The Passivhaus balancing act

Iain Fairnington of the A. Proctor Group highlights some of the key technical factors relating to performance in Passivhaus design, and best practice for managing the balance of movement of heat, air and moisture within the building envelope

Since time immemorial, basic protection from the environmental climate, rain, heat and cold have been key to the design of buildings we occupy. Add in fire and security, and we can see the range of factors affecting how building methods have evolved, and will hopefully continue to as new materials come to market.

Basic construction methods have evolved progressively throughout the centuries, leading up to the introduction of cavity walls in the late Victorian period, to protect from rain penetration. This has progressed further into the modern day, where Passivhaus is used by many as a yardstick for the balance of moisture issues, coupled with sustainable temperature control and a healthy building envelope.

In all this evolution, the key principles still need to be addressed, and the basic instincts of protection remain key.

We are living in unprecedented times and due to a pandemic, most are spending more time at home not just for leisure and relaxation, but for work too. The vaccination programme gives us all hope, but there is no doubt that these challenging times have increased people’s appreciation of the important things in life, including spending time with friends and family, perhaps over materialistic possessions.

Added to this, from the energy crisis in the early to mid-1970s to the current global climate crisis, there is a context which means we cannot rely on the earth’s existing resources for basic comfort goals.

In the design and construction of our buildings, it is crucial to take account of the HAMM (Heat Air Moisture Movement) principles, which consider the effect of insulation type and placement along with the vapour permeability of the various layers. This leads to a balance of key factors producing a healthy building envelope that protects the occupants.

Calculations & VCLs
One of the features of balancing these principles can involve calculations to assess condensation and/or moisture risks. Glaser calculations can do this, but these can have limitations. A WUFI calculation can help overcome these, e.g., rain and drying out of residual moisture in the construction.

A WUFI (BS EN 15026) calculation can show the effect different vapour control layers (VCLs) – with different performance levels – can have on the structure, and whether such a solution is critical.

It is difficult to argue that a VCL is not a good thing, but similar to a first Covid-19 injection, a VCL should be used to reduce the moisture risks to acceptable levels but not be relied upon as the sole method of protection. Remember a VCL is a vapour control layer – not as previously referred to in the antiquated terminology of a ‘vapour barrier,’ which led to over-reliance.

Generally, where the insulation is placed externally a VCL is less critical – compared with insulation solely between framing behind the sheathing board. This has led to the introduction of externally applied self-adhered vapour permeable yet airtight membranes.

It is the properties of these membranes in creating a self-adhered, airtight, yet vapour permeable layer, that allows it to be placed on the external face of the sheathing board. This allows for temporary water protection, airtightness and long-term vapour permeability. Along with the correct balance of insulation, this means a VCL may be less critical – yet it is important to assess this fully on a project by project basis.

There is little doubt that a VCL (especially variable resistance membranes) can be advantageous in several applications to reduce the moisture flow through a building envelope, but less critical in some areas, depending on the insulation placement.

For example, when placing insulation purely in between the frame of a timber frame building, the moisture risk may be too high to omit the VCL and therefore should be used to reduce vapour progressing through the structural frame. However, if this is balanced with insulation placed external to the structural frame, the dew point potential is reduced due to the warm frame and the VCL becomes less critical yet still good practice.

A good appreciation of the key balancing of heat, air and moisture movement can work together for the benefit of architects and contractors in continuing to improve the way that we design and construct buildings.

From an industry and design perspective, it is important to understand that VCLs should not be relied upon solely to reduce moisture build-up but can have benefits when balanced with other factors. The Passive House Institute (PHI) component database helps specifiers to identify certified components, which have been tested and passed the required standards. Innovative products offer solutions that can help to achieve the fine balance between heat, air and moisture movement.

Manufacturers are on hand to provide high-quality technical back up service around Passivhaus, including:

  • A Glaser and/or WUFI calculation service where appropriate to show the robustness of the proposed construction
  • Tool Box talks, virtual if required
  • Specification guidance
  • Site visits (dependant on Covid restrictions) that can give a compliance report showing areas that are well installed and areas needing improvement in the installation.

Iain Fairnington is technical director of the A. Proctor Group