Energy-efficient Building Envelopes: Reducing Heat Loss And Costs In Boston – Passive House is considered the most stringent voluntary energy-based standard in the design and construction industry today. Consumes up to 90% less heating and cooling energy than conventional buildings and can be applied to almost any building type or design, the Passive House High Performance Building Code is the only internationally recognized and proven construction standard to achieve this level. This is an energy standard based on science. of performance. Fundamental to the energy efficiency of these buildings, five principles are central to Passive House design and construction: 1) highly insulated envelope, 2) airtight construction, 3) high-performance glass, 4) thermal bridge-free details, and 5) heat recovery ventilation.

All these important principles are interrelated and influence each other in design. He cannot ignore one principle without adversely affecting other principles. To effectively create a Passive House building, the design must be considered holistically and incorporate all five design principles.

Energy-efficient Building Envelopes: Reducing Heat Loss And Costs In Boston

A building envelope is what separates the inside and outside of a building. Consists of exterior walls, roof, and floor. In cold climates like Canada, indoor air is heated to keep buildings comfortable, but some of that heat is lost (via conduction processes) as it travels through the exterior walls. To reduce this heat loss, insulation made of low conductivity materials is installed within the wall and roof assembly.

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Passive House makes the most of the envelope by insulating the building to minimize heat loss. The objective of Passive House is to use assemblies with sufficient insulation to provide double or triple the heat resistance compared to that required by current Canadian building codes. is. The result is a significant increase in the thermal performance expected from the building envelope. Insulating to Passive House levels also has the added benefit of increasing the resilience of the building, such as improved soundproofing, increased durability, and the ability to maintain internal comfort for longer periods of time even during power outages.

Achieving Passive House levels of heat resistance is not only about the amount of insulation, but also whether that insulation is used effectively. Insulation is most effective when it wraps around a building unencumbered by other materials, but there are always areas where this is not possible, such as around components used for structural reasons. When a material bypasses an insulator, it is known as a thermal bridge and can significantly reduce the effectiveness of the insulator, especially if the material is highly conductive like a metal.

Minimizing repetitive thermal bridging and aiming for as continuous insulation as possible, such as the assembly shown in Figure 1, helps maximize insulation within the building envelope.

(Left) Exterior insulated steel stud wall. (Medium) Precast insulated sandwich panel. (Right) Larsen truss wood frame wall. (Courtesy of Passive House Institute)

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Heat can also be lost from the envelope through air leakage. A building air barrier is a layer of material (membrane, tape, seal) around an envelope that restricts the movement of air in and out of a building. Gaps in the air barrier can allow air to enter and leave the building in an uncontrolled manner. This occurs when there is insufficient detailing during construction, when the air barrier has a large number of ducts and other penetrations, or when the quality of construction is generally poor.

Large amounts of uncontrolled air exchange with the outside world can lead to increased energy usage due to the need to repeatedly reheat the air, discomfort due to cold air gaps near walls, problems with localized moisture and condensation, etc. , which can cause various problems. Although air exchange is necessary for ventilation and the provision of fresh air, it is much more effective to tighten the envelope and use mechanical ventilation to control air exchange.

There are strict design and construction requirements for a Passive House project to be certified airtight. Quantitatively speaking, Passive House certification means that the building must have less than 0.6 air changes per hour (ACH50) at the time of testing. This stringent value can be compared to other high performance building codes such as the R2000 program which allows up to 1.5 ACH50 due to air leakage. As an additional quality assurance for Passive House projects during construction, at least one on-site air leakage test must be completed to prove that the building meets airtightness requirements.

Achieving this degree of tightness requires that the air barrier be continuous and clear on the drawing, that an effective air barrier material is used, and that clear details of penetrations and terminations are provided. This must be carefully planned during the design stage. When installing air-break walls, it is important to ensure the quality of construction through thorough quality control from the contractor to the contractor. The entire construction team must be aware of the important role airtightness plays in Passive House projects.

What Is A Passive House? [passipedia En]

The airtight construction of Passive House projects further reduces heating costs and local condensation problems, increasing comfort within the building. In Passive House buildings, these benefits cannot be achieved by simply strengthening the building’s exterior walls, but must be combined with appropriate ventilation strategies to deal with excess moisture within the building.

Although walls typically occupy the largest area of ​​a building façade, glazing systems (windows and glazed doors) can play an even bigger role in terms of contributing to heating energy. Due to their function (providing light and visibility), glass systems cannot insulate to the same extent as walls, making windows the weakest area of ​​the envelope in terms of heat flow resistance. Therefore, it is very important to use high-performance glass systems such as Passive House certified windows to reduce heat flow as much as possible.

As shown in Figure 2, the main characteristics of a high-performance passive house glass system include a non-conductive frame and a large thermal barrier. Insulated frame. Double or perhaps triple glazed units. Argon or krypton gas filling. Multiple Low-E coatings. Warm edge or non-conductive spacer.

In addition to specifying high-performance windows, it is important to carefully consider how they are integrated into the building design. Passive house designs utilize free passive heating from the sun. Solar heat gained from properly placed windows can help offset the amount of heat a building needs during colder months. During the summer, this should be countered by shading to prevent too much heat from the sun from entering the building and causing overheating. For each Passive House project, there is an ideal number of windows that balances the benefits of freely harnessing heat from the sun and minimizing heat loss due to too many windows.

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A final consideration for glass systems is surface temperature. When the outside temperature is low in winter, the inside surface temperature of low-performance windows can also be very low. Cold temperatures around windows increase the risk of condensation (and possible mold growth), and radiant heat loss and temperature drafts can make you feel colder when you’re closer to a window. To reduce these risks, certified windows must be evaluated according to hygiene and comfort standards that establish minimum allowable surface temperatures around the window.

A final envelope consideration is minimizing thermal bridging. This was previously discussed about repeating thermal bridges in typical wall and roof assemblies, but Passive House design also aims for no thermal bridges when it comes to architectural interface details. These are the parts of a building where different architectural features intersect that require additional attention during construction. Examples include how windows are attached to walls, how walls meet balconies, and how walls meet corners, as shown in Figure 3. The way these building features are connected and designed can also create thermal bridging, which is not always easy. recognize.

Thermal bridging from interface details can have different effects on building performance. For highly insulated exterior walls, such as those in Passive House projects, thermal bridging can significantly reduce insulation effectiveness by allowing heat to flow around the insulation and out of the building. It can also create localized cold spots, increasing the risk of condensation and mold growth. About these details.

The easiest way to avoid thermal bridging is through architectural design changes, such as using free-standing decks or canopies in low-rise buildings, or reducing the number of cantilevered balconies or articulated structures (with many corners) in larger buildings. (if possible). . This is not always practical or achievable. In such cases, you should pay special attention to these interfaces. It is important to reduce direct conductive connections between inside and outside. Examples include installing intermittent connections at shelf angles, over-insulating before certain connections around the foundation, wrapping insulation around protruding details, and special materials such as thermal breaks. Examples include using .

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It may not seem obvious, but thermal bridging caused by window-wall interfaces can have a huge impact. Depending on the project, the total perimeter of all window-to-wall connections can extend to several kilometers, so how the window is installed in the opening is critical to minimizing heat flow. play a role. To reduce thermal bridging at this connection, you may need to adjust the window position.

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