Guideline I.42019-08-16T15:47:40+00:00

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Guideline I.4: Thermal Comfort

Intent

To promote occupant thermal comfort through active and passive means, and to provide occupants with the ability to control the conditions in their space.

Required Performance Criteria

Guidelines apply to all New Construction projects and for Major Renovations that include replacement or alteration of relevant assemblies.

  1. Passive thermal comfort
    1. Avoid high solar heat gains: For regularly occupied spaces with at least one exterior wall facing within 45 degrees of due east or west, limit susceptibility to solar heat gains by employing one or more of the following on at least 95% of glazing within 45 degrees of due west or east:
      1. Specify glazing in those facades with low solar gain glass (solar heat gain coefficient [SHGC] < 0.4).
      2. Employ frit patterns that achieve SHGC equivalent no greater than 0.4.
      3. Implement exterior operable shading systems or southern horizontal and easterly and westerly vertical louvers.
    2. Avoid radiant temperature asymmetry
      1. For regularly occupied spaces above grade with at least one exterior wall, maintain an area-weighted average U-value for the exterior wall no greater than 0.2, including fenestration and doors.[1] The opaque wall and fenestration/door component U-values can be no greater than those listed in the prescriptive enclosure requirements section of the current version of the applicable energy code (International Energy Conservation Code [IECC] or ASHRAE 90.1).
    3. For naturally-ventilated spaces (i.e., without mechanical ventilation equipment), meet the design, operating, and performance criteria of the Adaptive Comfort Model from the current version of ASHRAE 55 Standard.
    4. Active thermal comfort[1]
      1. Mechanical systems
        1. All mechanically ventilated spaces must have mechanical systems designed to meet the comfort criteria of the current version of ASHRAE 55 during operating hours. Provide documentation of ASHRAE 55 calculations for summer, fall, winter, and spring conditions in a range of spaces including different space types and occupant activity levels. As long as I.2A Part 2i is met, the mean radiant temperature calculation is not necessary for these spaces. Garages, warehouses, vehicle maintenance and repair, ice rinks, and pools are exempted from this requirement.
        2. Ensure that the mechanical system including ventilation, heating, cooling, and distribution systems are commissioned as required under Guideline P.1 and the requirements of this section are included in the Basis of Design documentation.
    5. Occupant controls[2]
      1. Provide locally adjustable thermal comfort controls for at least 50% of single occupant spaces and a control for all shared multi-occupant spaces. Thermal comfort controls allow occupants to adjust at least one of the following: air temperature, radiant temperature, air speed, or humidity.

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.

Design:

  • 4A2: Submit building elevations showing the preliminary window and opaque wall surface areas. Submit the Design-Stage B3 Enclosure Thermal Comfort Worksheet, documenting the area-weighted average U-value of each exterior wall abutting a regularly occupied space and assuming code-compliant wall attributes.
  • 4A3: For all naturally-ventilated zones, submit preliminary building plans showing the planned airflow paths through each space.

Final Design:

  • 4A1: Submit documentation from the glazing manufacturer listing the SHGC value of the glass used on east- and west-facing exposures is less than or equal to 0.4. If shading devices are used to meet this requirement, submit building elevations showing shading devices instead of SHGC values.
  • 4A2: Submit building elevations showing the window and opaque wall surface areas. For window, door and spandrel components, submit manufacturer literature documenting the U-value for each of these components. Submit Final-Design B3 Enclosure Thermal Comfort Worksheet, documenting compliant wall assemblies.
  • 4A3: Submit ASHRAE 55 Adaptive Comfort Model calculations for all naturally-ventilated zones or sufficient representative spaces.
  • 4B1: Verify that the mechanical system can meet the comfort requirements of ASHRAE 55 for summer, fall, winter, and spring conditions in a range of different space types with different occupant activity levels using the CBE Thermal Comfort Tool, and document using the “LEED Documentation” option.
  • 4B2: Submit building plans showing the location of all local thermal comfort controls, individual occupant spaces, and shared multi-occupant spaces, indicating which of the individual occupant spaces contain thermal comfort controls and identifying the thermal comfort control for all shared multi-occupant spaces.

Appendix I-4: Enclosure Thermal Comfort Worksheet

CBE Thermal Comfort Tool for Use with ASHRAE 55: http://comfort.cbe.berkeley.edu/.

IECC 2015 U-value Requirements for Opaque Walls and Fenestration: https://codes.iccsafe.org/public/document/IECC2015/chapter-4-ce-commercial-energy-efficiency and https://codes.iccsafe.org/public/document/IECC2015/chapter-4-re-residential-energy-efficiency.

Morrison Hershfield – Thermal Bridging in Exterior Insulated Steel Stud Assemblies: http://blog.morrisonhershfield.com/insights/solutions-mh-thermal-bridging-in-exterior-insulated-steel-stud-assemblies.

Parallel Paths Method Wall U-value Calculator, for Wood-Framed Walls Only: https://www.appliedbuildingtech.com/fsc/calculator.

RDH Technical Bulletin – Cladding Attachment Solutions for Exterior Insulation Commercial Walls: https://www.rdh.com/resource/cladding-attachment-solutions-for-exterior-insulated-commercial-walls-bulletin/.

Steven Winter Associates – Cladding Attachment Thermal Bridging Guide for Exterior Insulation: https://www.b3mn.org/swa-cladding-attachment-thermal-bridging-guide-for-exterior-insulation/

Regularly Occupied Space:

Any space that is occupied by one or more persons for more than one hour during days the building is in use. Note that this includes spaces which may be irregularly occupied but, when occupied, a typical occupant would spend more than one continual hour in the space. Excluded from calculation of continuously occupied spaces are:

  • Spaces with uses that only require minimal lighting and in which the primary activity intended for the space would be harmed by daylight (this exclusion does not apply to spaces with ultraviolet light concerns) – note that this only applies to the calculation of regularly occupied spaces with respect to the daylighting requirements.
  • Spaces that do not meet the minimum occupancy outlined above during daylight hours) – note that this only applies to the calculation of regularly occupied spaces for the daylighting criteria with respect to the daylighting requirements.
  • Spaces where no individual occupant spends at least one continual hour during days the building is in use.