Since I specialice in high-performance homes, people often ask why I don’t use NatHERS star rating software. Here’s why.
In cooler climates, NatHERS star ratings tend to favour dark colours for external walls, roofs, and window frames, as the simulation software prioritises passive solar heating and cooling through sunlight exposure and cross-ventilation. However, this design philosophy doesn’t align well with airtight construction, which is why I don’t rely on NatHERS simulations for my Gold Star method. Instead, Gold Star takes an energy conservation approach that emphasises insulation, shading, airtightness, and continuous 24-hour mechanical ventilation for optimal efficiency and comfort.
An airtight passive solar house with poor shading, dark-coloured exteriors, and minimal insulation is a recipe for overheating, as each of these factors compounds the others, making the house poorly equipped to manage heat. Here’s why these elements combined lead to significant overheating and poor performance:
1. High Heat Absorption from Dark Exterior Colours
Dark-coloured walls and roofs absorb much more solar radiation than lighter colours, heating up rapidly in direct sunlight. In a passive solar design, this effect becomes particularly problematic when shading is limited or absent, as the walls and roof surfaces are exposed to the sun for long periods. This absorbed heat then radiates into the building, and with little insulation, this heat readily transfers through to the interior, raising indoor temperatures.
2. Insufficient Insulation Allows Heat Transfer Indoors
Insulation acts as a barrier, slowing down heat flow through the building envelope. However, when insulation is minimized (as is sometimes promoted in the NCC Deemed to Satisfy tables that emphasise solar heat gain), there is less resistance to external heat entering the home. In warm seasons, or on sunny winter days, the heat from dark exteriors and direct sunlight transfers quickly indoors without adequate insulation to buffer it. The result is a rapid increase in internal temperatures, creating uncomfortable and sometimes extreme indoor conditions.
3. No Shading to Moderate Solar Gains
Shading is a key strategy in passive solar design to reduce solar heat gain, especially during warmer months. Without effective shading (such as wide eaves, awnings, verandah, or vegetation), sunlight directly enters windows and strikes exterior walls throughout the day. This continual solar exposure warms up the building envelope and interiors, adding to the heat load. Without shading, dark exteriors and insufficient insulation amplify this effect, letting heat pour into the building and allowing surfaces to radiate heat back into the rooms. (Have a look at the NCC2022 Part 13 energy efficiency tables to see that less insulation is prescribed with dark external colours).
4. Airtightness Traps Heat Inside
Airtight construction is effective at retaining heat in winter, but it also traps excess heat during warm periods or on sunny days. When heat builds up inside an airtight home, it has very limited avenues for escape. Ventilation systems, if not adequately sized or used, may not release all the excess warmth, leading to a stuffy, uncomfortable indoor environment. In this situation, the house effectively acts like a sealed greenhouse: heat enters but cannot easily leave.
5. No Thermal Buffering Due to Reduced Insulation
Insulation doesn’t just keep heat out; it slows the rate at which heat enters or exits the building, helping to stabilise indoor temperatures. With less insulation, the home lacks this buffer, allowing temperature swings that result in excessive daytime heating and uncomfortable conditions. Insufficient insulation means that the house cannot regulate indoor temperatures effectively, letting daytime heat gain from dark walls and unshaded windows quickly warm the interiors without a means to moderate or delay it.
6. Increased Cooling Demand and Energy Inefficiency
In climates where overheating is a concern, these factors create a constant need for active cooling to maintain comfort. The very purpose of passive solar design—to reduce energy use by maximising solar heat gain in winter—is undermined in such a setup, as the home requires significant cooling. The dark exteriors, minimal shading, lack of insulation, and airtightness mean that the building’s heat load spikes during warmer periods, driving up energy use and negating any passive benefits intended by the design.
7. Comfort and Health Implications
Overheating, especially in airtight homes without adequate mechanical ventilation, poses serious comfort and health risks. Stagnant, overheated air can lead to sleep disruption, fatigue, and even heat stress, particularly for vulnerable occupants. If windows remain closed to maintain airtightness, indoor air quality may also suffer as fresh air is limited, contributing to a stuffy, uncomfortable environment.
In conclusion, an airtight passive solar house with dark colours, poor shading, and reduced insulation is highly susceptible to overheating, as dark surfaces absorb and radiate heat, unshaded windows increase solar gain, and low insulation allows for easy heat transfer. To avoid these issues, passive solar designs should be balanced with adequate shading, lighter colours, and sufficient insulation to ensure comfort, energy efficiency, and a stable indoor environment throughout the year; this is not what the NatHERS simulation does. I find that The NatHERS simulation software is not well paired with airtightness and is potentially problematic.
Gold Star takes an energy conservation approach that emphasises insulation, shading, airtightness, and continuous 24-hour mechanical ventilation for optimal efficiency and comfort. External colours are accounted for but are NOT relied upon for heating the home.