Recently, we have had multiple clients request thermal modeling for their prospective new builds. These homes were designed to meet the current New Zealand building code, and the clients were eager to understand their thermal performance before construction began. One project was further along in the planning process, with finalized plans submitted to the council and requests for further information (RFIs) already returned.
The client, concerned about building a warm and healthy home, began to question whether the design was optimal. Was it too late to make changes, or could they still improve the house’s thermal performance before the consent was granted? They approached us for guidance, requesting our design team to evaluate their plans and suggest improvements that could enhance the home’s energy efficiency and comfort.
Initially, the home was designed with 140mm walls and an assumed airtightness of 3.0 air changes per hour (ACH). However, based on recent testing of similar builds, which showed airtightness at 4.5 ACH, we opted for a more conservative estimate for a home adhering to the current building code. The thermal model revealed the following results for the original design:
Original Plan
| Heat Gains | Heat Losses |
| Annual heating demand: 91.1Kwh | Walls, ceiling and floor: 68.4Kwh |
| Solar through windows: 36.9Kwh | Windows: 39.9Kwh |
| Heating through appliances: 13.4Kwh | Ventilation Kitchen/bathroom etc: 32.1Kwh |
We then revised the design by reducing the wall thickness to 90mm, adding an external moisture barrier, an internal vapor variable barrier, and a 45mm internal cavity. Windows were upgraded from thermally broken aluminum to UPVC, and a heat recovery system was added along with improved floor insulation. The new thermal performance model showed significant improvements:
Revised Design Performance:
| Heat Gains | Heat Losses |
| Annual heating demand: 32.9Kwh | Walls, ceiling and floor: 56.8Kwh |
| Solar through windows: 48.6Kwh | Windows: 28.5Kwh |
| Heating through appliances: 13.4Kwh | Ventilation Kitchen/bathroom etc: 5.9Kwh |
Next, we adjusted the home’s orientation, reduced overhangs from 1200mm to 600mm, and reworked the design of windows and doors. These adjustments further optimized the thermal performance:
Performance After Orientation & Design Adjustments:
| Heat Gains | Heat Losses |
| Annual heating demand: 24.7Kwh | Walls, ceiling and floor: 57.9Kwh |
| Solar through windows: 54.9Kwh | Windows: 22.7Kwh |
| Heating through appliances: 13.4Kwh | Ventilation Kitchen/bathroom etc: 5.9Kwh |
For the final design, we used a 140mm wall with an external moisture barrier and an internal vapor variable barrier, along with a 45mm internal cavity. Additionally, we adjusted the sizes of some of the north-facing windows to further optimize the home’s thermal performance.
Final Adjustments Performance:
| Heat Gains | Heat Losses |
| Annual heating demand: 22.1Kwh | Walls, ceiling and floor: 51.9Kwh |
| Solar through windows: 53Kwh | Windows: 22.4Kwh |
| Heating through appliances: 13.4Kwh | Ventilation Kitchen/bathroom etc: 5.9Kwh |
These results demonstrate how small design changes can significantly impact a home’s energy efficiency and running costs. When we design new homes, we emphasize that performance should be a primary focus. Our expert designers not only create aesthetically appealing homes but ensure they are comfortable and Energy-Efficient.
