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What is Passive House?

Updated: Sep 29, 2023



What is Passive House?

If you're new to Passive House, you're probably aware of the insulation requirements and high-performance glazing components within the building elements. Whilst these are integral, they aren't the most exciting results of Passive House design and architecture. The constant oxygen-rich air, the reduction (elimination) of internal condensation and the eradication of mould within our homes is by far the most significant impact the Passive House revolution holds, and quite frankly, it's the only way we should be building.


This post will run through a high-level summary of Passive House with the following blog posts to dive into detail-specific breakdowns of a design ideology, results and impacts.


There are five main principles of Passive House design:

  1. Airtight construction

  2. Thermal insulation

  3. Mechanical ventilation heat recovery

  4. High-performance windows and doors

  5. Thermal bridge-free construction

It could be argued that no principle outweighs the other. Unless all five principles are executed, you risk losing a benefit of the design and the impact can go beyond thermal comfort and dive back into unhealthy buildings. If the building is not airtight, you lose all efficiencies of the Passive House components and risk odours, chemicals and dust coming back into the building. Without insulation, the thermal comfort levels fall outside of the target comfort levels (20-25 degrees). Without a heat recovery ventilation system, you lose the healthy and constant supply of clean, oxygen-rich air and the exhaust of chemicals, odours, steam and chemicals from the internal spaces. Windows and doors are often one of the most expensive building elements within your envelope, which is strange to think that they are one of the worst performing. Glazing systems of Passive House design are typically double-glazed at minimum, often tripled, and thermally broken (goodbye single-glazed aluminium systems!). The elimination of thermal bridges halts the transfer from external temperatures to the internal elements of your home. It helps maintain internal surface area (not air temperature) at a constant level to eradicate the formation of draughts between hot and cold surfaces. It's noted that an internal temperature difference of 5 degrees from the air temperature is enough to feel a draught forming. Let's run through each principle.


Airtight Construction

Passive House airtight construction is crucial to achieving the certification and achieving the energy-saving and health benefits that Passive House buildings offer. Airtightness refers to a structure's ability to prevent air leaks through the building envelope. Achieving proper airtightness in a building is critical to reduce energy consumption, as leaks can result in significant energy waste. Moreover, airtightness enhances indoor air quality, making the indoor environment healthier and more comfortable for occupants. Air sealing can help protect the building from moisture, noise, and other external pollutants that can harm the structure and the occupants' health.


Source: https://proclima.com/products/internal-sealing/intello


Installation of airtight and vapour-permeable membranes serves as a critical element in realizing an effective Passive House airtight construction. (Vapour permeable is so critically important that a future blog post will be dedicated to this subject). These types of membranes are designed to provide a highly effective air sealing solution that can significantly reduce air leakage and heat loss whilst allowing moisture particles to pass through to eliminate the negative impacts of condensation, preventing the occurrence of condensation, mould growth, and insulation damage. The installation process entails covering all joints, penetrations, and other vulnerable areas around windows, doors, and ventilation ducts.


As a key component of Passive House construction, the sealing of penetrations is critical to achieving airtightness and reducing energy consumption. Penetrations, such as pipes, electrical wires, and ventilation systems, create points of weakness or gaps where air can seep in or out. Proper sealing of these penetrations is essential for maintaining an airtight building envelope. In fact, testing has shown that even a small gap or hole can significantly impact a building's airtightness. Therefore, it is important to use high-quality sealing materials that are appropriate for the specific penetration type and location. This is particularly important in areas where multiple penetrations intersect, as they present multiple opportunities for air leakage. Proper sealing of penetrations can help achieve airtightness, which is critical to achieve energy efficiency in Passive House construction.


Thermal Insulation

The Passive House standard is one of the most energy-efficient building standards in the world, and it requires a very low amount of energy to maintain a comfortable indoor temperature. Thermal insulation is a key component of Passive House design, and it plays a crucial role in maintaining energy and improving energy efficiency through your walls, floors and roof structures. The effectiveness of different insulation systems will be evaluated by measuring the thermal conductivity, R-values, and U-values.


Source: https://greenmagazine.com.au/product/insulate-naturally-pty-ltd/steico-flex036-flexible-insulation-made-from-natural-wood-fibre/


Applying the appropriate thickness of insulation layers is a crucial component of designing and constructing a Passive House that is optimized for thermal efficiency. The choice of insulation materials and their thickness must be carefully planned for maximum efficiency. It is essential to determine the insulation requirements based on the specific climate of the region where the building is located, as an insufficient thickness of insulation may cause heat loss and increase energy consumption, whereas an excessive thickness of insulation may lead to heat buildup in warmer climates. Without insulation, continuous insulation, that is, the envelope has little hope of fighting off your external conditions.


Mechanical heat recovery ventilation

MHRV is a ventilation system that recovers the heat from the exhausted air and uses it to preheat the incoming fresh air (in a cool-climate example). By doing so, MHVR reduces the amount of heat that needs to be supplied to the building and improves indoor air quality by constantly exchanging potentially dangerous indoor air (odours, chemicals, CO2) with fresh outdoor air. MHVR is essential in Passive House because the buildings are so air-tight that natural ventilation - opening windows and doors is often not required. It doesn't mean you can't open your windows, but you actually don't need to. But without the constant ventilation, you risk creating a home even unhealthier than the way-too-easy-to-pass NCC standards.


The Passive House standard requires buildings to maintain a comfortable indoor temperature without relying on traditional heating or cooling systems. Instead, the goal is to achieve a continuous supply of fresh air through mechanical ventilation systems. For the really, really hot/cold days, you can get away with installing an additional (very) small 2kw system instead of forking out tens of thousands on a 20kW system that simply escapes through the building envelope. The installation of mechanical heat recovery ventilation systems in Passive House buildings is an essential component to achieving the standard for energy efficiency and reducing the carbon footprint of buildings. However, not all MVHR systems are created equal, and there are several factors to consider when choosing a system for your Passive House project.


The first is the system's energy efficiency, which should be high to ensure minimal energy consumption in operation. The second is the system's airtightness, as a poorly sealed system can compromise the air quality inside the building and lead to energy losses. The third factor is the system's noise level, which should be low to ensure a comfortable indoor environment. By carefully selecting an appropriate MVHR system, you can help to create a Passive House that is energy-efficient, healthy, and comfortable for occupants. It is important to note that a performance solution is often required for the use of MHVR in Australia, as the rules around kitchen and bathroom exhausts are not explicitly complied with by recovery ventilation units.


High-Performance Windows and Doors

The building code (and BASIX system) are incredibly easy to pass to gain certification to build your project. A common theme in the NCC/BCA and the Passive House debate is just how much further the Passive House system goes to secure both comfort and health. A poor-performing window allows an incredibly simple, unwanted transfer of energy between internal and external conditions. A single-glazed, aluminium (note: un-broken!) window combines 2 very poor building elements together (single glazing & a highly conductive frame) and essentially creates a wide open door for the uncomfortable external conditions (too hot or too cold) to walk right into your home.



Source: https://www.neuffer.de/en/upvc-windows.php


So what should you be looking for in a window or a door?

Firstly, treat the worst-performing element of the component, and it is not the glass. The frame is the poorest performer, and when it comes to windows and doors, it is the most expensive. We should be looking to limit the thermal conductivity of the unit by moving away from un-broken metals towards timbers, uPVC or thermally-broken products. This severely limits the transfer of energy between internal and external conditions. If you only address the glass, the frame will act as the weakest link and show up as a drastic failure point on thermal imaging scans.


To go from a single-glazed to a double-glazed system does not double the cost of your window quote. As mentioned, a lot of cost comes from the frame itself. For this reason, I firmly believe single glazing should not be an option in Australian practice as the marginal price increase on the glazing makes a considerable difference in the performance of the window. All glazing is not the same, and the u-Values and solar heat gain coefficients (SHGC) need to be critically assessed for each project. The benefit of the Passive House calculations via the PHPP is that you can find a good equilibrium between budget and performance in order to satisfy the maximum energy demands of the house itself.


Thermal-bridge Free Construction

Thermal bridges, which are specific points or paths in a building envelope that allow energy to bypass the building envelope's protection, significantly diminish the overall performance of a passive house. Examples include the meeting point between the glazing unit of the window and the window's frame itself, a structural steel beam which continues from external to internal spaces (think of an exposed roof beam) and the intersection (or change of thickness) of materials.


Source: https://blog.passivehouse-international.org/what-is-a-thermal-bridge/


The temperature differentials allow unwanted energy to transfer and increase the risk of moisture accumulation, leading to issues such as mould growth and structural damage. By using the structural beam example above, if it is very cold outside and nice and warm inside, the cold chills the beam externally and the high conductivity of the steel beam transfers heat through your wall (bypassing your nice thermal insulation layers) and begins radiating the unwanted cool energy into your home. Think about on a cold day how cold your aluminium windows are to touch internally, even though the heating is on. By identifying and eliminating these thermal bridges, a passive house can maintain a comfortable indoor temperature while reducing energy consumption by up to 90%. WUFI calculations may be required at critical intersection points in order to nullify thermal bridges.


It is important to choose materials with low thermal conductivity to prevent heat loss at these junctions, and if that's not possible, it's important to reduce the transfer by ample insulation techniques. The impact of thermal bridging can be substantial and should not be underestimated. This is why Passive House construction provides guidance on identifying and addressing potential areas of thermal bridging. It is important to ensure that all components of the building, including cladding, foundation, and walls, are designed and constructed to avoid thermal bridges so that the building can function at maximum energy efficiency.


Conclusion

Through this high-level exploration of the 5 principles of Passive House construction, we can see that Passive House is not 'passive design', the term commonly associated with solar gains, thermal mass and cross-ventilation. It is a step beyond; a scientific experiment with proven results. What is great about Passive House is that each project is rigorously assessed during design, during construction and at the final stages of construction. To get the plaque fixed to the wall and say, "My home is a Passive House home", the design must be executed as drawn and specified, and the on-site testing of airtightness must pass.



Source: https://passiv.de/en/02_informations/02_passive-house-requirements/02_passive-house-requirements.htm


The benefits severely outweigh feeling cold or warm inside your home and truly shine when discussing the health benefits of improved air quality and reduction of mould/condensation. There are proven results of the reduction of poor-building-enhanced conditions such as childhood asthma, and the list goes on and on.


Shore Architects are currently undertaking Passive House projects, so please feel free to reach out to discuss your project.


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