Everyone loves the Dursley Treehouse featured on Grand Designs; it’s quirky and beautiful, sensitive to its delicate surroundings, and lovingly crafted by our wonderful client, Jon himself. To top it all off, it is a Certified Passivhaus, widely recognised as the leading thermal comfort and energy standard not just in the UK but the world.
Passivhaus is not just a standard but a complete building methodology. For the professionals and clients that have embraced the standard, nothing else is quite the same. What makes it so good, what’s involved in completing a Passivhaus building, and why is it worth it?
The requirements of the standard are based not upon arbitrary or intangible constraints, but on decades of research into how people feel comfortable. Much of this work is codified in two documents, ISO 7730 and ASHRAE 55. Passivhaus is arguably the simplest and most robust way of meeting this guidance. In practice, this means Passivhaus buildings are amazingly comfortable. Feeling is believing – why not go and stay in the Treehouse, which will soon be operating as a B&B? The icing on the cake is that the energy consumption is 80-90% less than the typical UK housing stock, meaning a space heating bill of about £100 per year for an average house.
The technical requirements are stringent – insulation levels must be very high, good quality triple glazed windows and Mechanical Ventilation with Heat Recovery (MVHR) are both necessary, and the level of airtightness must exceed typical building regulations compliant houses by up to ten times. This leads to a very comfortable and efficient building on paper, but arguably the most significant difference between Passivhaus and building regulations is a very thorough and robust Quality Assurance process that ensures that the design and the calculations are aligned at design stage, and that what happens on-site matches the design (or any deviations are properly accounted for).
The earlier a commitment is made to Passivhaus certification, the less costly it will be, and the more likely it will be successful. Ideally, some input should be sought from a Passivhaus Designer before a planning application is submitted, because some factors that are fixed at this stage will strongly influence the energy performance. These include the form, complexity, orientation, materials and glazing design. Passivhaus is not a prescriptive standard so there is no specific requirement on any of these things, but good choices early on will create an easier path to success. Throughout the design, the Passivhaus Designer (who may also be the architect, mechanical engineer or other team member) should regularly update the calculations and report back the status to the team. It’s advisable to aim for a better performance early on to allow some margin for error and to accommodate design changes.
When the build begins on site, the construction team should have a good understanding of what is necessary, typically achieved by detailed discussions with the design team, attending a Passivhaus Tradesperson training course and so on. The biggest risk on site is airtightness, and it is usual for one person to be designated responsible, and given a title such as Airtightness Champion, Passivhaus Champion or ATIC (Airtightness and Thermal Integrity Champion, credit to Mark Siddal). Throughout the build, simple pieces of evidence must be collected to demonstrate adherence to the design, including invoices or delivery notes for the correct materials and photographs to show, for example, properly lapped and butted insulation. At the end, the Passivhaus Certifier will check all the evidence and calculations, and issue a certificate.
There are hard financial benefits to low heating bills, and the tangible benefits of thermal comfort discussed above. On top of this there is some early evidence to suggest that Passivhaus adds value to buildings. Hastoe Housing Association’s Surveyors now routinely add 5% to the stock value of certified buildings.