Built from traditional stone and slate, Preston Springs, a four bed home in Leyburn, North Yorkshire, may look like a typical new-build property, but it is far from it. Designed by architects LEAP, Preston Springs is a Passivhaus, meaning it is ahead of its time in terms of air quality, comfort and reduced energy use. It is one of the only properties of its type in the local area, with just over 1,000 Passivhaus properties in the whole of the UK.
Passivhaus is a voluntary standard for designing and building very low energy properties that offer high levels of comfort for their occupants. To achieve this, the levels of air tightness and thermal performance far exceed those mandated by building regulations.
To meet planning constraints, Preston Springs has been built using a conventional cavity wall construction. The 300mm cavity was fully-filled using two layers of Knauf Insulation Earthwool® 150mm DriTherm 34 Cavity Slabs. Manufactured from Glass Mineral Wool, DriTherm is specifically designed for cavities and uses a water repellent additive to prevent water ingress, making it ideal for its exposed location close to the Yorkshire Dales.
DriTherm 34 was chosen over rigid board insulation. Its structure meant the stone masons could cut and fit the insulation easily on site to ensure it fully-fills the cavity, touching both leaves continuously. This prevents undesirable air gaps that reduce thermal performance. It is worth noting that masonry construction with partially-filled cavities are particularly prone to air gaps between insulation boards or breaks in contact between the insulation and the wall surface.
Secondly, because detailing at junctions between the building elements and the openings meant thermal bridging was virtually eliminated, a more cost effective insulation specification could be chosen.
Principal architect Mark Siddall was confident that DriTherm 34 would deliver the 0.1 W/m2K U-Value required to help the project achieve Passivhaus certification.
For the loft, Supafil Loft Blowing Wool was specified after extensive consultation with Knauf Insulation’s Technical Support Team. While Supafil Loft is very popular in Scandinavian countries, it is not widely used in the UK. However, Siddall believes that blown insulation has a distinct advantage over other insulation types when working towards very high levels of thermal performance.
He said: “The advantage of Supafil Loft insulation is that it can get it into all the nooks and crannies very quickly and easily. This makes it a low-cost solution as far as labour is concerned because installers don’t have to cut and scribe around individual trusses at 400 and 600mm centres – something that would take a huge amount of time. By avoiding these air gaps the roof can be expected to deliver the thermal performance that was specified.”
It was not just the use of Supafil Loft that was unusual on the Preston Springs project, but its depth; 700mm of Supafil Loft was installed, delivering a U-Value of 0.06, which Siddall believes makes it one of the most highly-insulated lofts in the UK.
Siddall explains that to achieve Passivhaus standard, installation is hugely important. He said: “The only way to close performance gaps is through high quality workmanship. I have used the same construction team on several projects. I ensured that the installer had received the necessary training on how to install the Supafil Loft insulation correctly, so I had the quality assurance that it would perform as specified in practice.”
Steve Wise, Programme and Product Development Manager at Knauf Insulation added: “It is always exciting to work on projects that feature products not commonly used in the UK or in applications that differ from the norm, such as the 700mm Supafil Loft at Preston Springs.
“Passivhaus projects allow us as manufacturers to explore what is possible. Preston Springs demonstrates how environmentally-focused design, high performance products and high quality installation can deliver and meet even the strictest thermal standards in a real-world environment.”
Typically, Passivhaus properties achieve a 75% reduction in space heating requirements, compared to a standard UK new build. While monitoring is ongoing, anecdotal evidence from the owners show that Preston Springs is performing as designed, maintaining a temperate climate throughout the year. During ‘The Beast from the East’ the internal temperature was a reported 19°C without heating, while the external temperature plummeted to -6°C.
Furthermore, the owners report they are delighted with the low energy bills thanks to the very high thermal performance of the insulation combined with the heat recovery system.