Thermal Comfort Surveys - Post Occupancy
Copyright (c) 2010, Robert Bean, All rights reserved

Several building programs such as LEED™ can require ‘post occupancy surveys’ to assess the building, its systems, performance and operation.

Surveys that relate specifically to the occupants are performed to develop an accurate representation of the environmental conditions perceived by the occupants against those criteria defined by standards.

In the case of thermal comfort surveys, the benchmark standards are:

1. ANSI/ASHRAE Standard 55-2010, Thermal Environment Conditions for Human Occupancy, and

2. ISO 7730:2005, Ergonomics of the Thermal Environment—Analytical Determination and Interpretation of Thermal Comfort using Calculation of the PMV and PPD Indices and Local Thermal Comfort Criteria.

To ensure a correct thermal portrayal, the sample size and response rate from survey participants has to be high enough to lessen the risk of generalizing conclusions from a few respondents over those who did not reply.

Though there is no response rate specified in the ASHRAE standard, it is suffice to say that there is minimal value in assessing indoor environments and their enclosures if hypotheses are developed on a small fraction of the overall available opinions. This is not to suggest that there is no value in just a few responses, but only to emphasize there will be correspondingly, increases in value in developing building diagnostics or forensics hypotheses through greater occupant participation.

As discussed through our website, thermal comfort is by definition a “state of mind”, as such there will be as many subjective opinions in the survey as there are participants; and these individual perceptions can be surveyed at a “point-in-time” or ideally over several points in time - which would then cover different and diverse environmental conditions. These types of point-in-time surveys are done to correlate environmental factors to thermal comfort indices such as those included in Fanger’s PMV/PPD model which considers personal factors such as metabolic rate and clothing insulation, plus general factors including air temperature, radiant temperature, air speed, and humidity. Within these parameters are localized considerations such as floor temperatures, vertical air temperature differences, radiant asymmetry and drafts. In these surveys, respondents are asked to rate their perceived sensation on a seven point scale ranging from hot to cold (-3, -2, -1, 0, +1, +2, +3). Extrapolated from this data, based on the occupants predicted mean votes (PMV), will be the predicted percentage dissatisfied (PPD).

It should be expected that responses over a period of time may provide different results from several “period-in-time” surveys, and surveyors are cautioned to not conclude that differences are exclusive indicators of changes to the building and its systems.

In addition to the thermal comfort survey is the “satisfaction survey” which does not rely so much on general thermal sensations and specific environmental factors, but rather on an individual’s level of satisfaction ranging from very satisfied to very dissatisfied in relation to specific instances or periods of time measured over months or seasons.

Thermal satisfaction surveys required or recommended by building programs should be completed following at least six months of occupancy – a time period deemed suitable for addressing commissioning issues.

Acceptability in this type of survey is determined by the percentage of respondents who based on recall, expressed an overall “neutral” or “satisfied ” opinion (0, +1, +2, or +3) with their environment.

Like the “point-in-time” survey, results from one survey should not be interpreted as being representative of all un-surveyed periods.

Instrumentation

Sometimes, it is necessary to use indoor environmental instrumentation to assist surveyors in the diagnostics and confirmation phases of the surveys.

These tools are equipped with specialized sensors which represent each of the criteria established in the thermal comfort standards. Data recorded from the sensor inputs can be then analyzed as ‘point-in-time” values or more importantly provide "thermal trends" during both occupied and unoccupied times. These data points and data sets can then be compared to the recommended values in the standards as well taken into consideration with the data collected from the occupants.

If necessary, remediation can be done to the various elements influencing occupant comfort including exterior factors (usually related to solar loads), enclosure performance (usually related to infiltration/exfiltration and radiant transfer), interior finishes (usually related to conduction and non thermal concerns) and the HVAC systems (often related to oversized, improperly balanced or inadequate systems).

Thermal Comfort Survey - sample.
Harvard University Divinity School, Rockefeller Hall; LEED ™ EQ Credit 7.2 – Thermal Comfort: Verification

IEQ Survey - sample
Peretti, C., Schiavon, S., Goins, J., Arens, E. A, De Carli, M., Evaluation of Indoor Environment Quality with a Web-based Occupant Satisfaction Survey: a Case Study in Northern Italy, Center for the Built Environment, Center for Environmental Design Research, UC Berkeley, 2010