Passive House

Passive House (Passivhaus) is a building standard designed to significantly reduce the energy intake of buildings and ensure comfortable homes. This rigorous standard uses physics as a design tool to calculate extremely efficient building fabrics that on average reduce heating and cooling demands by 90%.

The idea of Passivhaus in the Australian environment is often received with scepticism as it was initially designed to cater to colder climates. The first Passivhaus was completed in 1991 in Germany, to date there are 4570 Buildings worldwide. Although a relatively new concept in Australia the standard has been increasingly adopted and tailored to suit our environment, at present, there are 11 certified buildings in Australia and many more on their way. Since its inception almost 30 years ago it’s proved an asset for many climates all over the world.

In Australia, 40% of energy consumption in buildings is evenly attributed to heating and cooling and with the largest homes in the world, it is telling that a huge portion of environmental change lies in improvements in the thermal efficiency of our buildings. On average Australia (per capita) uses almost as much energy as Germany (per capita) does on heating, this alone indicates that Passivhaus is not only suitable for Australian buildings but it’s important. An average household uses 200kWh/(m2a) whereas a Passivhaus uses 15kWh/(m2a), that’s more than a 90% saving in energy costs and a decrease on your carbon footprint.

The Passivhaus standard is built upon 5 scientific and environmental building principles, the basis of these are appropriate for any climate with or without Passivhaus certification. The five principles of Passivhaus design are a calculated level of thermal insulation, a continuous airtight layer, high-quality windows, thermal bridge free design and the implementation of a mechanical heat recovery ventilation system (MHRV).

Thermal Insulation

A Passivhaus must have a continuous thermal envelope, essentially encasing internal spaces facilitating the retention of warmth during the winter and coolness throughout the summer. Sufficient thermal insulation won’t always replace the need for heating and cooling appliances, rather it improves a buildings ability to retain optimum temperatures which significantly reduces energy intake. If a Passivhaus is ideally located receiving sufficient solar gain in winter and capturing breezes in summer, passive methods of heating cooling will be sufficient. Depending on the climate zone, Passivhaus walls are often slighting thicker than traditional Australian construction, however, if thin walls are a priority system such as SIP panels or vacuum insulation may maximise floor area.

Airtight Construction

An airtight layer is about introducing a high level of thermal control to the indoor environment. An airtight layer ensures heat is not lost to accidental ventilation, this can occur through leaks, holes, or cracks in the building’s construction or insulation. A certified Passivhaus is subject to a blower door test, positive results will confirm that the building has been constructed free of imperfections within the airtight layer. There are numerous ways to achieve an airtight layer, the easiest of which is as simple as a coat of plaster supported by membranes and tapes. In a well-designed Passivhaus the only way air should enter the building is when it’s been invited through the manual opening and closing of windows or doors or as filtered air from a mechanical ventilation system.

Passivhaus Windows

Passivhaus windows are glazing units that are analysed to ensure a controlled level of thermal transmittance. The Passivhaus level of thermal insulation becomes counterintuitive if the temperature can escape through gaps in a windows hardware or transfer through low performing glass. In Australia, double glazing is usually sufficient in meeting Passivhaus standards. Similar to traditional building design glazing on façades should be located and sized to optimize winter solar gains and avoid undesirable heat gains in the hotter months. As a rough guide glazing should total 25% of north-facing walls. The total area of glazing should make up 15-20% of a buildings total floor area and skylights should make up no more than 10% of this. Contrary to common misconceptions around passive houses it is important that windows are openable to encourage cross ventilation. Tilt and turn windows are popular in Passivhaus design as they have the ability to capture and direct breezes. If large areas of glazing are desired external shading devices should be introduced to protect glazing from substantial solar heat gains during the hotter months. The solar co-efficient of glazing can be used to assist in the control of these solar gains.

Thermal Bridge Free Design

Passivhaus construction must be thermal bridge free. Thermal bridges are points in a building's fabric that transfer heat at varying rates than the majority of that building, these usually occur at junctions that involve a change in direction, in material or in structure. If these bridges penetrate or interrupt the thermal envelope, they’ll create a bridging effect transferring heat into or out of the building.

“A Passivhaus wall is like a road. Just as ‘speed bumps’ decrease the speed of cars travelling along a road, insulation in a wall slows the movement of heat (which is generally from the inside to the outside). Thermal bridges in a wall create a highway which bypasses the speed bumps and allows heat to travel much faster through a wall”

Ventilation with Heat Recovery (MVHR)

Mechanical heat recovery ventilation systems (MHRV) are energy recovery ventilation systems. They take outside air, filter, circulate, and then remove stale or contaminated from internal spaces, such as bathrooms. Prior to air extraction in colder months the system recovers and recycles up to 90% of heat. MHRV incorporate a summer bypass for warm months when it becomes unnecessary to recovery heat. HRV systems don’t replace the need for heating or cooling appliances however in extreme climates they can be paired with coil accessories to assist in the conditioning of air temperature.

Australia is lucky to have a comparatively high level of air quality, however, people suffering from conditions such as asthma, respiratory diseases or allergies will know that our air is in fact more imperfect than is common knowledge. As the average person spends 90% of their time inside, air quality in our buildings is extremely important. In Australia we have an aging and growing population, we are also one of the highest producers (per capita) of greenhouse gasses in the world, accountability, improvements to and coping mechanisms around air pollution are becoming impedingly important.

“Contribution to global warming by Australia. Australia has one of the highest per capita emissions of carbon dioxide in the world, with 0.3% of the world's population it produces 1.3% of the world's greenhouse gases”

MHRV systems help to control humidity reducing the risk of condensation. In humid or cold climates particularly in uninsulated buildings, there is a significant risk of condensation, this moisture causes dampness and increases the chances of mould spores occurring, these tiny spores are transported in the air and when inhaled have the potential to cause both minor and more serious health issues, with 1 in 9 Australians suffering from asthma, respiratory health is important. The effects of moisture on building fabric can cause deterioration, reduce the life of a building and add additional maintenance costs.

Water Savings

Drought in Australia is a concept we’re very familiar with, so how does a Passivhaus encourage water savings. Although not a focus of Passivhaus it’s worth noting that insulating hot water pipes help to save ensuring water runs hot quicker. If your pipes are sufficiently insulated you could save 1000s of Litres a year per user.

What materials are suitable for a passive house?

Most materials are possible in Passivhaus design; however, some are more appropriate.

Masonry
Unlike traditional masonry in construction, In Passivhaus design, it’s preferable to externally insulate masonry so its thermal capabilities can be used, for example, reverse brick veneer. If the appearance of masonry on the exterior is desired its purpose becomes purely aesthetic and you will end up with extremely thick walls. And as heavy cladding systems need the be thermally broken at ground level detailing a masonry Passivhaus can often be more complex. Window placement has to be carefully detailed within the thickness of the wall.

Cavity
Traditionally in Australia, we do not insulate cavity’s and the uncommon nature of doing this trade may incur additional fees as it’s an unfamiliar construction method. Furthermore, wider cavities are required to provide space for insulation and wall ties between the cavity act as thermal bridges – wall ties with low thermal conductivity are expensive.

Timber
Ecologically sourced timber is the ideal material for the Australian environment as it is economical, minimises wall thickness, and is ecologically friendly when sourced properly. It’s a recommended material if you have minimal floor area to work with. Their lightweight qualities mean it’s easier to detail at reduced requirements of being thermally broken at the ground. Termite avoidance relating to timber is the trickiest part about the material. Timber has low embodied energy.

Steel
Steel frame construction can introduce considerable thermal bridges when not integrated or considered from the very beginning. Steel column to concrete foundation details need particular attention to ensure the thermal envelope isn’t compromised. Equally, anywhere else where structure needs to pass through the thermal envelope will require close attention to eliminate thermal bridging. This material has a high embodied energy.

Concrete
Concrete often allows for thermal bridge free design, is airtight, it has a high embodies energy and will usually need to be used with another material, for example for roofing.

Mixed
This is much more complex constructing and therefore designing junctions between different systems.

SIP Panels
SIP Panels are structurally insulated boards with inbuilt insulation. They are a thermal-bridge free option, airtight, and thermally efficient and allow for relatively thin construction, they’re worth considering if space is an issue. It is a less ecologically friendly option as the interior layer consist of rigid plastic foam, and cannot be recycled.