zones with daytime activities face toward the north for better utilization of natural irradiation by the sun. The East and west façades need to be small in comparison to the north façade. The introduction of light through east and west façades is always bad news in summer and also not very helpful in winter.

Adapt at the beginning!

Building layout design is a critical stage and taking steps on a way to a more passive house, is essential at the initial design stage. A smart architect arranges the rooms in a way that more daylight is available in the daytime for living areas, avoiding them to be adjacent to unconditioned rooms. As bedrooms contribute more to heating demands in winter and at night, it is wise to shrink their external walls and exposed fabrics as much as possible. Also, high raked ceiling is not a good idea for a night-time conditioned zone. The heating of such high-volume zones is very demanding mainly due to an elevated infiltration.

More mass towards the sun?

Depends on the climate you are in; high thermal mass walls may help. In cold climates, you need to face a wall with a high thermal mass (e.g. concrete wall, brick cavity walls or earth walls) towards the north and face the sun. Storage of heat in the daytime would be beneficiary at night-time when the ambient is colder. However, it is reasonable to avoid this to a certain degree in hot climates. Thermal inertia (a fancy word for a delay in cooling demand occurrence!) is tightly associated with thermal mass in hot climates which will lengthen the period for artificial cooling even hours after the sun is set.

Let it sit on the ground!

Based on our experience, a suspended floor is an awful choice if you are seeking higher star ratings. In a hot climate, a suspended floor deprives you of 0.5 to 1 star compared with the concrete slab on ground. Floor construction does not affect heating demand since you can insulate suspended floors enough to mitigate the difference; however, cooling demand is going to be an issue here. Note that the earth is cooler than the ambient air by a few degrees, additionally, building energy is transferred to the ground and hence, the ground temperature below the house is very moderate and near the comfort temperature. This facilitates decreasing the cooling demand in the hot season and beneficiary to the overall energy saving of the house services.

Block the flow of heat at all costs!

A high degree of insulation is worth its cost. More insulation means more buffer to have cheaper windows with a low thermal performance. Try and diversify your choice of insulation. Reflective insulation works best in summertime and hot climates. The use of roof blankets the flow of heat further in cold climates. Also, slab edge insulation and underslab insulation work very well in reducing heating demand in those climates. Remember, Adding R6.0 insulation to the ceiling is not the same as adding an R1.5 blanket to the roof and R3.5 to the ceiling; the latter is more effective (the lesser the cold faces the better and also less thermal bridging). Also adding a moderate amount of insulation to the internal walls adjacent to the unconditioned areas goes a long way in terms of heat transfer and acoustics.

Go a little easy on aesthetics!

Colour of external building elements matters, perhaps the most in extreme hot or cold climates. You need to maximize solar absorptance (solar heat gain) of building elements in cold climates. Therefore, the use of dark-coloured external roof or wall claddings (similar to Monument or Basalt) are useful. On the other hand, for hot climates, you need to use light-coloured external elements (similar to Surfmist or Shale grey). Understandably, these choices may conflict with the aesthetics of the building and the choice of material. You need to assess the effect and see if it is worth your while.

Let there be more daylight!

The cost of a project mainly depends on the overall U-value of its windows. Windows play a major role in one’s sense of belonging and attachment to the environment as well as well-being. But it’s reasonable to ask to what degree? For example, the BCA requirement for natural light is that the light transmitting area of a window (i.e. glazing) to be at least 10% of the floor area served. Also, an opening area of 5-10% to the floor area served is certainly ok for Climate zones 6-8 and probably sufficient for other climate zones as well. Going beyond the above thresholds comes with a price as you will need to decrease the overall U-value of windows.

As for window SHGC (Solar Heat Gain Coefficient), you generally need to maximise it on north-facing windows (i.e. clear windows). Also try to minimise it on the east or west-facing windows (i.e. tinted windows). Note that south-facing windows do not matter much in terms of solar gain.

Optimize shadings

Perhaps, the best options here to obtain an additional star is to optimize your shadings and eaves. Here’s how it goes:

  • North-facing windows do not like shadings with long projections. At summertime, sun is directly above the house and therefore short shading projection is enough to blocks the unnecessary irradiation. As a rule of thumb, do not allow the shading projection (P) to be more than 50% of the window height (h) plus the distance from top sill to edge of shading (G).
  • East and west windows do not help you in winter, however, it increases the cooling demand significantly in summer. Therefore, you need longer shading projections on them. Choice of retractable shading devices instead of fixed shadings is a very helpful choice. Generally, this solution pays back and makes it easy to go beyond a NathERS 6 star rating.

Get a professional

We recommend involving an ESD consultant in the early stages. What you need is a trusted company with specialists in Energy systems and ESD consulting. This gives you the ultimate insight as to what necessary and cost-effective solutions are to achieve a 7 star rating. It is wise to ask for a sample energy report and see how they are handling a project. Also, you need to know how feasible are their propositions and how inclusive is their energy report.