How passive house design handles electrical wiring and switchboards
Electrical work is one of the most common causes of airtightness failure in passive house construction – not because electricians do poor work, but because standard electrical installation practice wasn’t designed with airtightness in mind.
Cables punched through walls, lights cut into ceilings, switchboards mounted on external walls – all of these are routine in conventional construction, and yet all of them can create significant air leakage in a passive house if they’re not carefully managed.
Airtightness in passive house design
In a passive house, the airtight layer runs continuously around the building envelope, through walls, across ceilings and under floors. Every time an electrical cable penetrates that layer, it creates a potential air leakage pathway. And every time a fitting is mounted on or cut into the airtight layer, it creates a junction that needs to be sealed.
In a large home with dozens of power points, light fittings, switches, data cables and appliance connections, the cumulative number of potential penetrations through the airtight layer is significant. Each one is small, but together they represent one of the most common sources of the air leakage that shows up in a disappointing blower door test result.
The solution isn’t to limit electrical installations – it’s to design and install them in a way that keeps the airtight layer intact.
Using a service cavity
The most effective strategy for managing electrical installations in a passive house is the service cavity. This is a secondary layer of wall framing or battens installed on the warm side of the airtight membrane, creating a dedicated space for electrical cables, conduits, power points and switches that sits entirely within the conditioned envelope rather than penetrating it.
In a typical service cavity wall, the structural frame and insulation form the outer part of the wall assembly. The airtight membrane is applied to the inside face of that frame, carefully sealed at all joints and junctions. A secondary row of battens – typically 40 to 50mm deep – is then fixed to the inside of the membrane, creating the service cavity. Plasterboard is fixed to these battens, and all electrical work runs within the cavity between the membrane and the plasterboard.
This approach means that power points, switches, data outlets and most cable runs are installed entirely within the service cavity, with no penetrations through the airtight membrane at all. The membrane remains continuous and unbroken, and the electrical installation proceeds largely as it would in a conventional home.
Ceiling penetrations
Ceiling penetrations are where electrical installations most commonly cause airtightness problems in passive house construction. A recessed downlight cut into a plasterboard ceiling creates a hole that bypasses the airtight layer, and a conventional downlight housing has gaps around its perimeter that allow air to move freely between the conditioned space and the ceiling cavity above.
In a passive house in Australia, recessed downlights require careful management. The options include:
- Using airtight downlight housings that incorporate a sealed backing box preventing air movement
- Locating the airtight layer above the ceiling level so that downlights don’t penetrate it
- Avoiding recessed downlights entirely in favour of surface-mounted or pendant fittings that don’t require ceiling penetrations.
Surface-mounted LED fittings – which attach directly to solid ceilings or walls without requiring cutout holes – are the simplest solution and are increasingly common in passive house designs in Australia. They deliver excellent light quality and eliminate one of the most common sources of airtightness failure in residential construction.
Where recessed downlights are specified, passive house builders in Australia will typically require airtight backing boxes to be installed for every fitting during construction, with the airtight membrane sealed to the perimeter of each box before the plasterboard is fixed. This adds time and cost to the electrical installation but is the only reliable way to maintain airtightness through recessed ceiling fittings.
Switchboard location
The location of the main electrical switchboard is a decision that needs to be made early in the passive house design process. A switchboard mounted on an external wall requires cable penetrations through the building envelope – both the cables entering from the street and the distribution cables running to different parts of the home. Each of these penetrations needs to be sealed for airtightness. The switchboard enclosure itself can also be a source of air leakage if it’s not carefully detailed.
The simplest approach is to locate the switchboard on an internal wall within the conditioned envelope, away from the external building fabric. This eliminates the external wall penetration challenge and simplifies the airtightness detailing considerably. Where a switchboard on an external wall is unavoidable – for example, where the network operator requires street-side access – the cable entry points need to be carefully sealed and the switchboard housing detailed to prevent air movement between the outside and the internal space.
Cable entry from outside
Every cable that enters the building from outside, like the mains electrical connection, telecommunications cables, antenna cables and EV charger connections from an external garage, creates a penetration through the building envelope that needs to be treated with the same care as any other airtightness penetration.
In a passive house design, these cable entries are planned during the design phase, located in positions that make sealing practical and detailed using appropriate products – gland seals, airtightness collars or purpose-made cable penetration systems. The casual approach common in conventional construction – drilling a hole through the wall, pushing the cable through and filling the gap with expanding foam – is not adequate for passive house airtightness requirements.
