Service cavities deliver multiple benefits They make your Passive House building work

16 June 2021 by Jason Quinn

Architects and designers need to create plans that are buildable. That seems obvious, but it’s not always the case. When it comes to Passive House buildings, it’s even more important because of the high standards of performance and durability required and the fact that these attributes are independently verified through the certification process.

Insulated service cavities don’t sound super-exciting but they are found in most high-performance buildings for good reasons. They both protect the air vapour control layer (AVCL) and make it easier to construct. They increase thermal performance by adding a second layer of insulation.

A quick detour into the world of AVCL

  • The air control layer prevents air leaking through the building enclosures.
    This stops heat escaping and reduces heating demand. It also prevents moisture being carried on that airflow into the walls and roof structures, where it can condense and cause damage. This increases building durability.
  • The vapour control layer moderates vapour flow due to vapour diffusion through the building assembly.
    Most vapour control layers also increase the assembly’s drying potential by becoming more permeable to vapour when the local relative humidity is high. Vapour control layers moderate how moisture transfers directly through materials, as distinct from moisture carried on air leakage.

3D illustration of a timber-framed house with narrow ceiling service cavity and insulated wall service cavity. Image: Sustainable Engineering Ltd

The air and vapour control layers can be separate but very often, one material delivers both these functions. Examples of AVCL include specialist membranes, plywood, most OSB and wet plaster. Every penetration of the AVCL needs to be sealed or its performance will be compromised.

The use of service cavities minimises the penetration of services through the AVCL and makes it faster, easier and cheaper to run wiring, plumbing and ventilation ducting. Service cavities are common in walls, but are also used in ceilings and occasionally in floors. The cavity is usually inside the structural elements and the AVCL.

When thicker wall assemblies are called for (eg in colder climates), there is the risk of water vapour penetrating from the interior of the building, getting colder and eventually condensing inside the wall. Service cavities allow the AVCL to be optimally positioned within the wall assembly.

A wall service cavity is typically constructed by fixing timber battens to the inside of a flexible or rigid AVCL. Electrical cabling is usually loosely tacked to the battens. The battens serve no structural purpose and can be spaced to ideally suit the installation of insulation and mounting the interior finish lining. Insulation is very often a fibre insulation batt, but rigid insulation can be used. In a northern climate, it might be acceptable to skip insulating the service cavity but this is rare. No more than a third of the assembly’s insulation should be inside the AVCL without specific analysis. A greater proportion can bring risks of moisture build up. There are cases where more insulation is called for in the service cavity but we suggest you seek guidance.

If running wiring only, 45mm is the most practical minimum depth of the cavity. Cavities to accommodate ventilation ducting may be much deeper, up to 140mm.

This ceiling service cavity neatly contains wiring, plumbing and MVHR ducting. It is a relatively deep cavity to accommodate the MVHR terminal (outlet to the room).

The integrity of the AVCL should be blower door tested as early as possible during construction, when there is still an opportunity to repair it if necessary.
Without a service cavity, every penetration of the AVCL would have to be sealed. Special airtight boxes would be required behind every electrical outlet and light fitting. Wall linings would have to be fixed (directly to the AVCL) before the first blower door test. If air leakage levels were unacceptable, it would be impossible to identify the sources of leaks or repair them without removing the wall linings. Further, any subsequent damage to the wall or ceiling lining would likely also damage the AVCL.

An earth plaster serves as the AVCL in the Hiberna strawbale Passive House! While rare in New Zealand, wet plasters are used for masonry buildings, a common Passive House construction type in Europe.

More resources
Service cavities are discussed in two publications just released. Sustainable Engineering’s Passive House Primer is full of straightforward and helpful advice for architects and designers who are beginning to design higher-performance homes, all the way up to Passive House standard.

The long-awaited High-Performance Construction Details Handbook, substantially written by Sustainable Engineering’s Jason Quinn, has been released by PHINZ. It is a highly technical guide full of construction details from successfully built New Zealand homes.

Both are available for free download as digital documents.