Green Sky Thinking: Why choose Passivhaus?
The Passivhaus Trust and bere:architects held a talk on the benefits of Passivhaus
The session was chaired by Dan Phillips and the speakers were:
- Kirsten Priebe and Tanisha Raffiuddin, Passivhaus Trust
- Justin Bere, bere:architects
- Marion Baeli, Paul Davis + Partners
- Bruce Tofield, University of East Anglia
The delivery of predictable and consistently performing buildings was the central message of this compelling set of talks, which was attended by an audience of about 45, with approximately one-third architects. Setting certification criteria that relies on measured performance makes Passivhaus a robust standard for low energy design.
According to Kirsten Priebe of the Passivhaus Trust, there are over 200 completed projects in the UK and 37,000 Passivhaus buildings worldwide. To be certified as Passivhaus, a building needs to meet a number of detailed design criteria. Certification has been proven by performance in a number of UK studies which indicates that the gap between the designed performance and the measured performance is addressed by Passivhaus design methods.
Key Passivhaus performance criteria:
- Space heating demand: < 15 kWh/m²/yr
- Primary energy demand: < 120 kWh/m²/yr
- Airtightness n50: 0.6 ach minimum
Tanisha Raffiuddin of the Passivhaus Trust (and ex-AJ Footprint intern) spoke about how the Passivhaus standard ensures predicted performance, stating that with this standard the gap between designed and actual performance is minimised. The performance criteria are proven and measured, and the outcome is a building that is low-energy, draught-free and easy to run. The 15kWh/m2/yr is set at a level that can save on the cost of a conventional heating system. Attempting to add more insulation to go lower would require more investment in the building fabric for the savings in operational energy to pay for the investment.
The 0.6 airtightness requirement is more than ten times better than building regulations; not only does it reduce heat loss but it eliminates drafts and ensures consistent comfort throughout the building. A heat recovery ventilation unit is installed to a rigorous specification and maintains high air quality, allowing internal gains to be captured to heat the building. According to Raffiuddin, filters must be changed once a year - a five minute job. The U-value of installed windows is 0.8W/m2 which means that even sitting immediately adjacent to a window is comfortable.
The cost of achieving Passivhaus relative to conventional construction has been put at an additional 3 to 8 per cent, although in some none domestic new build projects it is claimed that it is possible to achieve the standard at no extra cost. The three main points are that it is simple and robust; it is certified so it will perform as expected, and the MVHR system means better air quality.
The talk was held at the Mayville Community Centre, a building that was retrofitted to the Passivhaus standard by bere:architects. The practice’s office now occupies the lower level. Justin Bere talked through the process of converting an existing building to this standard including the potential pitfalls. The architects managed to avoid a Building Management System (BMS) and the simplicity of the controls means that there have been no issues with the servicing of the building. In addition, the actual energy use of the building is slightly better than the PHPP software predictions.
Marion Baeli of Paul Davis+Partners spoke about the Princedale Road scheme. The performance of three terraced houses on the same road in west London was compared: one unrefurbished, one to Decent Home Plus and a final one to Passivhaus. The Passivhaus retrofit made an 83 per cent improvement on the primary energy use compared to the baseline. Despite this significant energy saving, the transmission losses from the National Grid means that the Princedale Road terraced house does not quite meet the primary energy requirements of the Passivhaus standard.
Bruce Tofield, a lecturer from the University of East Anglia, provided an overview of the economic and social benefits of Passivhaus. He emphasised that Passivhaus was ideal as an investment opportunity and that its measurement and certification procedure was the only way to ensure the reliable delivery of low energy buildings. He highlighted the socially positive aspects of the standard saying that ‘it is virtually impossible to have fuel poverty in a Passivhaus, even if the grid energy supply fails, you only have to run about a bit to get it warm.’
According to Tofield, Britain is the worst country in Europe for fuel poverty and Passivhaus can help address this. Tofield proposes a funding model in which mortgage lenders would lend 10 per cent to all borrowers to retrofit their properties to Passivhaus standard. He believes this money would be easily recouped at the point of sale, especially as energy prices rise.
Tofield also showed convincing figures from UEA research that show that converting the country’s entire building stock to Passivhaus would be almost three times cheaper than the extra power stations needed for the ‘business as usual’ scenario.
The talk made a compelling case for wider adoption of Passivhaus in the UK, but there was very little discussion about embodied energy in construction or any wider issues of sustainable design because Passivhaus is an energy standard. The standard does not address these aspects of a building’s environmental impact, and it is essential that a responsible designer considers the energy going into the building’s fabric. The high performance requirements of the Passivhaus may mean that designers choose energy-intensive materials and products to construct the building which compromise the overall environmental credentials of the design. In the hands of a good designer, these pitfalls can be addressed. Currrently, the cost premium for lower embodied carbon products may mean that designers opt for less expensive products with a carbon premium to keep project costs down. As the UK develops its own low carbon manufacturing, costs are likely to come down.
For those not familiar with Passivhaus, this was an excellent overview. For those already in the know, the event was a testament to the effectiveness of the Passivhaus Trust in honing the argument FOR Passivhaus in the UK. Best of all was the venue - the opportunity for an informative visit to Mayville.