By: Daniel Overby, AIA, NCARB, LEED AP BD + C In building design and construction, energy efficiency does not necessarily equate to good thermal comfort for occupants. In fact, human thermal comfort is rather difficult to analyze because it is as much psychological as it is physiological. The notion of “comfort” is a personal feeling – not a quantifiable metric like the kilowatt-hour is for electricity. Think about it – who has not been in a room where one person claims to be uncomfortably hot while someone close by exclaims to be cold? So, how can we empirically evaluate occupant comfort? Enter Professor Povl Ole Fanger. Predicted Mean Vote (PMV) Professor Fanger was an expert on the health effects of indoor environments. During the 1980’s, he introduced a seven-point scale to assess the thermal comfort of a large number of individuals. This scale provided the basis for taking surveys of large samples of people regarding how they felt under varying activity levels, clothing levels, and interior environmental conditions. Fanger also determined that the average value of the numerical survey results could be predicted. He established the concept of Predicted Mean Vote, which is based on the following equation: PMV = f (M, Rc, Ta, Tmrt, V, RH) where:
- M = metabolic rate due to activity (e.g., lying = 0.8 met; sitting = 1.0 met; running = 8.0 met).
- Rc = resistance of clothing (e.g., shorts and t-shirt = 0.1 clo; suit and tie = 1.0 clo; winter gear = 3.0 clo)
- Ta = air temperature
- Tmrt = mean radiant temperature
- V = air velocity
- RH = relative humidity
Predicted Percentage of Dissatisfied (PPD) In 1995, the British Standards Institute determined a correlation between the PMV and the predicted percentage of dissatisfied people (PPD) in a given interior environment. This correlation was defined as follows: PPD = 100 – 95 x e – (0.03353 PMV4 + 0.2179 PMV2) This correlation can be viewed graphically on the graph shown to the left. One can clearly observe that even a slightly uncomfortable interior environment will prompt considerable percentage of occupants to deem the space unsatisfactory. One very interesting feature of this equation is the minimum PPD of 5% for a thermally neutral vote (i.e. PMV = 0). This suggests that for any thermal comfort survey, at least 1 out of every 20 individuals will be dissatisfied. With data such as the PMV and the correlating PPD, design teams can better assess the notion of thermal comfort. Furthermore, design performance modeling may be leveraged to make considerable improvements to the thermal comfort of an environment. However, it is important to keep in mind that when it comes to comfort, there is no perfect solution.