The standard specifies the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.

Environmental factors include temperature, thermal radiation, humidity and air speed, while personal factors are activity and clothing. The standard contains a new predicted mean vote (PMV) calculation method, which is used where different levels of comfort may be specified. The equation will allow engineers to calculate the effects of different clothing and activity level, according to Gail Brager, vice chair of the Standard 55 committee.

"The PMV method is easier to use, and hopefully will result in engineers being more likely to use it rather than defaulting to the simpler graphic comfort zone, where the assumptions might not match their particular conditions," she said. "The use of this method can potentially improve occupant comfort by allowing engineers to more easily fine tune their comfort analysis to the particular needs of the occupants in the building."

The method also makes the standard more consistent with other international standards.

The standard also contains a new optional method for determining acceptable thermal conditions in naturally conditioned spaces. Field studies have shown that occupants' thermal responses in such spaces depend in part on the outdoor climate and may differ from thermal responses in buildings with centralized HVAC systems. This is due to the different thermal experiences, changes in clothing, availability of control and shifts in occupant expectations.

To use this method, spaces must have operable windows that can be opened by occupants.

"The adaptive model gives engineers a new tool, giving them more flexibility in deciding when and where full HVAC is required in a building and under what circumstances they can incorporate more energy-conserving strategies without sacrificing comfort," Brager said.