PassivHaus is an approach to energy-efficient building design developed in Darmstadt, Germany during the 1980’s. The first Passive Houses were designed and built in 1991. Since then, many thousands of buildings have been constructed based on Passive House design principles all over the world. The objective of Passive House is, put simply, to provide a comfortable and healthy internal environment year-round and in all climates, whilst consuming very little energy. In fact, a Passive House uses approximately 10% of the heating/cooling energy used by today’s “standard” houses.
The secret to the success of Passive House is the so-called ‘energy balance’, whereby heat energy losses (through walls, floor, windows, roof and air drafts) are almost entirely replaced only by ‘passive’ energy gains, including the sun, waste heat from electrical appliances and even human body warmth.
In a Passive House, the energy balance is accurately calculated to maintain an internal temperature of 22⁰C. The relative humidity must stay around 50%, and a constant supply of fresh air maintains comfortable, healthy living conditions for the occupants.
To create an energy balance, heat losses must be minimized. To achieve this, Passive House focusses on five key elements in the building design:
1. An optimal amount of insulation in walls, floor and ceiling.
2. Elimination of thermal bridges.
3. High-quality glazing.
4. An air-tight construction.
5. Mechanical heat-recovery ventilation.
The role of insulation is simply to retard the flow of heat from the warmer side to the cooler side. To be effective, the insulation needs to be sufficiently thick, correctly installed, and continuous. The thicker the insulation, the more it prevents heat from escaping. But we only need just enough, depending on the type of insulation being used. Any more is a waste of money. Installation needs to be rigorously in accordance with the manufacturer’s instructions. Otherwise the insulation won’t work as effectively as we expect, and its insulation effectiveness may decrease sooner than the normal lifespan of the product. Finally, there should be no gaps in the insulation. Even small gaps can significantly reduce the effectiveness of the overall insulation layer.
Just like a regular bridge creates a point where people or traffic can move across a river, a thermal bridge is a place in the construction where heat can flow through the insulation layer. Examples include details like balconies that are part of a suspended slab, metalwork that extends from the interior to the exterior, and even bends in or changes to the insulation material. In a well-designed building, thermal bridges will be either removed from the design altogether, or their effects cancelled out in other ways.
Windows let light and heat into our rooms, but they can also be a significant point of heat loss. Whilst even basic double-glazing can reduce heat loss, good quality double or triple-glazing is required in most Australian climate zones to achieve the Passive House energy balance. Another point to consider is that the window frame can also allow heat to escape if poorly designed or constructed from inappropriate materials. An example of this is cheap double-glazed windows available on the market with solid aluminium frames. Another important point to consider about glazing is shading. To avoid over-heating in summer, it is imperative that sunlight cannot reach the exterior glass. This can be achieved by correctly-designed eaves, pergolas or louvres. Even a well-placed deciduous tree or grapevine can work.
Drafts, whether or not you can feel them, are another major source of heat loss. Drafts can occur around poor-sealing doors and windows, old-style wall vents or ducts, and around window reveals. Even a tiny crack can allow a significant volume of air to flow, and can cause humidity or condensation problems within the building structure. A well-designed building has a defined airtight / humidity barrier. This is typically created by a product installed specifically for this purpose, and it should be tested during and at completion of construction to ensure correct functioning.
Mechanical heat-recovery ventilation
An adequate supply of fresh air is essential for comfortable living. The Passive House standard requires a compliant MHRV system to be installed as part of the design. Such systems use efficient electric fans to draw in fresh air from outside, and duct it into living rooms. Simultaneously, stale air is drawn out of wet rooms (kitchen, bathrooms and toilets) and ducted outside. In Winter, the heat-recovery component can extract heat from the stale air, and use it to warm the fresh air on the way in. MHRV systems also have inline filtering to remove dust and other pollutants from the air. They are an extremely efficient way to ensure a constant supply of fresh, clean air, and maintain a healthy humidity level inside the building.