How to Hit a 7-Star NATHERS Rating Without Blowing Your Budget

From late 2022, NSW lifted the BASIX target to align with 7-star NATHERS. Most new builds need to hit it. The question is whether you hit 7 stars cheaply or expensively. The answer depends on choices made early in design.

The free wins

• Orientation: Long axis east-west. Main living and key bedrooms on the north. Bathrooms, laundry and garage buffer the south. This alone is worth 0.5 to 1 star.
• Eaves: 600mm to 750mm eaves on north windows. Blocks summer sun, allows winter sun. Worth 0.2 to 0.4 stars.
• Window distribution: Glazing area on the north should be greater than on the south or west. Keep east and west glazing modest. Cuts cooling load.
• Roof colour: Light to mid-tone colours rather than charcoal or black. Worth 0.1 to 0.3 stars.
• Light external wall colour: Same logic. Reduces summer absorption.
• Ceiling fans: Specified in main rooms. Cheap to install during build (around $250 per fan), worth 0.2 to 0.4 stars combined.
• Cross-ventilation paths: Windows on opposite walls in main rooms. Allows night-time purging in summer.

The cheap upgrades

• R5.0 ceiling insulation: Standard now in most builds. Compared to R3.5: $1,500 extra, worth 0.3 to 0.5 stars.
• R2.5 wall insulation: Standard. Compared to R2.0: $1,200 extra, worth 0.2 stars.
• Slab edge insulation: 40mm rigid foam to slab perimeter. $1,800, worth 0.1 to 0.2 stars.
• Air sealing: Sealing tape and gaskets at penetrations, around windows. $1,500, worth 0.2 to 0.4 stars depending on existing detailing.
• Low-e single glazing: Coated single glass with low-emissivity surface. $1,500 to $3,000 extra, worth 0.2 to 0.3 stars.

The expensive upgrades

• Double glazing throughout: $8,000 to $25,000 extra, worth 0.5 to 1 star depending on baseline.
• Thermally broken aluminium frames: Additional $4,000 to $10,000 over standard aluminium, worth 0.3 to 0.5 stars.
• External shading systems: Adjustable louvres or operable awnings on west glazing. $5,000 to $12,000.
• Mechanical heat recovery ventilation: $8,000 to $15,000 system. Helps for tight homes, less critical for 7 stars.

Priority order for spending

1. Get orientation and design fundamentals right (free).
2. Specify standard insulation packages (cheap).
3. Specify ceiling fans throughout (cheap, high value).
4. Specify slab edge insulation (cheap).
5. Air seal carefully (cheap, high value).
6. If still short, upgrade to low-e single glazing on west and east (moderate cost).
7. Last resort: double glazing throughout (expensive).

Most homes hit 7 stars by step 5. Going beyond requires either tighter spec or compromised design (more shading, smaller windows).

Where the model overstates and understates

NATHERS modelling assumes standard occupancy patterns and a 24-hour 21 degree internal temperature. Real homes are not heated and cooled to that exact target. So actual energy savings between two rated homes may differ from the modelled difference. The relative ranking is reliable, but absolute kWh predictions are approximate.

Worked example: a 200 sqm home

Baseline design hits 6.0 stars. The path to 7 stars without breaking budget:

• Adjust orientation, swap living to north-facing (0.4 stars, $0).
• Add 600mm eaves on north windows (0.2 stars, $1,500).
• Ceiling fans in 5 main rooms (0.3 stars, $1,250).
• R5.0 ceiling, R2.5 walls instead of R3.5 / R2.0 (0.3 stars, $2,800).
• Light roof colour swap (0.1 stars, $0).
• Air sealing detail at penetrations (0.2 stars, $1,200).

Total spend to lift from 6.0 to 7.0+ stars: approximately $6,750. Payback period through energy savings: 5 to 7 years. After that, pure saving for the next 25+ years.