Scots take the Passivhaus road with new standard

This Summer, the Scottish Government is due to consult on the “Scottish equivalent to the Passivhaus standard,” as a compulsory requirement for all new homes. This represents a significant shift, with modelling from the Passivhaus Trust suggesting that heating demand from homes completed to the Passivhaus standard could be 79% lower than an equivalent home built to the current requirements in Scotland.

The Passivhaus standard requires properties to be highly insulated, with thermal bridges eliminated and air leakage reduced to very low levels. Delivering this at scale will require the entire industry to look carefully at its practices and the construction approaches being adopted. In particular, offsite solutions such as structural insulated panels (SIPs) offer a number of advantages which make them highly suited to these requirements.

Understanding Passivhaus requirements

While the exact format of the Scottish equivalent is yet to be confirmed, the Passivhaus standard itself provides an indication of what we may expect.

Passivhaus was developed to close the performance gap and provides an approach that is quality assured throughout the build process, ensuring that certified buildings really deliver reduced energy use, good indoor air quality and comfortable temperatures throughout the year.

It sets out a range of ambitious energy performance requirements which certified properties must meet:

  • Airtightness n50: ≤ 0.6 ACH @ 50 Pa
  • Space heating/cooling demand: ≤ 15 kWh/m².a
  • Primary Energy Renewable (PER): ≤ 60  kWh/m².a
  • Surface temperature: ≥ 17°C
  • Overheating: Max 10% > 25°C

The space heating/cooling demand is far lower than for an average existing UK home – estimated to be around 130 kWh/m².a. The PER metric, meanwhile, is designed to encourage the use of onsite renewables, applying fuel factors to different sources of energy. These factors are typically set lower for low carbon technologies and onsite renewables, essentially making it simpler to achieve compliance with these approaches.

The air leakage rate is calculated using an n50 measurement. This differs from the measurement used in Part L and Section 6 of the Building Regulations and standards meaning the rates are not directly comparable. However, this rate is significantly more stringent, requiring close attention to details.

Above and beyond these targets, the Passivhaus standard also requires that buildings be independently assessed post completion to ensure they meet the performance targets not only on paper, but also in practice. Only once this has been confirmed will the building achieve certification. This essential step has helped ensure that there is little if any performance gap in Passivhaus properties.

Passivhaus approach

In addition to these targets, the Passivhaus standard sets out a clearly defined process for delivering them. Properties need to be highly insulated with careful attention paid to all details to effectively deliver a ‘thermal bridge-free’ construction. Air leakage rates through the fabric should also be effectively limited using a range of measures – including specialist triple-glazed windows.

A Mechanical Ventilation Heat Recovery (MVHR) system is used to ensure a good level of ventilation is maintained within the property. These systems use a heat exchanger to take the heat from stale outgoing air to warm fresh incoming air, further limiting overall heating demand.

Offsite solutions

The high performance criteria of the Passivhaus standard raise obvious questions on how this can be accurately and affordably delivered at scale in Scotland. One solution already used on a large number of UK Passivhaus projects is the adoption of structural insulated panels (SIPs).

SIPs are fabricated by bonding two sheets of particle board or oriented strand board (OSB) on either side of a rigid insulation core. This creates a highly insulated panel which can be used to create both pitched roofs and walls, whose thermal performance can be easily raised further with an additional insulation liner.

The panel’s arrangements, dimensions and interactions are designed in CAD. They are then pre-cut at production facilities using CNC cutting machinery. This helps to ensure the cut panels precisely match the designs including openings for windows and doors. This supports accurate detailing to eliminate thermal bridges and limit air leakage around these junctions.

Some SIPs also feature a cassette joint which slots into a routed channel between the panels, ensuring excellent insulation continuity across the envelope and further limiting air leakage. Once an airtight membrane is fitted internally and tape is applied to junctions, the air leakage rate can be reduced to the requirements for Passivhaus.

In addition to simplifying installation processes onsite, the offsite production process for SIPs also helps to provide scalable scheduling benefits. The factory cutting process allows identical panel kits to be produced for multiple units without the errors and inaccuracies common with traditional onsite processes. Production timescales can be easily predicted,
allowing for accurate scheduling and improved site logistics.

The shells of individual housing units can typically be erected in around two weeks by a small team of site operatives with lightweight lifting machinery. Once a breather membrane is applied and windows and doors fitted, work can then begin on internal fit-out, further speeding up the construction process.

SIPs can also support a wide range of facades including timber cladding, brick slips and renders to meet project requirements.

Looking ahead

In March, the Passivhaus Trust published its recommendations for the Scottish standard. These called for the standard to be based on the full Passivhaus, adapted into a Scottish-owned version including local weather data. It also suggested a phased transition, allowing time for upskilling, with buildings being accepted as “deemed to satisfy” the new standards by January 2025. Use of established technologies, such as SIPs, can support this transition – helping project teams to meet these ambitious timelines.

Jonathan Ducker is head of regulatory affairs at Kingspan Insulation