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Guideline I.4: Thermal Comfort

Intent

To promote occupant thermal comfort through ambient temperature and operative temperature control, which includes wet bulb, dry bulb, and globe temperatures, relative humidity (RH), mean radiant temperature (MRT), and air velocity.

Required Performance Criteria

  1. Maintain continuous indoor exposure to ambient temperature in continuously occupied spaces less than 80°F and greater than 64°F. For transition spaces (entries, hallways, exterior walls) temperatures may fall outside the limits for continuously occupied spaces to save energy.
  2. In continuously occupied spaces where MRT asymmetry could be a problem (e.g., spaces such as glass atria, rooms adjacent to boiler rooms, and areas under an exposed roof structure), maintain the wall, floor, and ceiling surface temperatures within 20°F when taken from all continuously occupied positions, OR maintain no continuous indoor exposure to greater than 0.30 asymmetry in MRT across three body plane hemispheres (front-back, side-side, top-bottom).
  3. Design air velocity greater than or equal to 10 feet per minute (fpm) for continuously occupied spaces. Exception: Spaces with natural ventilation or mixed-mode ventilation are exempt from I.5C during the times that they are operating in a natural or mixed-mode ventilation mode.
  4. Design interior relative humidity (RH) within most recent version of ASHRAE Standard 55 for continuously occupied spaces. Exception: Spaces with natural ventilation or mixed-mode ventilation are exempt from I.5D during the times they are operating in a natural or mixed-mode ventilation mode.

Recommended Performance Criteria

  1. Vary Dry Bulb Temperature (DBT) via building control system to avoid thermal boredom. Produce ramped drifts of up to + 2.0°F/hour in peak-to-peak variation around neutral temperature. Note: Operative Temperature (OT) is also known as Wet Bulb Globe Temperature, (OT or WBGT = 0.7 Natural Wet Bulb Temperature + 0.3 Globe Temperature).

Calculate or simulate thermal comfort using guideline performance criteria and other appropriate thermal comfort indices. Operative Temperature is determined by DBT, relative humidity, and mean radiant temperature (DBT, RH, MRT), and air velocity. Calculation can also include the effects of Clo value (the insulation value of clothing), physical activity, and time. See especially the most recent version of ASHRAE Standard 55 and the Human Factors and Ergonomics Design Handbook for explanation of conditions and measures to provide for thermal comfort. See other references, particularly Engineering Data Compendium and NASA MSIS for handling special condition problems. See Handbook of Environmental Psychology for discussion of thermal issues for particular settings (e.g., offices, industrial environments) and for perceived control of thermal variables.

Implementation in the Design Process:

During Predesign, determine special thermal comfort requirements or problems that may be encountered in the building due to work activities or siting or design considerations.

Review conditions that affect thermal comfort using the most recent version of ASHRAE Standard 55 or Human Factors and Ergonomics Design Handbook. Perform any baseline studies on thermal problems or issues that may exist in current facilities if the project involves a move or remodel.

During Design, estimate thermal comfort performance measures using ASHRAE Standard 62 occupancy limits for spaces and comfort zone and other thermal conditions in ASHRAE Standard 55. Ensure that no major design characteristic of the building required by these guidelines will push these variables outside general comfort ranges as defined by the guidelines.

As the Design phase progresses, consider additional calculations of thermal comfort indices as appropriate to specific project conditions. Additional measures may include operative temperature, new effective temperature (which combines air temperature and relative humidity,) or Wet Bulb Globe Temperature (which combines dry bulb, wet bulb, and globe temperature measures.) The latter is the effective index under potential heat stress conditions.

During construction administration, verify the project is achieving thermal comfort as designed and required by this guideline.

Final Design:

  • I.4A: Verification of designed temperature range within guideline limits.
  • I.4B: Verification of mean radiant temperature asymmetry within guideline limits.
  • I.4C: Verification of designed relative humidity within guideline limits.
  • I.4D: Verification of thermal variables within limits set forth in the most recent version of ASHRAE Standard 55.
  • I.4E: Verification of variation of DBT included in building system operation specifications.

ASHRAE Standard 55-2013: https://www.ashrae.org/resources–publications/bookstore/standard-55-and-user-s-manual University of California Berkeley CBE Comfort Tool: http://comfort.cbe.berkeley.edu/

Tillman, Barry. 2016. Human Factors and Ergonomics Design Handbook, Third Edition, Barry Tillman. McGraw-Hill, NY.

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