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facts and myths on radiant barrier

Technical definition:

Operative temperature is defined as a uniform temperature of a radiantly black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual non-uniform environment.

Some references also use the terms 'equivalent temperature" or 'effective temperature' to describe combined effects of convective and radiant heat transfer.

In design, operative temperature can be defined as the average of the mean radiant and ambient air temperatures, weighted by their respective heat transfer coefficients.

The instrument used for assessing environmental thermal comfort in terms of operative temperature is called a eupatheoscope and was invented by A. F. Dufton in 1929.

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I say, if building codes dropped the reference to controlling air temperatures and switched the requirements to controlling mean radiant temperature, building performance specifications would have to change overnight. Bad buildings have low MRT’s in winter and high MRT’s in summer; this contributes to large differences in the vertical air temperature; creates excessive drafts and increased radiant asymmetry. Bad buildings also have uncomfortable floor temperatures and are challenged to maintain reasonable levels of humidity.



Operative Temperature (
To): What happens when MRT and Tdb mate?
Copyright (c) 2012, Robert Bean, R.E.T., P.L.(Eng.) and

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When it comes to thermal comfort, operative temperature (To) is what humans experience thermally in a space. It is the combined effects of the mean radiant temperature and air temperature (Tdb).

Mathematically, operative temperature (To or Top) can be expressed as;

To = ( hrTmr + hcTdb ) / ( hr + hc )


hc = convective heat transfer coefficient

hr = linear radiative heat transfer coefficient

Tdb= air (dry bulb) temperature

Tmr = mean radiant temperature

In its simplest form
operative temperature can also be expressed as;

To = (Tmr + Tdb) / 2

The predominant representation of the simple form for operative temperature is found on the psychrometric chart published in ANSI/ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy (Figure 1).

The chart is familiar to virtually all seasoned Architects and HVAC designers except most designers fail to note the x-axis in thermal comfort analysis is not dry bulb (
Tdb or Ta, air or ambient) temperature but operative temperature (To).


Thermal comfort zone using the ASHRAE graphical method

Figure 1. Modified psychrometric "thermal comfort" chart as published in ANSI/ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy considers humidity (y-axis), operative temperature (x-axis), metabolic rate, clothing and air velocity (reprinted with permission from ANSI/ASHRAE Standard 55, see 

Mean Radiant Temperature: An intimate part of operative temperature

Most people understand the concept of dry bulb or air temperature but few really grasp the concept of mean radiant temperature. Unfortunately the focus on air temperature has led to bad indoor climate solutions comprising of blowing hot and cold air instead of focusing on the mean radiant temperature which is first and foremost a building enclosure performance element. This element can if necessary be augmented with radiant cooling and heating systems designed using the graph below.


Figure 2. ASHRAE Design Graph for Sensible Heating and Cooling with Floor and Ceiling Panels1 reprinted with permission from Section/page 6.8, Panel Heating and Cooling, 2012 ASHRAE Systems and Equipment Handbook (see


Figure 3. During the hot summer months, watching an outdoor sports match or concert can be tantamount to baking uncomfortably in the sun -- but it doesn’t have to be. At the TEDxSummit in Doha, physicist Wolfgang Kessling reveals sustainable design innovations that cool us from above and below, and even collects solar energy for later use.


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