Energy Efficient Building Envelopes
Space heating and cooling accounts for over 30% of global energy consumption. With a 150% predicted increase by 2050 for cooling alone, it’s hardly surprising that the question of how to design energy efficient buildings is echoing around the planet. Recently, the focus has been on the building envelope. The new construction phase offers the best and most practical opportunity to incorporate passive heating and cooling designs that take advantage of energy efficient building materials. Several European Union countries have adopted policies that mandate the construction of net-zero energy buildings; these policies will be enforced by the end of the decade.
Whether one chooses to call it the building envelope, shell, or enclosure, the boundary between a building’s conditioned interior and the outdoors plays a critical part in how much energy the building will require for heating and cooling, as well as comfort levels, lighting, and ventilation. While emphasis is placed on the building envelope’s role in energy-efficiency, it shouldn’t overshadow its other purposes, to protect the building’s occupants and give them shelter, privacy, fire protection, and comfort. The challenge is to design a building with an envelope that reduces energy consumption without compromising occupants’ needs.
Walls, roofs, and floors represent the largest external areas of buildings, and they are also the routes by which heat escapes the building envelope. While proper insulation mitigates heat loss in winter and keeps it out in summer, the type and amount of insulation should be governed by the building’s use. For example, office and service-sector buildings need less insulation than residential buildings since they have a higher internal thermal load due to the density of people, and the use of electrical equipment and artificial light.
Vapor and/or Air Membranes
Ironically, insulation can be both beneficial and detrimental. Modern techniques of insulating walls, floors, and roofs reduce energy loss, but it is difficult for moisture to evaporate, resulting in condensation and eventual damage. One solution is the use of vapor retarders such as air and vapor membranes. Air barriers resist air movement, and therefore the movement of moisture by increasing air tightness. Vapor barriers allow any moisture that finds itself within the building to escape through diffusion. Depending on the membrane, both resist water penetration, keeping exterior moisture from leaking in, even with wind-driven rain. Regardless of the energy that it saves, insulation installed without any moisture protection is unacceptable. A proper air and vapor membrane will prevent future moisture and mold from forming inside insulation.
Windows should let in as much light as possible, maximizing heat gain in winter, but minimizing it in summer. Appropriate sizing, orientation, and glazing are the primary ways to balance heat and light flow. The trick is to maximize window performance while achieving energy balance. In cold climates, windows specified in heated buildings should retain heat (low U-value) while admitting solar radiation (high G-value). In warm climates where cooling is needed, windows need to keep out the sun’s heat (low G-value) and, ideally, enable the building to shed heat. Such energy balance can be achieved with double or triple glazed well insulated windows combined with architectural shading.
In warm climates as much heat as possible needs to deflect off the roof. This will prevent heat build up in attics or air conditioned spaces where it can result in condensation. The simplest way to do this is to specify a white or light color roof. Such cool roofs have the ability to reflect both visible and infra-red light. The effectiveness of cool roofs, however, varies depending on the type of roof, the insulation used, and the climate. In general, reflective surfaces can improve the building’s energy efficiency, reduce urban islands of heat, and contribute to the cooling of the planet.
For information on the importance the International Energy Agency (IEA) places on energy efficient building envelopes, see their Technology Roadmap.
Cosella-Dörken delivers innovative, high-performance air and moisture barriers for commercial and residential construction sold under the DELTA® brand name. A North American manufacturer based out of Beamsville, Ontario, Cosella-Dörken Products, Inc. is a subsidiary of Ewald Dörken AG, a leading European developer and manufacturer of waterproofing and drainage products sold worldwide. Cosella-Dörken is known for delivering premium products while providing educational programs and full technical support. For more information, call 1-888-4DELTA4 (433-5824) or visit www.cosella-dorken.com.