What is the role of psi values in Passivhaus?

Minimal thermal bridging – and therefore low psi values – are an important part of the building fabric measures required to achieve the Passivhaus standard. Indeed, some new-build Passivhaus buildings aim to achieve a ‘thermal bridge-free’ envelope, meaning psi value heat losses are all but eliminated.

The complete elimination of thermal bridges is much harder in an existing property, where it can be harder to work with the existing structure. Nevertheless, the Passivhaus equivalent for retrofit projects – EnerPHit – still requires thermal bridging to be addressed and modelled.

What is the Passivhaus standard?

The Passivhaus standard is often considered as a low energy standard, because it aims to minimise the energy required for heating and cooling. Nevertheless, it is more accurately described as a comfort standard, as it seeks to maintain stable internal temperatures and avoid the draughts that usually cause people to feel uncomfortable.

As the name suggests, the Passivhaus standard uses passive building design techniques to reduce the need for mechanical heating and cooling.

It makes use of compact building forms, with junctions kept as simple and few in number as possible. That way, there is less overall surface area through which heat can be lost, and what surface area there is can be constructed to be highly thermally efficient.

The standard also seeks to make the best possible use of solar gains, ensuring the effect of the winter sun is maximised while preventing summer sun from causing extended periods of overheating in the building.

To achieve lower energy consumption and more reliable levels of comfort, the Passivhaus standard goes beyond the minimum requirements of national building regulations.

While building regulations – such as the current Part L 2021 – include requirements for thermal bridging, the typical detailing able to satisfy those requirements is not enough to meet Passivhaus. Addressing thermal bridging heat losses contributes to both the low energy and comfort aspects of the standard.

How do low psi values help to achieve comfort?

There are three main areas of Passivhaus building design where well-designed junction details and linear thermal bridges really help to make a difference.

High levels of insulation and minimal thermal bridging

Constructing building fabric to meet the Passivhaus standard requires achieving very low U-values. That includes using high performance windows that achieve much lower U-values than typical glazing units.

For that performance to be delivered in reality, insulation must be designed and installed to be continuous where different building elements meet (including where windows sit within an external wall or roof). Otherwise, heat energy is simply lost at these thermal bridges between elements.

A U-value cannot measure this thermal bridging heat loss, so instead psi values must be calculated. When well-designed thermal bridges with low psi values are achieved along with low U-values, heat loss is minimised across the whole building envelope. This provides consistent and predictable performance.

Equally importantly, there are no localised areas of lower temperature that cause building users and occupants to sense a temperature difference and feel uncomfortable. The Passivhaus standard requires a minimum internal surface temperature of 17 deg.C to be maintained throughout the building.

To ensure thermal bridges are designed to help achieve that target, the thermal modelling that establishes their psi value also gives a surface temperature factor (fRSI).

High levels of airtightness

As building users, we not only perceive differences in surface temperature, but also the movement of air. Poorly constructed junction details can act as a path for warm air to escape the building, which creates uncomfortable draughts and causes the heating system to work harder.

While psi values themselves cannot measure airtightness, good thermal bridge detailing and correct construction on site is a critical part of helping to achieve the airtight building envelope required for Passivhaus.

Indoor air quality

The Passivhaus standard seeks to create good indoor air quality through a number of ways. The main one is the use of mechanical extract ventilation, often with a heat exchanger so that outgoing warm air is used to warm incoming cool air.

The design and specification of the building fabric also plays a part in achieving good air quality. If a linear thermal bridge is responsible for localised excess heat loss, then condensation could occur at that detail.

When warm, moist air makes contact with a colder surface, the air cools. The air can no longer hold the same quantity of moisture, and any excess is deposited as condensation. Cool, damp surfaces are ideal conditions for mould to form, with a potential corresponding increase in the risk of respiratory problems for occupants.When fRSI is calculated as part of modelling a thermal bridge, it can be used to predict the likelihood of condensation and mould growth occurring. As an example, an fRSI of 0.75 or greater is the target for residential buildings to help ensure that condensation will not occur.

Calculating psi values for Passivhaus projects

There are a variety of sources for thermal bridge detailing and calculated psi values. However, libraries of standard details usually only deal with ‘typical’ detailing associated with building regulation compliance.

It may be possible to obtain some standard detailing to the level expected for Passivhaus, but then your project will need to be able to accommodate that detailing exactly as given.

For many Passivhaus projects, bespoke detailing and calculation is required, to address the individual nature of each project. That’s where the AutoPSI thermal modelling tool from BRS Technology can help.

The online software allows you to calculate psi values quickly and simply, all in accordance with relevant standards. Its drag and drop interface means you can create bespoke, customisable junction details. Both the psi value and surface temperature factor are calculated instantly, so you can easily check whether a proposed detail will help you to achieve Passivhaus certification.

Find out more by visiting the AutoPSI website.