Mean radiant temperature

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Mean Radiant Temperature (MRT) - Part II
Copyright (c) 2011, Robert Bean, R.E.T., P.L.(Eng.) and

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Mean Radiant Temperature (MRT) - Part I: Introduction/theory
Mean Radiant Temperature (MRT) - Part II: Application of theory
Mean Radiant Temperature (MRT) - Part III: Spreadsheet tool for thermal bridging
Mean Radiant Temperature (MRT) - Part IV: Project examples & ASHRAE research
Mean Radiant Temperature (MRT) - Part V: Calculation example (REHVA GB #7)

click to enlarge (MRT)

Explanation (idealized graph - above): as the building performance goes from terrible/poor (floor surface flux > 30 Btu/hr/sf) to terrific/high (floor surface flux <10 Btu/hr/sf) the MRT approaches the conditions of a hypothetical perfect enclosure. Another way of saying this is the further one moves away from the hypothetical perfect enclosure the lower the MRT in winter and higher the MRT in summer and thus the greater the discomfort. When it comes to calculating fluid temperatures for heating and cooling systems the closer one moves toward the hypothetical perfect enclosure (see operative temperature) the lower the fluid temperatures in winter and higher the fluid temperatures in summer and thus the greater the system efficiency.

Transitional and terrific buildings enable maximum efficiencies from heating and cooling appliances like boilers, heat pumps, chillers and solar systems due to the necessarily lower fluid temperatures in heating and higher fluid temperatures in cooling.

Message: building enclosures, HVAC efficiencies and thermal comfort are not exclusive items unto themselves - they are in fact one and the same.

Above slide: from our course on building science for cold climates, effects of insulation on thermal bridging and the mean radiant temperature.  You can excessively feed a mechanical system fuel and power (calories) to compensate for a bad building or you can build a good building and reduce the calorie input. Insulation, caulking and good choices in windows go a long way in solving energy problems plus these "three amigos" have no moving parts, will never need a technician and will last longer than the mechanical systems.

See overview of housing performance categories in North America.

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