Guideline I.4: Thermal Comfort

Post-occupancy evaluations of existing B3 buildings have identified occupant thermal comfort as an area in need of significant improvement. High thermal performance enclosures are the first step to improving this comfort with minimal use of additional energy. Better design of mechanical systems to increase occupant control over thermal conditions is also necessary, combined with enhanced commissioning to ensure those controls and systems are working properly.

Early in the design phase, begin evaluating the appropriate window-to-wall area ratio (WWR) abutting regularly occupied spaces. Achieving a WWR above 40% to 50% in those spaces while meeting the area-weighted average U-value requirement under 0.2 will be difficult without using fenestration that surpasses code-level thermal performance. Fixed and operable windows, storefront, and curtainwall systems are available that can achieve U-0.2, but they remain a more expensive option compared to limiting the WWR.

Identifying or calculating the U-value of opaque wall assemblies and fenestration can be difficult in some cases. A simple center-of-cavity or center-of-glass U-value is not adequate to determine thermal comfort because of the strong influence of thermal bridging. In the early design stages, it may be more helpful to use code-prescribed U-values for calculations since actual assemblies and fenestration products may not yet be known. Code U-values for Climate Zones 6 and 7 are provided in the Design-Stage B3 Enclosure Thermal Comfort worksheet. For the final design-stage submission, actual whole-assembly or whole-window U-values must be calculated or provided for the specific assemblies and components which have been selected. Both ASHRAE 90.1 and the IECC Energy Conservation Code contain tables listing the U-values for a range of different opaque wall assemblies and insulation thicknesses. These values are particularly important to use for walls incorporating steel framing elements.

It’s important to note that while the ASHRAE 90.1 and IECC tables account for thermal bridging between steel studs, they generally assume any exterior insulation is perfectly continuous. In practice, exterior insulation installed outboard of the sheathing is rarely continuous since the cladding needs attachment points to transfer wind and gravity loads to the structure. Often, attachment is provided via a system of metal girts or clips or both, with very significant impacts on the R-value of the exterior insulation. For example, a typical system of horizontal z-girts installed 24 in. on center may reduce the additional R-value of exterior insulation by more than 50%. If exterior insulation is used, the impact of such thermal bridges must be accounted for in the U-value calculations as well. In some cases, manufacturers of proprietary cladding mount systems will have methods or tables to help calculate the R-value reduction due to their girts or clips. If such resources are not available for a particular project, use the Steven Winter Associates Cladding Attachment Thermal Bridging Guide to find the appropriate degradation factor for the exterior insulation (see Additional Resources section, below).

Whole-window U-values for storefront and curtainwall products can also be difficult to provide since they depend strongly on the spacing of the mullions and resulting frame-to-glass ratio. High amounts of window frame tend to drive up the window U-value and decrease the thermal performance of storefront and curtainwall systems since they act as thermal bridges. Some manufacturers will provide a range of U-values representing a range of frame-to-glass ratios. In that case, it is the responsibility of the design team to determine an appropriate U-value from that range, given their fenestration design, or to ask the manufacturer for assistance. Other manufacturers may only provide a single U-value based on a specific window test size and frame-to-glass ratio. In that case, an adjustment to the U-value may be appropriate using experience, judgement, and/or recommendations from the manufacturer. Special, project-specific U-value calculations are possible and encouraged but not required.

Compliance with I.4A Part 2 is verified by submission of a calculation performed for each regularly occupied space with one or more exterior walls. Nonregularly occupied spaces (e.g., hallways, stairways) are exempted from this requirement. Below grade spaces and spaces with less than 30% window and door area can be omitted from the calculations, as long as walls, windows, and doors meet the minimum U-values prescribed in the applicable energy code. A group of identical spaces, or nearly identical spaces, require only one calculation demonstrating compliance for the group. If design values for the opaque wall and fenestration are already known in the design stage, they can be used in conjunction with the Final Design-Stage B3 Enclosure Thermal Comfort Worksheet and submitted in place of the design-stage calculator.

Improving passive thermal comfort with a high quality enclosure is the first step to achieving thermal comfort for occupants. Mechanical systems must be well designed and commissioned in order to maintain comfortable conditions. Anticipated compliance with ASHRAE 55 comfort zones should be verified for a range of representative spaces in summer, fall, winter, and spring conditions. The Center for the Built Environment (CBE) Thermal Comfort Tool, hosted by the University of California, Berkeley, may be used to set desired operating conditions, activity levels, and clothing levels within a space to check for compliance with ASHRAE 55 comfort requirements. As long as Guideline I.4A Part 2 is met, the mean radiant temperature of all regularly occupied space will be sufficiently close to the indoor air temperature. In this case, the indoor air temperature can be used as the operative temperature, eliminating the need to calculate mean radiant temperature. A link to the CBE Tool is provided in the Additional Resources section.

Since thermal comfort is subjective and dynamic, a mechanical system that can provide the conditions to satisfy ASHRAE 55 is not a guarantee of thermal comfort for all occupants all the time. Research studies have shown that occupants feel more comfortable and satisfied with their space when given the means to control at least some of the factors that influence thermal comfort. These include air temperature and radiant temperature, air speed, and humidity. Ideally, every occupant should be given thermal comfort controls allowing them to adjust at least one of these factors, but this may be cost-prohibitive or not feasible given some types of mechanical systems. However, every mechanical system should be designed to provide locally adjustable comfort controls for every shared multi-occupant space and at least 50% of individual occupant spaces. Individual occupant spaces are enclosed or nearly enclosed spaces occupied most regularly by a single person. These requirements are best considered early in the design of the mechanical systems of the project.