Prescriptive approach
The simplest and oldest method of prescribing building energy performance is to specify the minimum required performance for each of the enclosure components in either U- or R-value (i.e. opaque walls, fenestration, roofs, below-grade components, etc.). The ‘installed insulation’ approach is the simplest and least flexible one. Designers choose the prescribed insulation R-value from a code table and create assemblies based on it. Today’s codes further prescribe how much must be installed within metal framing and the amount required to be installed as continuous insulation (ci) outboard of the metal framing.
This approach is very restrictive for design but has the advantage of relatively simple-to-read tables.
Trade-off analysis
Both simple and detailed trade-off methods are available. In the simple trade-off method, only enclosure components are traded off, whereas the detailed approach allows a more sophisticated analysis of solar gains for both reducing heating loads and increasing cooling loads. Like the prescriptive path, the trade-off approach requires meeting all mandatory parts of the code.
The simple enclosure trade-off method is very easy: provided the total heat loss/gain of the proposed building enclosure is equal to or less than a building built to the prescriptive minimum values, the building is code compliant. The total heat loss is simply calculated as the sum of the individual component areas times that component’s U-value.
Whole-building energy
Although specific characteristics of building enclosures (R-value, airtightness, and solar heat gain co-efficient [SHGC]) can reduce the demand for space heating and cooling, improvements to heating and cooling system efficiencies, lighting design, and mechanical ventilation systems can have a major impact on large commercial and institutional buildings. Thus, codes for larger buildings (e.g. ASHRAE 90.1 and NECB) often prescribe minimum performance levels for a wide range of mechanical equipment, lighting, and control systems.
Codes and thermal bridging
Photo courtesy RES Precast
Based on research conducted by numerous organizations nationally and internationally, the effect of thermal bridging is now understood to play an important role, especially in well-insulated enclosures. The R-value often does not include the impact of specific thermal bridges such as floor slabs, structural anchors, and balconies. Thermal bridges, or at least the major ones, are generally intended to be included in tabulated U-values and code language is currently being strengthened to make this clear.
Ci is a common terminology encountered in modern prescriptive codes. It was added to code language to minimize thermal bridging, primarily in steel- and wood-framed enclosures. Ci is defined by ASHRAE 90.1 as:
insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.
Calculating enclosure thermal performance
Many owners do not wish to provide more performance than the minimum code requirement. Hence, building professionals need to design buildings that ‘just meet’ these codes. This requires both an understanding of the code minimum performance and how to calculate the performance of the building enclosure. Other projects have different goals or a long-term perspective. In this case, designers are driven to make the best choices between many competing alternative enclosure systems and materials. In either case, an understanding of what level of thermal performance can be achieved is critical.
