Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
Not for profit educational resource on indoor environmental quality.
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Fundamentals of indoor environmental quality / thermal comfort and air quality solutions using radiant based HVAC


One of the myths perpetuated with radiant cooling systems is condensation risk on the cooling surface. This would be a real risk if there was no moisture control. Without moisture control numerous risk factors develop such as supporting the growth of bacteria, viruses, fungi (moulds, molds) and mites. According to ASHRAE Transaction Human Exposure to Humidity in Occupied Buildings and the 2011 ASHRAE Application Handbook humidity less than 30% or more than 60% can introduce higher multiple microbial risk factors. As noted in the EPA Indoor Air Plus program, "You have to control humidity to below 60% RH."

Clearly you can see that various risk occur through the relative humidity range but the optimum zone with the least risk occurs between 30% and 60%. According to the 2011 handbook this illustration describes, "Optimum Humidity Range for Human Comfort and Health". Within this controlled humidity range properly designed and controlled radiant cooling system can easily operate without condensation.

There is an exhaustive supply of research addressing this topic and readers are encouraged to seek out these documents for detailed study. For our purposes here, it is enough to say once again moisture must be controlled in habitable spaces which ultimately enables the successful use of radiant cooling. 

This content is a key component from our course, "Integrated HVAC Engineering: Mastering Comfort, Health, and Efficiency."


Update: An excellent discussion on humidity and microbial was recently authored by Stephanie H. Taylor, M.D. who states, "The movement and infectivity of bacterial, viral, and fungal organisms vary with the RH of the air…" This is supported by Dr. R.L. Dimmick from the  Naval Biological laboratory, Univ. CA, Berkeley who stated, “Moisture content may, indeed, be the most important environmental factor influencing the survival of airborne microbes.”  Taylor goes on to state, “Maintaining the relative humidity of hospital indoor air between 40% and 60% can significantly decrease healthcare associated infections.” As we have noted repeatedly humidity must be controlled and 60% as an upper limit serves the physicians and the occupants and the engineer.

See also:

Radiant Cooling - Part I, Fundamentals
Radiant Cooling Systems: Calculation Example
Tres Bien for Large Scale Radiant Cooling
Radiant Cooling for Sceptics: How to do radiant cooling in high humidity geographies
Radiant based HVAC systems - bibliography / resources
Radiant Cooling Systems: Condensation Concerns Part 1 of 6, Preservation of Materials
Radiant Cooling Systems: Condensation Concerns Part 2 of 6, Microbial
Radiant Cooling Systems: Condensation Concerns Part 3 of 6, Hydrolysis
Radiant Cooling Systems: Condensation Concerns Part 4 of 6, Dimensional Stability of Hygroscopic Materials
Radiant Cooling Systems: Condensation Concerns Part 5 of 6, Respiratory Discomfort
Radiant Cooling Systems: Condensation Concerns Part 6 of 6, Thermal Comfort



  1. Taylor, S.H. (2014) Infectious Microorganisms Do Not Care About Your Existing Policies. Engineered Systems < > accessed Nov 16, 2014


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