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Moisture - control it or it will control you
Humidity, water vapor and condensation influence both the occupant and the building
Image Credit: © Martyn F. Chillmaid / SPL

As it relates to comfort:

"At high ambient temperatures the degree of discomfort can be heavily influenced by the air humidity..." Fanger, P.O., Thermal Comfort: Analysis and Applications in Environmental Engineering, McGraw-Hill Book Company, 1970

As it relates to health:

"The patient physiological barriers in passages from the nose to the depths of the lungs are most functional at room RH of 40% to 60%. Mucus-lined passages prevent pathogens from invading into deeper body tissues by continually washing particles away. This critical defense mechanism is impaired by dry air, enabling harmful organisms to reach deep lung tissue and the blood stream." Stephanie H. Taylor, M.D.

On a hot day you'll generally feel more comfortable with lower humidity due to the cooling effect from greater evaporation of perspiration from your skin.

Useful Reference on Humidity and Design from:

Dr. J.Lstiburek, P.Eng. and Building Science Corporation

Please be patient with these downloads...takes up to 30 to 40 seconds on a high speed service.

Relative Humidity

 Radiant Heating and Hardwood Floors is about Controlling Humidity







Dehumidify Air

Dehumidification for Radiant Systems








Shown below is the steam humidifier connected to the editors ventilation system.

With radiant based HVAC systems the discharge temperature of the fresh air system is a nominal 70 °F which is generally less effective with evaporative type humidifiers.

Steam is the preferred choice not only for its effectiveness at low air temperatures but also for its air borne bacteria fighting benefits. This is why in the CSA HVAC Standards for Healthcare Facilities that steam is the only humidification system allowed. Doesn't it make sense to use it for your home as well?


Additional Resources

Learn more about humidity in general by reading the:

Total Comfort System
Radiant Cooling
Radiant Hardwood Floors

For an academic study of humidity and indoor climates visit:

Bio-Climatic Design

As it relates to comfort:

"It is often assumed that dry indoor air, i.e. low air humidity could cause a drying out of the mucosa of the upper airways and skin due to increased evaporative power of dry air. A number of laboratory and field studies show that the perception of “dry air” is due more often to the air being polluted or too warm than being physically “dry”. Since the sensation of dryness is strongly associated with the prevalence of SBS, it is therefore used as indicator of the health problems in buildings, but not to indicate that the air has low water content."
ref.: Indoor Environment- Health-Comfort and Productivity, B.W. Olesen, PhD, International Center for Indoor Environment and Energy, Technical University of Denmark

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Humidity: Effects on the Environment and Occupants

Notice: We're running it again, Integrated HVAC Engineering: Mastering Comfort, Health, and Efficiency. 

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Regardless of the HVAC system type, moisture must be controlled for biological concerns (bacteria, viruses, mites and molds), for hydrolysis and the control of VOC emissions (formaldehydes, phthalates, terpenes etc.), for the dimensional stability of hygroscopic materials (woods), for the preservation of materials, and for occupant respiratory and thermal comfort.

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Humidity: An Important Nexus in Indoor Environmental Engineering

Optimum zone for humidity

Figure 1. Optimum zone for humidity (for larger images and more detail see our ASHRAE San Antonio Presentation - note: you'll need the password - simply join our Linked-In discussion group and request the password - be sure to provide the URL of the presentation page).
Adapted:"Criteria for Human Exposure to Humidity in Occupied Buildings." Dr. Elia Sterling, 1985

"Relative humidity levels below 25% are associated with increased discomfort and drying of the mucous membranes and skin, which can lead to chapping and irritation. Low relative humidity also increases static electricity, which causes discomfort and can hinder the operation of computers and paper processing equipment. High humidity levels can result in condensation within the building structure and on interior or exterior surfaces and the subsequent development of moulds and fungi. In most Canadian cities, ideal indoor relative humidity levels are 35% in the winter and 50% in the summer."
Reference: A Report of the Federal–Provincial Advisory Committee on Environmental and Occupational Health , Technical Guide to the Investigation of Indoor Air Quality in Office Buildings, Department of National Health and Welfare, Cat. No. H46-2/93-166 Erev ISBN 0-662-23846-X Humidity and the Comfort Zone

Figure 1a. Window type performance at various humidities and outdoor  temperatures. Source: THERM5.2/Windows5.2 NFRC Simulation Manual, Fenestration Heat Transfer Basics, Condensation Resistance

Humidity and The Comfort Zone

Thermal comfort zone using the ASHRAE graphical method

Figure 2. Thermal comfort zone using the ASHRAE graphical method incorporates humidity and operative temperature (for larger images and more detail see our ASHRAE San Antonio Presentation note: you'll need the password - simply join our Linked-In discussion group and request the password - be sure to provide the URL of the presentation page).
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Figure 2a. Relationship between temperature, humidity and satisfaction. Slide from our course on Indoor Air Quality, see Master Program
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Humidity and hygroscopic materials:
Dimensional stability in wood is achieved between 40% and 60% rh +/-10% @ 70°F to 80°F (21°C to 27°C).

Humidity and its effects on hygroscopic materials

Figure 3. Humidity and its effects on hygroscopic materials such as wood
(for larger images and more detail see our ASHRAE San Antonio Presentation
-  note: you'll need the password - simply join our Linked-In discussion group and request the password - be sure to provide the URL of the presentation page).

Thermal and Moisture Regions of North America

Shown below are the moisture and thermal regions of North America. Your interior humidity is as much about your geographic location as it is about your building/ mechanical systems and the moisture producing activities inside.

Thermal and hydro-thermal regions of North America

Thermal and hydro-thermal regions of North America


Figure 4a. and 4b. Thermal and hydro-thermal regions of North America (Courtesy of Building Science Corporation)
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The following is an excerpt from, "Canadian Building Digests CBD-1. Humidity in Canadian Buildings, N. B. Hutcheon" originally published in 1960 but still valid today

Effects of Relative Humidity upon People

It has long been accepted that thermal sensations of comfort are affected by relative humidity and that for the same comfort a higher temperature is required to offset a decreased relative humidity. The research work on which these conclusions were based is under review and new evidence is being obtained. It is now evident that the conclusions drawn from the earlier work applied mainly to the thermal sensations experienced over limited periods of time after leaving one room for another at a different relative humidity. For conditions of prolonged exposure to the same relative humidity at temperatures within the comfort zone most people will experience only small changes in thermal sensations of comfort over a wide range of relative humidities so long as they are not actually sweating.

There is little conclusive evidence to show that either high or low humidities are of themselves detrimental to the health of normal people. It seems logical to assume that, in general, extremes of humidity, are undesirable and that it is desirable to keep relative humidities at values within a broad range of from 30 to 70 per cent. There is, however, no firm basis for establishing such limits so far as the health and comfort of most people are concerned.


Although it would be desirable to avoid high moisture contents to prevent deterioration of the building structure, and at the same time to provide constant humidity the year round to minimize dimensional and other changes in materials and furnishings within a building, this is difficult to achieve under Canadian winter conditions. Even moderate humidities inside buildings can produce wetting by condensation on and in the building structure because of the large temperature gradients within walls in winter. The best answer in many cases where higher humidities are not necessary will be to allow the humidities to fall to low levels. In buildings with large moisture sources steps may have to be taken to eliminate water vapour from them to avoid condensation. Consideration should always be given to the difficulties that may be created in any given building before humidification is added. When relative humidities in the medium to high range must be carried, the buildings must be specially designed and constructed.


John Straube and Chris Schumacher answer the question "how can I manage humidity levels in my home

12 Days of Building Science - Managing Humidity
Building Science Consulting Inc. | Building Science Laboratories

Additional reference material

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

  2. Jing, S., Lia, B., Tan, M., Liu,H., Impact of Relative Humidity on Thermal Comfort in a Warm Environment, Indoor and Built Environment, 2013, 22: 4, 598-607

  3. Fanger, P.O., Thermal Comfort: Analysis and Applications in Environmental Engineering, McGraw-Hill Book Company, 1970

  4. Human Comfort and Health Requirements, Washington State University, 2006

  5. Jan Vilhelm Bakke, Bente E. Moen, Gunilla Wieslander, Dan Norbck, Gender and the Physical and Psychosocial Work Environments are Related to Indoor Air Symptoms, Journal of Occupational and Environmental Medicine, 2007, 49, 6, 641

  6. J.L.M. Hensen 1990. "Literature review on thermal comfort in transient conditions," Building and Environment, vol. 25, no. 4, pp. 309-316.

  7. R.R. Gonzalez and A.P. Gagge 1973. ‘‘Magnitude estimates of thermal discomfort during transients of humidity and operative temperature and their relation to the new ASHRAE Effective Temperature (ET*),’’ in ASHRAE Transactions, vol. 79:1, pp. 88-96, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA.

  8. R.R. Gonzalez and L.G. Berglund 1979. ‘‘Efficacy of temperature and humidity ramps in energy conservation,’’ ASHRAE Journal, vol. 6, pp. 34-41.

  9. R.G. Nevins, R.R. Gonzalez, Y. Nishi, and A.P. Gagge 1975. ‘‘Effect of changes in ambient temperature and level of humidity on comfort and thermal sensations,’’ in ASHRAE Transactions, vol. 81:2, pp. 169-182, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA.

  10. C.H. Sprague and P.E. McNall jr 1970. ‘‘The effects of fluctuating temperature and relative humidity on the thermal sensation (thermal comfort) of sedentary subjects,’’ in ASHRAE Transactions, vol. 76:1, pp. 146-156, American Society of Heating, Refrigerating, and Air Conditioning Engineers, Atlanta, GA.

  11. J.A.J. Stolwijk 1979. ‘‘Physiological responses and thermal comfort in changing environmental temperature and humidity,’’ in Indoor Climate, pp. 491-506, Copenhagen.

  12. Palonen, J., Seppänen, O. and Jaakkola, J. J.K. (1993), The Effects Of Air Temperature And Relative Humidity On Thermal Comfort In The Office Environment. Indoor Air, 3: 391–397. doi: 10.1111/j.1600-0668.1993.00025.x

  13. Eccles, R., An explanation for the seasonality of acute upper respiratory tract viral infections. Acta Otolaryngol 2002; 122:183–191. <>

  14. Johnson, C., Eccles, R. Acute cooling of the feet and the onset of common cold symptoms. Family Practice 2005; 22: 608–613. < html>

  15. Eccles. R., Acute cooling of the body surface and the common cold. Rhinology, 40, 000-000, 2002 <>

  16. Skin Hydration: Ensuring Barrier Protection for the Healthcare Professional
    Lori F. Jensen, RN, and Pam Werner, RN, BSN, CNOR, MBA 05/31/2007

  17. Modeling the airborne survival of influenza virus in a residential setting: the impacts of home humidification

  18. Facts about your skin
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Related reading:

Do I need an engineer? A Guide to HVAC/Indoor Climate Design Services
Where will your indoor climate system score?
How to "ball park" your budget for indoor climate control.
Indoor environments: Self assessment
Built to code: What does it mean for consumer thermal comfort?
The Total Comfort System - The "Un-minimum" System
Thermal Comfort: A 40 grit perspective for consumers
Thermal Comfort: A Condition of Mind

Do-It-Yourself HVAC - Should you do it?
The Cost of HVAC Systems - Are You Paying Too Much for Downgrades?
Radiant Installations - The Good, Bad and Ugly
Thermal Comfort Surveys - Post Occupancy, Part I
Thermal Comfort Surveys - Post Occupancy, Part II
HVAC does not equal Indoor Environmental Quality (IEQ)

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