Thermal Comfort

“Good temperature is the one you don’t notice at all”

Our thermal comfort defines not only our well-being but our physical and intellectual performance.

There are many factors that effect human thermal comfort, such as air temperature, temperature of surfaces, humidity, and air movement. All of the above may be grouped under "environmental variables". "Personal" variables such as clothing insulation value ["clo" value] and the metabolism rate ["met" value] are also important components when calculation individual thermal comfort.

http://www.design.asu.edu/radiant/01_thermalComfort/comfortC_01variables.htm

Each of the above influences how we feel in a given volume of air. To reach a

desirable level of comfort some or all of the factors may be manipulated. Sometimes one of them may be changed instead of the other. For example, instead of raising the temperature to make the space feel warmer for people with a low physical activity, humidity may be raised. In order to understand what thermal comfort is and how it effects us we had to review some of the terminology:

Relative humidity - is the percentage of water in air compared to the maximum the air can hold. For ideal comfort relative humidity has to be kept low in hot temperatures/climates.

High humidity (60% or above) might become a serious issue in extreme climates as it could cause condensation, destroy/decompose materials that can hold water, and even promote growth of microbes. Low humidity (20% or less) can cause health problems such as dry nose and throat nosebleeds, or may cause problems with not very stable materials, such as shrinking, etc.

Air movement accelerates evaporation process when touching a surface. When touching human skin through evaporation it cools the body influencing our thermal comfort. Air movement may be increased when instant cooling is needed. To maintain a comfortable space without drafts the air movement should be kept between 10-15 cubic feet per minute.

In last class we also discussed such terminology as:

Thermal capacity – the ability of a material to sore heat proportional to materials mass and weight.

Thermal resistance – how fast heat transfers through a material. For example, materials with low resistance cool or hear rapidly, and material with high thermal resistance may be used to absorb the heat during the day and releasing it into the space during the night to heat it up (methods used in deserts)

Heat lag – explains gap in temperature, it is the time it takes for heat to travel through a material.

Core body temperature – 37C (98.6F).

· People perform best id the room temperature stays the same most of the time

· ¾ of the body heat is released by human body just to keep it warm. The rest is used to move it. At the same time, the more body moves – the more heat it generates. That means that the more body moves the warmer it stays and more the space is warmed up by the body, which must be considered when designing spaces for active physical activities by calculating space volume, nunber of people, and an average metabolic rate

Metabolic rate – rate at which energy is used based on activity

Thermal equilibrium – a material is at thermal equilibrium when there is no energy/heat transfer between the material and its surroundings

Heat transfer – a travel of heat from higher temperature surfaces to lower temperature surfaces. For example, in winter the cold surface of a window “sucks in” the heat from the material and bodies in the interior of the room

Thermal dynamics – heat movement

· Convection – movement of active warm air molecules to cooler areas. For example, when air moves pass our body it observes its energy/heat

· Conduction – heat transfer through direct contact with cool surfaces. For example, as we stem on cold stone floor our body heat quickly starts moving from out body to the stone material

· Radiation – heat radiates to cooler surfaces without any physical contact

· Evaporation – transformation of a material from liquid to gas during which gas absorbs heat energy

Reflectance/Absorbance – ability of heat to be reflected off light surfaces and absorbed by dark surfaces

Keeping in mind the discussed above building envelope and joining elements of a new building constructed bust be determined individually based on climate and site conditions. Thermal breaks must be correctly done between the building envelope and materials. Thermal bridge, using the qualities of heat discussed above might let the cold (or hot) air transfer from the outside of the building in its inside (which is not desirable) through such elements as metal joints. To make correct decisions designer must know the conditions he/she is building in, the qualities of the materials used, and the general physics of air and moisture movement

Thermal comfort is hard to visualize but it may be illustrated with mathematical formulas or charts. This is an awesome illustrated website with "simple" mathematical formulas that explains what is thermal comfort, its values, conditions, and how to calculate it. The website highlights what to measure and pay attention to when calculating thermal comfort and explains all the factors that have to be taken into account such as physical activity factors, clothing factors, temperature and humidity factors, etc: http://www.blowtex-educair.it/DOWNLOADS/Thermal%20Comfort.htm :

I personally like Willis Carrier's Psychrometric Chart that he developed over one hundred years ago trying to help people visualize the relationship between the air temperature and relative humidity that always exist in space. this chart is the basis of the air conditioning industry.

The chart outlines conditions at which most people are comfortable at. Winter and summer comfort zones are shown separately. It should be taken into account that different amount/kind of clothing is worn during this two different seasons.

It may be concluded from the chart above that the temperature where most people would be comfortable at year round would be around 74F which is about 23C.