“The fuel economy? Yes I have very good news on that front madam, you won’t be disappointed with your purchase. This car has been benchmarked against a 1992 red Ford Sierra driven by carefully by Geoff around Basingstoke – this Sierra is our notional average car against which all other cars are measured for consistency. Your new purchase has scored 35% lower carbon emissions than Geoff and his Sierra, in computer simulations.

“We think you’ll be delighted by the running costs when you get it out of the showroom….

“The actual MPG in real use? Hmmm, well, errr….. that is an excellent question, but while I don’t have a specific figure for that I can tell you that it scores 35% lower carbon emissions than Geoff, and his red Sierra.”

The Scottish Government’s decision to introduce a Scottish Passivhaus equivalent for new build housing has the potential to be world leading and could make a significant contribution to alleviating fuel poverty, addressing climate change and improving energy security in Scotland.

However, unless we use the right metrics we won’t get the homes that Scotland needs to meet this ambition.

The car analogy perfectly sums up why current proposals for a ‘Scottish Passivhaus equivalent’ for housing won’t deliver the energy efficiency we need. What is being proposed is to use a relative notional building metric (using comparisons with a standard house) rather than absolute targets that tell you how much energy the house will use to heat and power per m2 (comparable to miles per gallon in a car).

The policy could fall at the first hurdle because it is simply not possible to deliver a ‘Passivhaus equivalent’ unless the way you measure energy in houses relates to the Passivhaus methodology. Even before we get to setting the higher energy standards required for a Passivhaus equivalent, the proposals are effectively using the wrong language to express those targets.

It is important that we should all take the time to respond to the current consultation to help it get back on track by using the right methodology and metrics.

Absolute vs relative targets

The Scottish Passivhaus equivalent consultation is proposing using relative ‘notional building’ energy targets instead of the absolute targets used in the Passivhaus methodology.

Relative targets are based on percentage reductions in CO2 emissions over a ‘notional building’. They involve comparisons with fixed building specifications. Relative notional building targets can lead to variability in performance expectations, as they are based on comparisons and may not provide the same level of rigour and clarity. The methodology also doesn’t encourage designing buildings with a simple form factor and permits inefficiency in building form. As a result, the current proposals won’t deliver Passivhaus equivalent levels of comfort, indoor air quality and low energy bills. Only absolute targets can deliver a Scottish Passivhaus equivalent.

The Passivhaus Trust strongly recommends the use of absolute targets based on heating demand, peak heating load and EUI (energy use intensity). Absolute targets are one of the key recommendations in LETI’s Climate Emergency Design Guide, and have been chosen for RIBA’s 2030 Climate Challenge targets and for many Local Plans developed by many local authorities around the UK; they are also likely to be fundamental to the new UK Net Zero Carbon Building Standard.

Absolute targets are more effective than notional (or relative) targets for several reasons:

• Unambiguous standards: Absolute targets provide a clear, unambiguous benchmark that does not depend on comparisons with other buildings or changing conditions.
• Consistent performance benchmarks: By setting a fixed target, absolute standards ensure that performance is consistently evaluated against a set criterion, rather than relative to the performance of other buildings or evolving benchmarks.
• Encouraging innovation: Absolute targets challenge the building industry to innovate and find new, effective solutions to meet the set standards.
• Future-proofing: Absolute targets remain relevant over time as they are not influenced by the performance of existing buildings or changing benchmarks.
• Leadership and precedent: Adopting absolute targets positions Scotland as a leader in sustainable building practices and could influence broader national and international standards.

The consultation on the Scottish Passivhaus Equivalent policy ends on 23 October 2024. The Passivhaus Trust has written guidance on how to make the case for absolute targets in consultation responses.