Whole Building Energy Modeling
Spring, 2012
Assessing the energy footprint and performance of buildings is becoming an important part of building design, with energy performance exerting huge influence on building costs, occupant comfort, and environmental impacts.
Now more than ever, computers are being used to model the energy performance of building systems. As well as helping to determine accurate energy flow through assemblies, Whole Building Energy Modeling is being used as a means of assessing overall energy performance in both new and retrofit construction projects.
Embodied Energy
Embodied Energy is a topic that has been around for several decades. Together with operational energy, Embodied Energy provides a means for estimating the energy footprint of a building over its lifespan, taking into account resource consumption and environmental impact.
The primary criticism associated with Embodied Energy analysis is the accuracy of the values. The contributing activities included in Embodied Energy values are acquisition, processing, manufacturing and transportation – all of which can vary greatly depending on local industry practices and project location. As well, the values are subject to variances in modelling assumptions.
As such, Embodied Energy should not be taken as an absolute measurement of energy efficiency but rather as a comparative analysis tool to help select between design options with an aim to reducing the overall building energy footprint.
Thermal mass for energy storage
Thermal Energy Storage describes how a structure or high thermal mass component will absorb and release thermal energy over the course of a day, in order to maintain thermal equilibrium with its surroundings. Thermal Mass effects can be taken advantage of both passively and actively. Examples of passive applications would be having skylights shine onto an exposed slab area during the day and release that heat at night. Actively capturing that same thermal energy could be done by installing tubes within the same slab that carry a fluid which is warmed during the day and can be circulated to other parts of the building at night for heating.
When used correctly, a structural component with a high or active thermal mass can significantly reduce daily temperature swings. In turn, space heating and cooling demands are reduced for these areas; which reduces energy consumption.
Thermal bridges
Thermal bridging occurs at any location where there is a discontinuity in the thermal barrier (i.e. penetrations through insulation). Wood or steel stud locations, brick shelf angles, exposed concrete slab edges and cladding supports are all examples of thermal bridging. A recent study completed by RJC found that reductions of up to 7 per cent of the space heating costs can be realized by insulating exposed slab edges on highrise buildings.
Thermal bridging cannot be eliminated; however, it can be significantly reduced by constructing or retrofitting building envelopes with integrated thermal breaks at structural connection points. These breaks in turn assist in increasing the thermal performance of the whole envelope and become increasingly important as code mandated minimum insulation values increase.
Energy flows through glazing
Glazing systems, windows, window-walls and curtain walls lose heat in a similar way to opaque walls, however, glazing (windows) are special in also being able to bring heat energy into the building due to their transparency.
Glazing systems have the potential to be “net gain”, depending on their construction and orientation within a building. This has particular bearing in buildings where cooling loads dominate, such as in large office towers.
A number of technologies are available on the market today, that help improve the overall performance of glazing systems. These technologies (including coatings and films, gas fills for insulated glazing units (IGUs), aerogels and evacuated glazing systems) work to alter the emissivity, visual transmittance and U-value of the glazing system, to better suit the application. It should also be noted that the typical insulating value of a glazed area is usually lower than an opaque wall type which is part of the recent push by some organizations to limit the amount of windows on buildings, the window-wall-ratio, to 40 per cent to better control heat losses.
The primary purpose of surface coatings and films is to reduce the amount of heat transferred via radiation. Protective films are also available that filter out UV light and reduce visible light transmittance. The gas fill in an insulated glazing unit has a significant impact on the thermal performance of the glazing system. Aerogels represent an interesting area in window fill technology. An aerogel is a lightweight silica matrix consisting of 96 per cent air and is typically found where diffuse light is to be provided.
The aerogel also increases the insulating capacity as the window becomes more opaque which limits heat losses to the exterior. Finally, a new window fill technology evacuated glazing is created with double or multi-glazed units that have a vacuum in the interstitial space instead of gas fill. The technology is promising, however, it has limited case history on projects due to higher capital costs.
Energy and air leakage
Another mechanism for energy movement in buildings is air leakage. Not only is air leakage a means of transportation for moisture through walls, it also contributes significantly to energy loss. Several studies have shown observed energy losses in the 10 per cent range as a direct result of air infiltration. Air leakage will occur anywhere there are unsealed discontinuities in the air barrier. Key locations for air leakage are areas where dissimilar materials meet, wall-to-floor/wall-to-roof junctions and penetrations.
Air leakage in new construction can be reduced through conscientious detailing in concert with quality assurance and quality control procedures. In existing buildings air leakage can be improved as part of both large retrofits and on a component by component basis (e.g. windows, doors, etc.).
Conclusion
As the energy performance of new and retrofitted buildings becomes a higher priority both in code and in society at large, it is becoming increasingly necessary for engineers to consider how their designs address this issue. This article introduces some building energy use concepts to consider in both new construction and retrofit projects to assist in helping reduce the overall building energy use footprint.
The tools for quantifying energy performance are readily available and modelling software is extremely powerful; but can often be seen as overwhelming or confusing. Therefore, it is critical to have a clear understanding of the fundamental principles of energy performance and how these technologies can be integrated into the whole building as similar to any modelling tool, the quality of the results is highly dependent on the quality of the input data.
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