Construction system with 0.5m-thick walls aims to exceed forthcoming Scottish Passivhaus regulations and reduce heating bills by 79%
As the Scottish equivalent of the Passivhaus standard continues to work its way through parliament, an Aberdeenshire-based construction company has launched a range of prefabricated homes designed to exceed the new standard.
Coldwells Build’s Passivhaus range, designed by Glasgow-based John Gilbert Architects, features six different property types available in two, three, four and five-bed options and priced between £224,000 to £272,000.
Clients can opt either for a full turnkey service, from planning to final certification, or a self-build option, with a watertight and airtight Passivhaus shell handed over ready to decorate.
A Passivhaus panel construction system forms the structural framework for the homes, delivering high levels of insulation and airtightness and cutting heating bills by 79%.
According to Rupert Daly, associate director at John Gilbert Architects, the properties should comfortably exceed requirements set out under Scotland’s forthcoming Passivhaus regulations, currently in consultation.
‘Performance will be certified Passivhaus Classic, as a baseline, so whatever the new standard comes in as, we're going to either meet or exceed it,’ said Daly.
The 0.5m-thick panels are packed with large amounts of recyclable stone wool. ‘We're not building stick thin walls here, we're almost harking back to the Scottish vernacular of really thick stone walls, but instead of using heavy massive stones, they're really thick insulated walls,’ said Daly.
Targeted U-values for wall, roof and floor elements range from 0.085 W/m2.K to 0.09W/m2.K, eclipsing the 0.10 W/m².K to 0.15 W/m².K values targeted on a typical Passivhaus project.
Passivhaus homes must be enveloped in a single, continuous airtight layer. The Coldwells Build properties will achieve this using an airtight board that wraps around wall panels and roof cassettes. Small gaps between boards will be taped and sealed. Coldwells says this should also help minimise the risk of follow-on trades inadvertently piercing the airtightness layer as can happen with airtightness membranes.
One unique aspect of the construction system, Daly notes, is that homes don’t require an optimum site orientation to ensure thermal performance. ‘Having a very chunky and potentially over-insulated wall panel helps balance out the energy impact of other aspects, like solar gain,’ he said, adding that larger windows are possible where other Passivhaus projects might have to limit window sizes.
The homes are designed to look bespoke rather than mass-produced, with a number of external timber cladding options, slate or standing seam roofs and different internal colour palettes.
‘Customers want some choice, but not infinite choice, and nor do we, because it adds a lot of time to procurement,’ said Daly. Other elements of climate-friendly specification include sustainably-sourced timber and reduced use of high embodied carbon materials like cement and steel.
The cladding and high-performance windows and doors are pre-installed into panels in Coldwells’ Aberdeenshire factory before being delivered to site and craned into position. Once the floor slab and ground works are complete it will take two to three weeks to complete the build ready for fit out to begin.
Recent months have seen the collapse of several offsite manufacturers, but according to Daly, the relatively small scale of Coldwells’ operation and its targets should limit its exposure. ‘They're looking at tens of units, rather than thousands, within the year and that's no bad thing,’ he said. ‘Volume offsite manufacturers have tried to do it at mass scale and they've not had the throughput. As a small operation this is more sustainable,’ he concludes.