In February, schoolchildren from around the globe went on strike to demand urgent action on climate change. It followed stark warnings within a report from the Intergovernmental Panel on Climate Change (IPCC) stating that unprecedented measures are required within the next 12 years to limit temperature rises to 1.5°C above pre-industrial times – avoiding potentially catastrophic global impacts.
With the built environment estimated to account for around 40% of total UK carbon emissions1, improving the energy efficiency of our buildings must be viewed as a priority.
The Passivhaus Standard offers a proven model for minimising the energy usage of buildings via a fabric-first approach. By applying its principals with the precise design, improved predictability and outstanding thermal performance of structural insulated panels (SIPs), developers are now achieving Passivhaus Certification on projects of increasing scale and complexity.
At its core, the Passivhaus Standard aims to allow the creation of buildings which require very little energy to heat or cool, whilst also providing a high level of comfort for occupants. To achieve this, it sets clear energy performance targets which a building must meet:
Primary energy demand ≤ 120 kWh/m2/yr
Space heating/cooling demand ≤ 15 kWh/m2/yr
Specific cooling load ≤ 10 W/m2
Passivhaus performance targets for cooler climate buildings
To put these figures in context, the maximum space heating demand for a Passivhaus building is around 10% of that of an average home (estimated to be 140 kWh/m2/yr 2). As such, whilst these criteria do not specifically address a building’s carbon emissions, in practice they should significantly limit emissions when compared with a property built to current Building Regulations/Standards.
To meet these criteria, all areas of the external fabric of the property typically need to be insulated to a U-value of 0.15 W/m2.K, or lower. It is also a requirement of Passivhaus that the building be fundamentally ‘thermal bridge free’. To achieve this, close attention to detailing is crucial when designing the building and installing the insulation to ensure that potential thermal bridges around openings and at junctions (especially the wall / floor) are properly addressed. In addition, air leakage rates must be no higher than 0.6 ach@50 Pa. This is typically achieved by installing an airtight layer, such as oriented strand board (OSB), and airtight tape, which is applied to seal all junctions.
High levels of airtightness within Passivhaus buildings necessitates good ventilation via means of a mechanical ventilation with heat recovery (MVHR) system. MVHR systems extract the heat from outgoing stale air and transfer it to warm incoming fresh air, further reducing the heating demand and ensuring a fresh, comfortable environment within the home.
Whilst it is possible to attain Passivhaus certification with traditional construction methods, in many cases offsite construction approaches such as SIPs can provide a simpler, faster and more adaptable solution to meeting the demanding fabric requirements.
A typical SIP comprises an insulated core sandwiched between two layers of oriented strand board (OSB), with a jointing system that ensures excellent insulation continuity throughout the envelope, limiting repeating thermal bridging. The panels are precision cut to each project’s particular specifications in a production facility, including spaces for openings, such as windows and doors. This ensures an accurate fit, significantly reducing the need for onsite adjustments and waste. It also gives architects considerable freedom in determining the design for the property.
The panels offer excellent ‘out-of-the-box’ fabric performance with whole wall and roof U-values of 0.20 – 0.17 W/m2.K, or better. By assessing all junctions and openings within the building envelope, and carefully installing additional insulation, thermal bridges can be eliminated, and the U-values of all elements reduced to the required level.
The jointing arrangements inherent in SIPs can also support extremely airtight structures. Once an airtight membrane is fitted internally and tape is applied to junctions, the air leakage rate can be reduced to the 0.6 ach @ 50 Pa required by the Passivhaus Standard.
SIPs also provide a number of practical benefits. Their offsite production process supports greater predictability in scheduling, allowing project teams to accurately plan for panel deliveries, avoiding trade overlaps and maximising site efficiency.
The panels can be quickly installed by a small team of trained operatives with a dry construction process that is less dependent on weather conditions than other traditional approaches. When SIPs are used for both the walls and roof, the outer shell of domestic properties can often be erected in just two to three weeks. With the addition of a breather membrane to the panel exteriors, the construction is made weathertight — allowing internal fit-out to begin. The outer timber facing also provides a suitable substrate for a variety of cladding options including brick slips, render and timber cladding.
One project to take advantage of the benefits SIPs provide is the Norwich Regeneration Company’s Rayne Park estate. The development includes a mix of private and affordable housing, with 112 of the 172 properties, earmarked for full Passivhaus Certification.
The Kingspan TEK Building System was chosen to form the envelope of many of the dwellings based on its technical specification and value offered through its offsite production process. The first phase of the development completed this March, with the Passivhaus units expected to have a heating demand of just 11 kWh/m2/yr and a primary energy requirement of 77 kWh/m2/yr.
With over 65,000 buildings now certified Passivhaus around the globe, the Standard provides a clear route to dramatically reducing the energy performance, and consequently carbon emissions, from our buildings. Offsite approaches such as SIPs provide the ideal delivery method for this standard, allowing the cost-effective construction of entire estates.
1 UK Green Building Council – Climate Change www.ukgbc.org/climate-change
2 Why Choose Passivhaus? Passivhaus Trust www.passivhaustrust.org.uk