Area of work:
Homes in the UK are a significant source of energy demand and greenhouse gas, with those built recently on average requiring 86 kWh/m2 of space heating energy. For a typical house built in brick and blockwork and with concrete foundations, these core construction materials will cause around 350 kgCO2e/m2 to be emitted into our atmosphere.
With an average home size of 68m2, and about 200,000 homes completed in a normal year, that’s a one-off 4.7 million tonnes of CO2e emitted from the production of these construction materials alone (often known as embodied carbon) and nearly 1.2 TWh of annual space heating energy demand created. The Passivhaus standard reduces the space heating demand of a new home to very low levels, but it does not ensure low carbon construction, as it makes no comment on the embodied carbon of building materials. Passivhaus can also sometimes be both costly and complicated to build.
Passivhaus Homes (PHH) has simplified the process of building a home that meets the Passivhaus standard, while reducing cost, risk, and embodied carbon. This leap forward in housebuilding has been made possible by the PH15 system, which combines off-site manufacture and natural, low carbon, materials with simplified processes and thorough training.
The start of the journey to a PH15 home begins with the Passivhaus Handbook and the PH15 Architect’s Design Guide, which, together with training information and instructional videos, provide the guidance clients and architects need in order to design a home that will achieve Passivhaus standard while keeping costs under control. PHH uses the architect’s drawings to carry out an energy assessment using PHPP software (Passive House Planning Package) and create a 3D computer model of the building, which in turn leads to a cutting list for the timber components that are used to create a PH15 building. These components arrive flat-packed with clear drawings to guide the assembly process, enabling carpenters to get involved in housebuilding, and the clients can get involved too if they wish. Wood-fibre and other timber-based materials are used for cladding and floors, with non-fossil fuel based insulations such as cellulose and sheepswool, and the finish on the walls and roof specified by the architect – common wall finishes include lime render, silicate render and timber. Triple-glazed windows and doors are supplied, along with a heat recovery ventilation system – all suitable to meet the Passivhaus standard and with a range of design and finish options for the client to choose from. The result is a home that uses natural, low carbon materials as much as possible, while being guaranteed to achieve the Passivhaus standard.
Impact on customers
The benefits of a home that achieves the Passivhaus standard are well known: very low heating bills, more stable indoor temperatures, and improved indoor air quality. PHH sets a high bar on overheating risk as well, using thermal modelling at the design stage on every project, with the aim of ensuring overheating will occur for no more than 2% of the time, to address future warming due to climate change.
The key benefit that PH15 brings for a client is being able to control costs – after initial design work and before planning applications are made, an estimate can be given that is accurate to within 5%, and because PH15 is designed as a whole-house system there will be no risk of remedial measures to meet the Passivhaus standard. The cost for a PH15 system is between £450-850 per m² of gross internal floor area, depending on the design (form factor) and amount of glazing, with finished construction costs delivered at £1,800/m2 for a good standard one-off bespoke home, and for multiple homes much lower build costs can be achieved.
For the architect, PH15 simplifies the design process, provides all technical information needed and enables them to confidently assure clients that their work will achieve Passivhaus standard. From an environmental point of view, a reduction in energy use from the typical 86 kWh/m2 to as little as 10 kWh/m2 in a PH15 home is impressive, but even more important is the reduction in the embodied carbon. This drops from the 350 kgCO2e/m2 for a masonry-built home with concrete foundations, to around 230 kgCO2e/m2 for a PH15 home that is still built on a concrete slab. But the light-weight nature of the PH15 construction means steel screw pile foundations and a raised timber floor can sometimes be used instead of concrete, bringing the embodied carbon of the core construction materials down to around 55 kgCO2e/m2.
Demand for Passivhaus standard homes is growing, especially in the social housing sector, and the PH15 system from Passivhaus Homes demonstrates that this is achievable and cost effective, giving clients the confidence to specify Passivhaus for new homes they are building. PH15 also demonstrates how timber can be used cost-effectively in low-carbon construction, which the 2019 UK Committee on Climate Change report on housing stated as being the most effective way to use limited biomass resource to mitigate climate change.
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