ASHRAE 55 interpretation in Sefaira

ASHRAE 55 is a thermal comfort standard that is referenced by many Green Building Rating schemes as the means for testing whether a design achieves comfort criteria or not. 

The standard is quite long and although we believe it is fairly clear what the criteria are, this article explains how we've interpreted it and where we've chosen not to apply some considerations in the standard.

Why do I care

Ideally Sefaira could be used to demonstrate thermal comfort compliance against ASHRAE 55 for standards such as Green Star or LEED. You may want to know the underlying assumptions we've made in interpreting this standard if you choose to do this.

What constitutes a pass in ASHRAE 55

The ASHRAE 55 standard takes the form of a set of guidelines. In setting out these guidelines it defines reasonably clearly what the requirements for a comfortable building are.

ASHRAE 55 is fundamentally based a thermal comfort standard called ISO7730 which defines how PMV can be used as a metric for testing thermal comfort. This is a link to the ISO7730 standard. This is a link to EnergyPlus's reference manual which describes how these calculations are made. We take these results directly from EnergyPlus.

ASHRAE 55 makes it clear on pages 5-7 that it considers the acceptable comfort range to be a PMV score in the range of -0.5 to +0.5 (refer image below).

There are other places in the standard where comfort is represented on a psychrometric chart with operative temperature on the x-axis. This is simply a visual representation of the PMV criteria.


Assumptions Sefaira makes in calculating PMV

PMV is calculated each hour and based on 6 inputs:

  1. Dry Bulb Temperature
  2. Mean Radiant Temperature
  3. Relative Humidity
  4. Clothing Value
  5. Metabolic Rate
  6. Air Velocity

The first 3 are determined by EnergyPlus using the heat balance calculations applied to all other parts of the simulation.

The clothing value is calculated dynamically using the "Dynamic Clothing Model for ASHRAE 55" by Schiavon and Lee (2012). The calculation method is documented in the EnergyPlus manual here. The full paper is referenced here. It enables us to vary the clothing rate assumed seasonally based on the external temperature at 6am so that only one simulation for comfort is needed.


Metabolic rate is assumed to be constant and equivalent to 1.1 which is seated, typing. 

Air velocity is assumed to be constant at 0.1m/w (20ft/min). This is the lowest acceptable value we could use and maintain the intent of the standard. We used a low value because in testing it gave our models the best chance of passing. 

Alternative interpretations

ASHRAE 55 describes two comfort levels, 80% acceptability and 90% acceptability. It's not that clear which types of buildings pass the standard with an 80% acceptability criteria. You can easily change the criteria to be 80% acceptability by moving the PMV controls to ± 0.8. This represents 80% acceptability per the graph below from the standard.


Green Star Criteria Notes

Green Star offers points for designing a project within a PMV range. It refers to the ASHRAE 55 Standard. The default settings represent the Green Star criteria for 2 points. 

The criteria for 1 point would be to have PMV within ±1.

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