Looking Beyond the Star Rating – Understanding How a House Actually Performs

One of the questions I am often asked is, “Why did this house achieve this particular star rating?” While the NatHERS certificate provides an overall measure of thermal performance, it doesn’t always explain why a building performs the way it does.

Recent work using HERO has reinforced the value of looking beyond the final star rating and examining how individual rooms behave throughout the year.

The hourly results viewer allows us to investigate how a room responds to changing weather conditions. Instead of only seeing an annual performance score, we can look at questions such as:

• Which rooms become hottest during summer afternoons?
• Which rooms lose heat most rapidly on winter nights?
• How much heating or cooling would be required to maintain comfort?
• Which design changes are most likely to improve performance?

This type of analysis is particularly valuable for projects that are already performing well. Once a design reaches around 8 Stars, further improvements often require a much better understanding of the building’s behaviour rather than simply adding more insulation or higher-specification windows.

One important lesson from this process is that the building should always be understood before proposing changes. Rather than making multiple design alterations and hoping for an improvement, it is usually more effective to identify the rooms that are driving the heating or cooling demand, develop a clear hypothesis about why they are behaving that way, and then test targeted design changes.

This approach combines climate-responsive design principles with detailed thermal modelling. The result is not simply a higher NatHERS rating, but a better understanding of how the building will perform for the people who will actually live in it.

For clients interested in optimising a design rather than simply achieving compliance, this deeper level of analysis can provide valuable insight into where improvements are likely to have the greatest benefit before construction begins.