Efforts to optimize energy performance in buildings often focus on improving individual components, like selecting high efficiency mechanical equipment. A more holistic approach includes a focus on driving down the basic demand for energy and understanding how all systems interact with one another. High performance building envelopes are key to this more holistic approach as they can significantly improve the overall comfort and energy performance across the lifespan of the building.
An airtight building is critical to efficiency and comfort. Airtightness is achieved by wrapping an air barrier around the entire building and ensuring that transitions between windows, doors, glazing units, air vents, and other envelope openings are well-sealed. An airtight building helps keep out noise, odors, and pests, and improves energy performance by reducing air leakage and drafts.
Airtightness requires attention to detail, where each and every transition needs to be successfully sealed. Building plans should incorporate air barrier materials at every point along the envelope assembly and specify how these materials will be joined up to create an unbroken, airtight layer.
Airtightness and proper ventilation are closely connected. Neither is effective without the other, and if either is suboptimal it is virtually impossible to provide a quality interior environment or to use energy efficiently. In airtight buildings with proper ventilation, filtered fresh air enters the building and stale air is exhausted at specific, controlled points, such as vents and operable windows.
A building’s airtight layer is made up of a series of materials that fall into the following three categories:
sick building syndrome
Many people mistakenly identify airtight buildings withsick building syndrome (SBS), in which mold and other pollutants are allowed to infest a building. While buildings with SBS typically have no operable windows, they are not actually airtight—in fact, their leaky envelopes are often the point of entry for moisture, mold, and other pollutants. SBS also often results from mechanical systems that lack fresh air supply or proper filtration and were poorly installed and/or maintained. High performance envelopes are the cure for sick building syndrome, not the cause.
Constructing a high performance building envelope can require more precaution, closer attention to detail, and additional training for contractors and sub-contractors.
Continuous, uninterrupted insulation that wraps the entire building and is coordinated with thermal breaks in windows, doors, and cladding systems, will effectively reduce pathways for heat to travel in and out of the building, thus minimizing the need for heating and cooling.
Insulation effectiveness is measured in terms of its thermal resistance or R-value, with higher values representing greater effectiveness. A material’s R-value depends on its thickness, density, and, in some instances, its age and moisture accumulation.
When upgrading existing buildings, it is best to apply insulation from the exterior in the form of a rainscreen or External Insulation and Finish System (EIFS). However, due to costs, lot line restrictions, historic preservation restrictions, and other limitations, building renovations often require applying insulation from the interior, leading to scenarios where it is more difficult to effectively insulate the whole building. Nevertheless, even imperfect interior insulation upgrades can vastly improve building performance.
Different types of insulation materials such as rigid board, batts and rolls, loose fill, and spray foam, are each suited for specific interior or exterior applications. For building retrofits or challenging architectural conditions, innovations in insulation technology have resulted in high preforming products that provide high R-values relative to weight and bulk, or additional insulating capacity to standard materials, potentially improving their utility in problematic scenarios.
Many high performance building strategies are passive, using little to no technology, and remain effective throughout the life of the building. Examples of passive design strategies include consideration for a building’s orientation, form, location, and window-to-wall ratio.
Incorporating passive design strategies early in the design process diminishes reliance on add-on technologies and mechanical systems that increase life-cycle costs of the building.
Most glass buildings of the past are highly inefficient, but new, sophisticated glazing technology can far outperform them. Combined with automated shading systems and thermally broken, airtight frames, high performance glazing reduces heat gain and minimizes glare while providing building occupants with attractive views and ample daylight.
The amount of glazing used on the building, known as window-to-wall ratio, the orientation of the glazing, and the type of glass, are all important variables affecting indoor comfort and energy performance. These variables will impact a building’s heating, cooling, and lighting loads, and define access to daylight, natural ventilation, and views.
New innovations in glazing manufacturing have improved the insulation quality of glass. Combined with indoor and outdoor shading strategies, high performance glazing can reduce unwanted solar heat gain while allowing daylight to enter spaces. One of the most notable benefits of high performance glazing is that indoor spaces directly adjacent to the glass are much more comfortable and therefore better utilized.
When specifying glass, consider properties such as U-factor, Solar Heat Gain Coefficient, visual transmittance, and airtightness. To control heat gain and reduce glare, use applications such as fritting or laminated coatings to provide shading, as well as elements like louvers, overhangs, and automated interior shades.
Daylighting is the controlled admission of natural light into a building to reduce electric lighting, save energy, and create stimulating and productive indoor spaces.
Daylighting systems pair fenestration with interior or exterior shading devices and a responsive lighting control system to minimize glare, balance heat gain and loss, and respond to sunlight’s natural variations.
Mitigating thermal bridges is key to supporting the effectiveness of insulation and maintaining energy performance. Thermal bridge mitigation strategies include careful architectural detailing and coordination of building components, as well as specifying thermal breaks where needed.
The most common type of thermal bridge is caused by structural elements used to connect the building envelope back to the building’s structural frame, such as metal anchors and ties. Other common thermal bridges occur at junctions between two or more building components, such as window- and door- to-wall junctions, as well as balcony-to- wall and parapet-to-roof junctions.
Identifying and mitigating all thermal bridges in a building can be a tedious but critical task, as even small thermal bridges, such as masonry ties, can add up to significant energy loss.
The objective of most thermal bridge mitigation strategies is to reduce
the number of building components spanning from exterior to interior and ensuring that each include a thermal break material that reduces heat flow through the component.
A vast number of thermal break materials and products are available on the market. Typically composed of fire- resistant fiberglass or resin composites, these materials are low conductivity and often have a high compressive strength for use as structural thermal breaks.
On the occasion when a thermal bridge is necessary, specifying materials with the lowest possible thermal conductivity can help reduce its impact.
The architect plays a key role in the successful design and construction of a high performance building envelope. Architects must detail each building condition to ensure that all envelope components are correctly coordinated.