9.6 Stars in 70 Square Metres: How 11 Design Decisions Worked Together

Achieving a very high NatHERS rating in a compact dwelling is not simply a matter of adding more insulation or specifying better windows. The result usually comes from numerous decisions working together: building form, orientation, glazing, thermal mass, internal planning, shading and construction all affect the final heating and cooling loads.

A recent 70 m² SIPs dwelling assessed in NatHERS Climate Zone 13—Perth—achieved an exceptional 9.6-Star thermal performance rating.

This is the highest NatHERS result I have so far assessed for a Structural Insulated Panel dwelling.

There was no single feature responsible for the result. The rating arose from an unusually successful alignment of 11 design, construction and site factors.

1. A compact floor area

Small dwellings have proportionally more external wall, roof and floor area relative to their usable floor area. Without an adjustment, this higher envelope-to-floor-area ratio could place them at an inherent disadvantage.

NatHERS therefore applies an area correction factor to allow dwellings of different sizes to be compared more fairly. At approximately 70 m², this dwelling benefits from that correction.

The adjustment helps, but it does not by itself produce a 9.6-Star result. The underlying heating and cooling loads must still be exceptionally low.

2. An efficient, nearly square building form

The compact, approximately square plan encloses the required floor area with relatively little external wall.

Compared with a long, narrow or highly articulated floor plan, this reduces the amount of building envelope exposed to outdoor conditions. Fewer external corners and a lower wall-to-floor-area ratio also help limit unwanted heat transfer.

This is one of the simplest principles of energy-efficient design: a compact form is generally easier to keep comfortable than a sprawling or geometrically complicated one.

3. A well-judged proportion of glazing

The dwelling has a window-to-floor-area ratio of approximately 21%.

For this particular design, that falls within a very effective range. There is enough glazing to provide daylight, ventilation and useful winter solar gain without creating excessive summer heat gain or winter heat loss.

More glass does not necessarily produce a better-performing house. Its area, orientation, shading and thermal properties must all be considered together.

4. High-performance glazing

The windows and glazed doors use some of the highest-performing glazing available within the project’s standard product range.

Even in Perth’s relatively mild climate, windows can be a major source of winter heat loss and summer heat gain. Improving the glazing performance helped the dwelling retain useful winter warmth while controlling unwanted heat transfer through the building envelope.

The effectiveness of the glazing also depends on where it is placed. High-performance glass cannot entirely compensate for excessive or poorly oriented windows.

5. Insulation levels well above the minimum

The SIPs wall and roof construction provides insulation levels substantially above minimum compliance requirements.

Structural Insulated Panels can create a highly insulated building envelope with fewer repeating framing-related thermal bridges than conventional lightweight stud construction. Their actual performance nevertheless depends on careful detailing and construction, particularly at panel joints, openings and service penetrations.

The high insulation levels greatly reduce heat flow through the walls and roof, helping the dwelling resist both summer heat and winter heat loss.

6. Useful internal thermal mass

A highly insulated lightweight envelope can benefit from thermal mass placed inside that envelope.

This dwelling combines its SIPs construction with a slab-on-ground floor incorporating exposed polished concrete and tiled areas. The exposed slab moderates internal temperature changes by absorbing heat when the interior is warmer and releasing it as temperatures fall.

Where winter sunlight reaches the slab, the concrete can store some of that solar energy and return it to the room later. During warmer conditions, the slab also helps moderate short-term indoor temperature peaks.

In this particular NatHERS model, slab-edge insulation was not required to obtain the optimum result. This should not be interpreted as a universal rule: the effect of slab-edge insulation varies with climate, floor coverings, solar access, construction and the geometry of the dwelling.

7. A reduced conditioned volume

The main living area has a flat ceiling rather than a full-height cathedral or raking ceiling.

This reduces the volume of air associated with the principal conditioned space and limits the amount of roof envelope directly exposed above it. As a result, less heating and cooling energy is required to maintain the modelled comfort conditions.

High ceilings and raking forms can provide architectural character, but they also have thermal consequences. In this dwelling, the more contained living-area volume contributed to the strong result.

8. Very low and well-balanced seasonal loads

The model produced an unusually good balance between its heating and cooling performance.

A design may perform exceptionally well in summer but poorly in winter—or the reverse. In this case, neither seasonal load dominates the result. The insulation, glazing, thermal mass, shading and solar access work together to keep both loads very low.

This balance is particularly important in Perth’s Mediterranean climate, where a successful passive design must respond to hot summers while still admitting and retaining useful warmth during winter.

9. Beneficial overshadowing

Overshadowing from adjacent buildings did not penalise the result. In this case, it made a beneficial contribution.

Neighbouring structures helped limit unwanted solar exposure without preventing the dwelling from receiving the useful solar access needed for winter performance.

Overshadowing is sometimes treated only as a disadvantage, but its effect depends on direction, season, time of day and the location of the dwelling’s glazing. Accurate modelling is therefore important: removing surrounding buildings from an assessment can sometimes make the predicted performance worse rather than better.

10. Efficient internal planning

The dwelling avoids unnecessary corridors, enclosed hallways and very small internal spaces.

Although these spaces may be individually small, they can have comparatively high heating and cooling loads per square metre. They also consume floor area without necessarily adding directly to the usefulness of the principal living spaces.

The open, efficient plan allows the compact footprint to function with relatively little thermally inefficient circulation or incidental space.

This is an important lesson for compact housing: good performance is influenced not only by the external envelope but also by how the floor area is organised internally.

11. A particularly favourable orientation

The dwelling is positioned approximately 28 degrees away from a due-north alignment.

Orientation testing indicated that this angle was especially favourable for the particular combination of glazing, shading and surrounding buildings. Rotating the principal glazed doors to face due north produced a slightly lower result of approximately 9.5 Stars.

This does not mean that 28 degrees is generally better than north. It means the best orientation must be considered in relation to the complete design and its site. Solar access, window positions, shading and neighbouring obstructions all interact.

The result also suggests that the shading to the principal glazed doors deserves careful resolution so that the strong winter performance is retained without introducing unnecessary summer heat gain.

What the 9.6-Star rating means

A 9.6-Star NatHERS rating indicates exceptionally low modelled heating and cooling requirements under standard NatHERS occupancy and comfort assumptions.

It does not guarantee that the completed dwelling will never require mechanical heating or cooling. Construction quality, airtightness, occupant behaviour, window operation and the final installation of insulation and shading will all influence its real-world performance.

Nevertheless, the result demonstrates what can be achieved when a compact plan, efficient form, high-performance envelope, appropriate glazing, thermal mass, orientation and site conditions work together.

The broader design lesson

The success of this dwelling cannot be credited to SIPs, glazing, thermal mass or orientation in isolation.

Its performance comes from the relationship between all these elements.

That is the central value of thermal modelling during design. NatHERS can do more than determine whether a proposed dwelling reaches the required rating. When the model is examined carefully, it can reveal why a design performs as it does—and help the designer identify which features should be protected as the project progresses.

A high-performance home is rarely created by one spectacular feature. More often, it is the outcome of many sound decisions quietly reinforcing one another.