What are the 5 Principles of Passive House Design?

Over the last few years, we have seen a significant increase in interest in Passive House design. For some people this is part of their ongoing commitment to sustainable living and reducing their environmental impact. To others, it is the efficiency of a Passive House, and the significantly reduced ongoing costs, that is most appealing.

But whatever is driving your interest, it is important to understand what makes a Passive House so unique. Here we explore this, taking a closer look at the key principles guiding the design and build of these structures. We will also touch on the benefits they provide and why so many homeowners are so interested in this approach.

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What is Passive House?

The Passive House concept is considered by many to be the gold standard in energy efficient home design. It has been established to promote good building practices that have a focus on superior management and retention of heat energy. It also provides a set of standards to assess construction works against and a framework for certifying the most efficient structures.

On a more practical level, Passive House design provides a range of benefits. Most significantly, research has shown that homes that meet the Standard use up to 90% less energy on climate control. In fact, to be certified as a Passive House, a home must use less than 15kWh/m2 per year for heating and cooling.

Additionally, a Passive House provides a high level of thermal comfort, maintaining an internal temperature of 20 – 25oC for most of the year. The humidity level must also be kept below 12g/kg (or approximately 60% at 25oC) for most of the year. And the advanced ventilation system helps maintain high air quality and health, while preventing odours and the growth of mould.

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The 5 Passive House design principles

Passive House design is a holistic approach to maximising the thermal management and energy efficiency of a structure. Five key principles are used to guide design and build activities, ensuring they meet Passive House’s high performance standards. These are heavily interconnected and all five must be adopted to achieve the full range of benefits.

#1: Airtight construction

The “building envelope” is a key concept in passive house design. This represents everything that separates the inside of a home from the outside – the exterior walls, the roof, and the floors. In a Passive House, this needs to be an almost impenetrable structure that stops almost all air leakage.

This is because, when air escapes from inside a structure, it takes heat energy with it. As a result, more energy is required to keep the internal climate at a consistent and comfortable temperature. Air leakage can also cause unpleasant draughts, creates cools spots within the structure, and can lead to condensation issues.

To prevent this, a membrane – or air barrier – needs to be installed throughout the building envelope to stop the flow of air. While most new houses will have something like this, it is usually not consistently applied and will have multiple penetrations (for windows, doors, etc.). This is particularly true for lower quality new builds, where an effective air barrier is not considered an important feature.

However, in a Passive House, the air barrier is a key consideration during both the design and build stages. Passive House builders use a combination of materials to ensure one continuous membrane encases the whole building envelope. Specially designed products (e.g. Passive House compliant windows and doors) are also used to minimise the impact of the required penetrations.

#2: Superior insulation

Much like an air barrier, all new homes are required to have some form of insulation installed. This is usually done in the external walls and ceiling, and provides some thermal protection for the home. However, the volume and quality of material used varies greatly, with lower-cost builders often choosing to do the bare minimum.

By contrast, Passive House designs prioritise insulation, installing throughout the building envelope – including under the floor. This adds another layer of thermal protection to the home and further limits the potential for heat exchange.

Importantly, the material used needs to be high quality – a good custom home builder should be able to recommend suitable products. It also needs to be applied consistently, with minimal breaks for structural elements and penetrations. In addition to helping prevent heat lost, this also acts and soundproofing and makes the structure more resilient and durable. In fact, a Passive House should be able to maintain a comfortable internal climate, even without power.

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#3: Ventilation and heat recovery

Because the building envelope is so airtight, a mechanical system is required to manage airflow. This helps bring a steady stream of fresh air into the home, and exhausts out pollutants and moisture. It should also have a filtration system that helps maintain the quality and health of the air coming in.

The Passive House design principles also require ventilation systems to have a heat recovery component. In the cooler months, this captures the heat from the dirty exhaust air and transfers it to the clean incoming air. In certified Passive Houses, this helps retain at least 75% of the internal heat energy.

#4: High performance glazing

Glass windows and doors are generally the source of the greatest heat losses in most homes. This is particularly true here in Australia, where thin single-glazing – which provides minimal thermal protection – is used widely.

The Passive House design principles address this by requiring higher quality glazing that is designed to significantly reduce heat exchange. For example, many Passive House designs feature windows and doors that are triple glazed, and thermally broken. There is also a range of Passive House certified products that have been proven to meet the required performance standards.

The number, size, and location of windows and doors also needs to be given careful consideration in a Passive House. This is because designs are required to maximise natural light and make the most of free passive solar heating. Though this needs to be balanced against the potential for heat loss at night and during the cooler months.

Here in Australia, Passive House designs also need to consider how to protect the home from overheating in the height of Summer. For example, many homes have a retractable shade structure, like an awning, fitted around each window. This allows them to take full advantage of the sun in the cooler months and minimise its impact in the warmer ones.

#5: No Thermal bridges

A thermal bridge is a weak spot in the building envelope that allows heat to escape. This is usually where there is a break in the insulation, like for a window, door, or structural elements (eaves, corners, balconies). The Passive House design principles require potential thermal bridges to be identified early and addressed during the build phase.

The best way to do this is to avoid the common causes of thermal bridges. For example, a self-supported balcony will require significantly less penetrations of your insulation than a cantilevered balcony. Similarly, as corners are difficult to insulate, minimising the number of them will reduce the number of thermal weak spots.

Where breaks in your insulation are unavoidable, a material that is less conductive to heat (e.g. timber) should be used. These materials can also be used between more conductive materials (e.g. metal) to create a thermal break.

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Other considerations when building a Passive House

In addition to the key design principles, there are a few other factors to consider when planning a Passive House. This includes:

• Upgrading existing structures: With a few relatively minor changes, it is possible to upgrade an existing home to meet Passive House design principles. This usually requires the windows and doors to be replaced and more, higher quality insulation to be installed throughout the structure. While this should improve the performance of your home, a knockdown and rebuild is usually required to realise all the benefits. A knockdown and rebuild also allows you to increase the size and update the style of your home.

• Architectural style: Modern home designs are the most common choice for those looking to build to the Passive House Standard. That said, it is possible to apply the Passive House principles to the design and build of almost any style of home. For example, if you would like a French Provincial home, there are ways to upgrade the structure to meet the Standard. That said, most Passive House products suit modern home designs and products for French Provincial homes will be more expensive.

• Cost of construction: As Passive House designs are becoming more popular, the price of the specialised products they require is steadily coming down. However, a Passive House is still more expensive to build than a comparable home using traditional construction methods. The exact difference in price will depend on a range of factors, but could be as low as 3% – 5%.

More information?

If you would like to discuss Passive House principles further, or are considering building your own Passive House, contact Carmel Homes. We are one of the leading custom home builders in Melbourne and are deeply committed to promoting efficiency and sustainability. As part of this, over the last 30+ years, we have helped design and build homes that minimise their footprint, without impacting their liveability.