The thermal envelope is a crucial aspect of Passive House design, which aims to create highly energy-efficient buildings. It has three main elements: high insulation levels, a design that eliminates thermal bridges, and a tightly sealed envelope. These components not only contribute to energy efficiency but also ensure high-quality building physics and long-term structural durability.
The concept of Passive House design emphasises prioritising the performance of the building’s shell. This includes carefully selecting and using materials and components that make up the envelope. The thermal envelope acts as a protective barrier, keeping the indoor spaces comfortable and reducing energy loss.
Identifying and addressing any potential weak points in the thermal envelope is essential. This can be more challenging when there are basements, garages, or other connected spaces. However, by carefully designing and constructing the envelope, it’s possible to minimise heat loss and maintain optimal indoor conditions.
The thermal envelope of a Passive House works similarly to a vacuum flask, ensuring that indoor areas remain at a pleasant temperature. These buildings are designed to provide consistent temperatures on all internal surfaces and maintain a stable indoor climate. They effectively prevent temperature swings and drafts, regardless of whether it’s the cold winter months or hot summer periods.
Wherever you have identified building elements as part of the thermal envelope, you need to be able to detect three functional layers:
• an airtightness layer on the inside of the insulation
• the insulation layer
• a wind and weather tightness layer
air tightness layer
The airtightness layer is a very important part of Passive House buildings. It helps to achieve Passive House certification and ensures that the construction is of high quality.
In simple terms, airtightness means controlling the movement of air within the building. This is affected by factors like air pressure and temperature. It’s about making sure there are no drafts or leaks around windows, seals, or other areas where air can get in or out.
It’s worth noting that airtightness is not the same as a building’s ability to handle moisture. Moisture can still move through materials even if the building is airtight. So, a building can be airtight and still deal with moisture-related issues effectively.
Creating an airtightness layer requires careful planning. The first step is to draw a continuous line inside the building to mark the airtight envelope. This line is an important part of the overall insulation of the building.
When different materials come together or when installing doors, windows, or building services, it’s important to pay attention to detail. Proper design and sealing of any gaps are crucial to maintain the integrity of the airtightness layer.
insulation
The insulation layer plays a crucial role in passive house construction by providing excellent thermal protection for the building envelope. It helps to keep the heat inside during winter and outside during summer, making the interior pleasantly cool.
The amount of insulation needed for a passive house depends on factors like the climate, shape of the building, and its orientation. The quality of other building elements, like windows, also affects the insulation requirements. Efficient windows, for example, can reduce the amount of insulation needed. However, passive house insulation levels are generally higher than what building codes require. Although insulation can add some extra cost, it is a worthwhile investment due to the energy savings it brings, even with current energy prices.
In passive house projects, it’s important to have accurate and independent information about the performance and durability of insulation products. Depending on the materials and local climate, the insulation layer may need to be thicker than projects built to minimum code standards.
Insulation comes in different forms, such as straw bales, batts, SIP panels, and vacuum panels. The choice of insulation type depends on personal preferences since passive house construction focuses on performance rather than specific materials.
The required thickness of insulation can also vary based on the density and location of the building. For example, a compact multi-story apartment building in Sydney may only need 90 mm of typical wall insulation, while a sprawling single-story house in Melbourne may require 300 mm of insulation in the walls. This shows how insulation needs differ based on specific project requirements.
Passive house buildings enhance energy efficiency not only by using high-performance windows but also by insulating the walls, roof, and floors with a continuous layer. This layer typically has U-values ranging between 0.1 and 0.15 W/m2K, indicating its effectiveness in reducing heat transfer.
Photography by The Guthrie Project.
window + weather tightness
Windows are an important part of a passive house building’s structure. They must be carefully selected and installed to ensure they are airtight and energy-efficient.
To achieve airtightness, high-quality seals, multi-point locking systems, and thermally broken frames are used in windows. The glazing choice also affects the window’s ability to maintain a tight seal.
Weather sealing is equally important because it prevents air and moisture from entering the building. If not properly sealed, this can lead to heat loss, dampness, and damage to the building materials.
To ensure effective weather sealing, high-quality materials are used, and attention to detail is crucial during installation. Weather-resistant materials like silicone or EPDM gaskets are used to maintain a tight seal, even in harsh weather conditions.