Science & Technology

Murky science of comfort part 2: Why we feel suffocated in a building even when it is pleasant outside

Humans are built to get used to local environments but unfortunately industry interests have come to define what ‘comfortable’ room temperature is  

 
By Avikal Somvanshi
Last Updated: Sunday 01 July 2018
People are likely to react differently to the same weather conditions due to their level of adaptation, living patterns, and eating habits. Credit: Author
People are likely to react differently to the same weather conditions due to their level of adaptation, living patterns, and eating habits. Credit: Author People are likely to react differently to the same weather conditions due to their level of adaptation, living patterns, and eating habits. Credit: Author

Do people in warm climate zones prefer warmer indoor temperatures than people living in cold climate zones?

Tropical nations have consistently used higher temperature threshold to define heat wave compared to their temperate friends. But P O Fanger, a Danish professor who is regarded as the father of comfort science didn’t believe that there was any dissimilarity in how different people were affected by temperatures; a view largely accepted globally.

(Read about Fanger's views in the previous blog).

But all of the research done by Fanger was done in the US and Denmark. I wondered if the same results could be observed in tropical environments.

India’s first thermal comfort index

In the mid 1980s, M R Sharma and Sharafat Ali, scientists at Roorkee-based Central Building Research Institute (CBRI), conducted thermal comfort studies in India in indoor environments typical to India. Sharma and Ali employed Fanger’s seven-point cold-hot scale to gauge people’s comfort perception.

They believed that there was heterogeneity in human response to the same temperature. “The purpose of a thermal comfort index is to estimate the influence of environmental factors…the fact that thermally equivalent conditions produce different subjective sensations due to the level of adaptation, living patterns, eating habits, etc... (is) the reason to look for an index of thermal comfort for Indian subjects,” Sharma and Ali wrote.

Published in 1986, the study found that Indians preferred warmer indoors than what Fanger considered comfortable. It proposed a Tropical Summer Index (TSI) to calculate thermal comfort conditions for Indians. They said that Indians were comfortable between TSI values of 25oC and 30oC with an optimum condition at 27.5oC. The National Building Code of India accepted these albeit with some caveats (more on that in the upcoming blogs).

Two years later, the World Health Organization (WHO)’s European office was to recommend 18-22°C as room temperature in its Healthy Housing guidelines for member states.

Finding of these studies showed people were comfortable beyond comfort limits prescribed in standards but still it wasn’t enough for me to disprove the universality of Fanger’s formula. 

Debunking Fanger’s theories

Results of experiments conducted in fields cannot be compared with ones carried out in a climate-controlled laboratory. Sharma and Ali conducted their study in a naturally ventilated building and WHO recommendations were based on field surveys while Fanger’s Predicted Mean Vote (PMV) is meant for an air-conditioned building.

The hypothesis is that if removed from external climatic reference, all humans will thermally behave the same, exhibiting compliance with laws of thermodynamics. In a 1973 paper published on the basis of a survey on subjects from tropical regions, Fanger categorically concluded that people “cannot become adapted to prefer warmer or colder environments.”

But evaluation of the PMV index is not easy as many of the parameters have to be estimated or require sensing modalities that may not be available. For this reason, international standards (ASHRAE Standard 55 and ISO 7730) introduced simplified prescription methodologies to define acceptable limits for thermal comfort.

Since Fanger’s PMV model has no relation with the outdoor climatic conditions, the seasonal variation in the standards’ simplified prescription is merely a function of the assumed change in people’s clothing between summer and winter, and associated relative humidity requirement for different clothing type (see table below).

Seasonal variation in thermal comfort prescription is solely a function of change in people's clothing

 

Winter

Summer

OPPTIMUM COMFORT TEMPERATURE

22°C

24.5° C

ACCEPTABLE COMFORT TEMPERATURE RANGE

20-23°C

23-26° C

Assumptions input in PMV model to calculate comfort temperature

 

 

RELATIVE HUMIDITY

50%

50%

MEAN RELATIVE VELOCITY

< 0.15 m/s

< 0.15 m/s

MEAN RADIANT TEMPERATURE

equal to air temperature

equal to air temperature

METABOLIC RATE:

1.2 met

 1.2 met

CLOTHING INSULATION

0.9 clo*

0.5 clo

Source: Thermal Comfort Conditions - ASHRAE Standard 55 (1992)

 * clo is a technical measure of clothing insulation

In the real world, most designers and building operators calculate comfort based on assumption that the people will wear suits and shoes year long and a relative humidity of average 55 per cent. This roughly works out to be 21.5 ± 1.5oC and buildings usually operate at this temperature throughout the year which is the winter prescription.

My New York apartment was in a centrally air-conditioned building, but I still wasn’t comfortable at 21-22oC. In fact, others often complain about new buildings being cold. As per Fanger, my internal thermostat was broken or was it?

Fanger’s diktat that all humans should be comfortable at the same temperature started getting scientifically questioned in the 90s. The results of multiple climate chamber experiments from around the world, in which clothing and meteorology were controlled as in Fanger’s, found comfort temperatures ranging from 24.9-29oC.  In general, the observation was people from warmer climates did prefer warmer indoor conditions.

But the status quo was unlikely to change soon.   

Standards were never questioned

The Heat Ventilation Air Conditioning (HVAC) industry and its professional association ASHRAE, which monopolised thermal comfort standards setting, rubbished most of these studies citing inconsistency in research methodology and overall quality control. And they had good reasons to do so.

By 1980s, HVAC industry had successfully lobbied to make its standard a legally binding legislative required for construction among the developed nations. In the absence of credible opposition at that time, it was a relatively easier task.

The most bewildering aspect of these legislations was that they assumed it would apply across all building types and to every human. This wide application of PMV can be attributed to vested interest of HVAC industry as the scientist community had been clear about its limitations. Fanger acknowledged in the introduction of his 1970 engineering guide that PMV is exclusively meant for centrally controlled HVAC buildings where occupants have little or no control over their immediate thermal environment.

Over the years, the industry narrowed the definition of thermal comfort to such an extent that it became almost impossible to design a building without HVAC system even in relatively mild climate zones where the weather is pleasant throughout the year. Any acceptance of flaws in the PMV model would call for broadening or introduction of exceptions to the standard that would have adversely affected the growth of the HVAC industry.

But a 1998 study sponsored by the industry was to change things as the results were contrary to their expectations.  

(This is the second article in a series which explains the evolution and politics of thermal comfort standards).

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