The UK Net Zero Carbon Buildings Standard Guide: Part 3: Operational energy

Over the past few years, various organisations including UKGBC, RIBA, LETI, DfE and the NHS have published operational energy targets for net zero carbon-compatible buildings. The types of buildings previously considered were few, including just offices, homes, schools and NHS buildings. 

The following chart shows the as-measured annual energy consumption along with the Standard's proposed limits for a range of completed Max Fordham projects. The Standard limits are for projects that would be newly built or retrofitted in 2028. That is, we’re looking at how those building designs would fare if they were being designed now and/or looking at what retrofit plans existing buildings might need to undertake. 

The Max Fordham house is a single family home building and was a new build project around 2017. It is Passivhaus certified using around 300mm of insulation in walls, roof and ground floor, triple-glazed windows, quad glazed rooflights with a measured air tightness of 0.3 m³/hr/m² @ 50Pa. The building also includes MVHR and insulated window shutters. Space heating is by direct electric and hot water generation using a two-stage air source heat pump. 

The as-measured energy performance has been around 50 kWh/m²/yr, which wouldn’t meet the Standard's target of 43 kWh/m²/yr for new build houses in 2028. If we were undertaking a similar project now we’d consider using a single stage (propane) ASHP for both space heating and hot water. These types of heat pumps weren’t available at the time the Max Fordham house was designed but are widely available new and are more efficient than the systems installed. 

A further issue to consider is the fact that for the monitored period (2019-2021) the Max Fordham house was being used, in part as a care home. The Standard's energy target for care homes is much higher (140 kWh/m²/yr), so perhaps the target should be adjusted to reflect this. 

The Max Fordham House does meet the Standard's Retrofit in One Go target in 2028 or in fact in any year up to 2050. Since Max’s house doesn’t use any fossil fuels, it would be able to achieve verified net zero carbon status without undertaking any actual retrofit works. 

Agar Grove 1a is a block of 38 flats and was a new build project around 2016. The building is Passivhaus certified and includes wall, ground floor and roof U values ranging from 0.1 to 0.25 W/m²/K. Window U values are 0.8 W/m²/K and air tightness was measured at 1 m³/hr/m² @ 50Pa. The building includes MVHR,  space heating and DHW are provided using communal gas boilers. Cooling is by natural ventilation. 

The measured energy consumption was 75 kWh/m²/yr which does not meet the UK NZC BS 2028 new build target of 39 kWh/m²/yr. However, if the gas boilers were changed for a communal ASHP with SCOP of 2.8 then the predicted EUI would be 39 kWh/m²/yr, meeting the new build target and reinforcing the simple idea that Passivhaus and heat pumps is the way to go for net zero carbon buildings. 

It might be tricky to achieve SCOP = 2.8 for a heat pump retrofit (due to existing building constraints). However, 2.0 should be readily achievable. In this case, the predicted EUI is 45 kWh/m²/yr, which easily meets the Standard's retrofit target all the way up to 2050. Hence Agar Grove 1a has a simple net zero carbon-compatible decarbonisation plan to implement when the boilers reach their end of life (estimated 2035).

St Johns College Library is a higher education building and was a new build project around 2018. The building has a heat loss form factor of around 1.1. The wall, roof and floor U values are around 0.25 W/m²/K. Windows are triple-glazed with a U value of around 1.1 W/m²/K. Air tightness has been measured at 4 m³/hr/m² @ 50Pa. The building is predominantly naturally ventilated (does not include MVHR) and is heated and cooled using ground source heat pumps with an estimated SCOP of 3.5. 

The as-measured annual energy consumption is 62 to 65 kWh/m²/yr comfortably meets both the 2028 new build and retrofit targets for higher education buildings which are 92 and 119 kWh/m²/yr respectively. 

Clearly, the building design (and management procedures) are adequate to meet the Standard's operational energy limits, however, it feels like the targets might need to be made a bit more onerous for buildings of this type to get equivalence with other types included in the Standard. A potential development of the Standard could be to include some higher education building subtypes. 

Ravelin Sports Centre is a university sport and leisure building and was a new build project around 2020. The building (and Standard energy target) has the following areas: 43% dry sports, 30% wet (pools) and 27% fitness suites. There’s also a car park but the area of this has been omitted from this analysis. 

The building includes a pre-insulated cassette type façade with U values of around 0.2 W/m²/K. Air tightness was measured at 3 m³/hr/m² @ 50Pa. The building has a mixed-mode ventilation system that makes use of both natural ventilation and MVHR in some areas. 

Low (temperature) grade heat is generated by air source heat pumps and in some cases then upgraded (heated further) by water source heat pumps.  The low-grade LTHW system provides heat to the pool water, the foyer, the sports hall and offices via underfloor heating, and other spaces including the studios and fitness suite via LTHW coils in fan coil units. The high-grade LTHW system provides heating for the domestic hot water for items such as showers. 

Cooling is provided through a mixture of natural ventilation, large ceiling fans and, in some cases, fan coils served by air-cooled chillers. The operational energy performance has been measured at 128 kWh/m²/yr (when car park GIA is excluded) - this comfortably meets the Standard's targets well into the future. 

If we were undertaking this type of project again, we’d keep the same operational energy strategy but give greater focus to embodied carbon emissions, specifically the heat pump refrigerant types. The equipment that was available at the time the Ravelin building was being designed relied upon refrigerant with high GWPs. Today there are many more heat pump products available on the market that use natural refrigerants with GWP < 10. 

Our Edinburgh and Cambridge offices

• Max Fordham Offices UKGBC Net Zero Carbon Buildings Report

The Civil Engineering Building, University of Cambridge

• Net Zero Carbon Guide: Civil Engineering Building, University of Cambridge

Cranmer Road, Student Residential, University of Cambridge

• Cranmer Road, Cambridge 
• Passivhaus Trust: Cranmer Road 

Two Rivers Primary School, Keynsham, Bristol

• Two Rivers Primary School
• UK Net Zero Carbon Buildings Standard Guide Part 1

The Hive Library, Worcester

• CIBSE Journal: Monitoring at the Hive