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For dehumidification solutions visit:

10 Reasons to Control Moisture:

To enable positive perceptions of thermal comfort.
To enable positive perceptions of indoor air quality and respiratory comfort.
To enable positive perceptions of indoor odour quality.
To enable comfort in mucous membranes.
To control hydrolysis (VOC emissions).
To control microbial.
To maintain dimensional stability in hygroscopic materials (woods).
To prevent condensation on hydrophobic materials (glass, aluminum).
To prevent condensation in hydrophilic materials (drywall).
To preserve moisture sensitive artifacts / collectibles / musical instruments.

For greater explanations go here.


Image result for Santa Fe RX
Figure 10. In-space dedicated dehumidification unit Therma-Stor Santa Fe RX

For more information on buildings, energy and indoor environmental quality visit the Building Science Podcast hosted by Kristof Irwin, M.Sc., P.E.

Feature Podcasts:
Water, Water, Everywhere

See also:

Allison Bailes, Ph.D. takes on moisture problems here.

Rudd, A. Humidity Control in Humid Climates, Prepared for: The National Renewable Energy Laboratory. Building Science Corporation August 2013

Rudd, Armin, Hugh Henderson, Jr., 2007. "Monitored Indoor Moisture and Temperature Conditions in Humid Climate U.S. Residences." ASHRAE Transactions (17, Dallas 2007). American Society of Heating Refrigeration and Air-Conditioning Engineers, Atlanta, GA.

Rudd, A., J. Lstiburek, and K. Ueno. 2005. Residential dehumidification systems research for hot-humid climates. US Department of Energy, Energy Efficiency and Renewable Energy, NREL/SR-550-36643.

Rudd, Armin, Joseph Lstiburek and Kohta Ueno, 2003. "Residential dehumidification and ventilation systems research for hot-humid climates," Proceedings of 24th AIVC and BETEC Conference, Ventilation, Humidity Control, and Energy, Washington, US, pp.355–60. 12-14 October. Air Infiltration and Ventilation Centre, Brussels, Belgium.

Building America Technical Highlight. Evaluation of Humidity Control Options in Hot-Humid Climate Homes

MacPhaul, D., Etter, C. 2010. HVAC System Design for Humid Climates. Whole Building Design Guide

IAQ: A Physicians View. Stephanie Taylor, M.D.,
M. Arch, CIC

Split whole house ventilating dehumidifier with no added sensible heat load — specifically designed for hot, humid climates.


What does Dr. Stephanie Taylor, M.D., M. Arch, CIC have to say about controlling temperature and humidity?

"Physicians are aware that indoor climates of 70˚F to 72˚F with 40% to 60% rh decrease respiratory infections, promote wound healing, and decrease dehydration from transdermal water loss. This is well established! Extensive research done by the military also shows that our skin coefficient of elasticity (Young’s modulus and shear strength) are determined by skin hydration, skin temperature, ambient temperature, and air humidity. Dry climatic conditions worsen skin inflammation and decrease essential barrier functions that protect underlying tissues from mechanical stress damage, skin irritants, and allergens."

Source: Engineered Systems, November 3, 2016

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

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

Facts about your skin














Humidity: An Important Nexus in Indoor Environmental Engineering
Copyright 2016, Robert Bean, R.E.T., P.L. (Eng.)

See also: Overheating Doesn’t Happen in Never Neverland: Peter Pan is in The House

Recently I was enroute to Halifax, NS to deliver a lecture on thermal comfort to the Nova Scotia Home Builders Association. Scheduled along the way I dropped into Madison, WI to participate in an appropriately tagged, “moisture mind meld” with a brain trust of humidity experts.(1) You might ask where a bunch of building geeks would hold such a meeting. Well in a State home to my paternal grandmother is the home of an enthusiastic American manufacturer of some of the best engineered dehumidification equipment made on the continent. Building folks know them under the brand of Therma-Stor and their ingenuity is found all over the world and in many devices used in conditioning people and spaces and in the building restoration and maintenance industry.

So why the get together?

That’s where Nikki Krueger, Industry Manager for Therma-Stor, and engineers Andy Ask and Tim O'Brien (Business and Product Development) enter the story.

Therma-Stors’ team flew several times onto my radar screen over the past few years at the Building Science Summer Camp hosted by Joe Lstiburek, Ph.D., P.Eng. of Building Science Corporation fame, then the Better Building Better Business Conference run by Seventhwave followed by the Humid Climate Conference in partnership with the Passive House Institute (Austin Chapter).

Seems fate was at work again because for the past few years I have also been studying moisture challenges in buildings including health effects on people and debunking the myth that condensation was the root of all evil in radiant cooling systems.

Funny how when there is a need to elevate an important message to society that a greater power manages to bring together those who can make a difference. Well the round up which included engineers, scientists and builders like Kristof Irwin, M.Sc., P.Eng., Allison Bailes, Ph.D., Armin Rudd, Chris Conway and John Semmelhack, was the work of Ms. Krueger. Nikki works with two visionaries in the industry, that being President Todd DeMonte and the legendary Ken Gehring. Gehring, affectionately known as Teddy Bear, invented the Ultra-Aire whole house ventilating dehumidifier back in 1996. If you need to know anything more about Todd and Ken is that they with their team, raised the company to the success story that it is today. Along with Nikki, Therma-Stor hosted the humidity event in part for the purposes of identifying how industry can do a better job of educating the home buyer and house building industry on the benefits of controlling moisture with simpler more effective solutions. A fundamental principle I fully endorse.

We discussed a lot of stuff that day but much of it boils down to this; as buildings become more energy efficient, their sensible cooling loads go down and that drives up the relative importance of moisture loads due to the ever present constants of occupant behaviour and ventilation air. There is a similar result with below grade heat losses having a greater consequence on total loads when above grade walls and roofs enclosures are improved.

So what does this mean?

It means people installing whole home A/C systems for thermal comfort in high performance homes with the assumption that effective dehumidification will occur, could be sadly disappointed as compressors cycle down or off due to the low and infrequent cooling comfort loads especially associated with shoulder seasons. Those shoulder seasons in the eyes of the mechanical system become longer as home performance increases. The result of low load homes is exacerbated further with oversized systems which regrettably is a systemic problem in the HVAC industry. Consider, in absence of smaller dedicated dehumidification systems, when the blower in the primary HVAC system is “off” as controlled by a thermostat, there is no air recirculation, no introduction of ventilation air, no filtration and no dehumidification. Herein lays the problems and why companies like Therma-Stor are positioned nicely as experts in the growing dedicated dehumidification solutions business. You see with these dedicated systems and if necessary, strategically selected and positioned in-space fans (ceiling, desk or floor mount); people can enjoy drier, cleaner and more thermally comfortable rooms. It also means people who want to use radiant cooling in homes can do so with a dedicated system. This ought to make the PEX pipe folks very happy because it simplifies the design process and it fits so well with the ethos of comfortable and efficient radiant cooling systems.

Check out Matt Risingers coverage of Therma-Stors SD12 unit. Matt has a great way of explaining why these stand-alone systems work so well.

Following our meetings in Madison and with additional input from physicians and industry engineers, I’ve revised my position on moisture control as noted below.

Beans’ Ten Best Reasons To Control Relative Humidity Between [35% and 55%] +/- 5%

1. To enable positive perceptions of thermal comfort

The authoritative document governing thermal comfort is ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy. The DNA for compliance includes ten prime metrics of which controlling humidity in one of them. To see the effect of humidity, readers are advised to visit our comfort simulator at or the CBE Comfort tool at .

Thermal comfort zone using the ASHRAE graphical method

Figure 1. 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).

2. To enable positive perceptions of indoor air quality and respiratory comfort

Research by Berglund and Cain (1989) and Cain et al. (1983) showed decreasing acceptability of air quality with increasing air temperature and humidity. Toftum (1998) showed that humidity controlled between 30% and 60% resulted in the least amount of occupants dissatisfied at a nominal 68F (20C). As temperatures increased for example to 72F (22C) occupants preferred humidity controlled between 30% and 40%. The challenge here is physicians would prefer the higher values (40% to 60%) but to do that the temperature would need to decrease. In traditional homes with high summer time MRT’s lower setpoints require energy for mechanical cooling; but in high performance homes with lower summer time MRT’s and with lower humidity, elevated air speeds achieved with ceiling fans can be used instead.

Figure 2. Upper limits of air humidity for preventing warm respiratory discomfort

Figure 3. Human satisfaction with the indoor air quality depending on relative humidity and air temperature.

3. To enable positive perceptions of indoor odour quality

Fang et al (1999) noted the effect of humidity on chemical and sensory emissions from building materials was found to be significant for waterborne materials such as floor varnishes and wall paints. In balancing between respiratory preferences and odour quality it is recommended to maintain humidity levels as prescribed towards the lower value and choose appropriate interior finishes.

4. To enable comfort in mucous membranes.

A healthy body is a hydrated body. When moisture content in sensitive organs such as eyes, nose, throat and skin is below or above optimum they become irritated leading to both physiological and psychological stress. For this reason moisture should not be too high nor too low and controlled with dehumidification or humidification as required. For further studies on effects from high humidity see hidromeiosis.

5. To control hydrolysis (VOC emissions)

Many building products especially interior finishes have components which are susceptible to breaking down into gases in the presence of a solvent such as water vapour. Formaldehyde is one such constituent and is commonly found in glues used in flooring, furniture and millwork. Maintaining lower humidly levels in the home reduces the probabilities of such lung irritants being released into the air (Corsi, 2013).

Figure 4. Relative humidity and paint emissions

6. To control microbial

When relative humidity is controlled within the prescribed range there is less risk of sustaining environments which support the growth of virus, bacteria, molds, mites, and some insects. All which can trigger negative response systems in the body.

Optimum zone for humidity

Figure 5. 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 and ASHRAE Handbook of Fundamentals 2013.

7. To maintain dimensional stability in hygroscopic materials (woods)

Building scientists demonstrate that the dimensional stability of materials such as woods are intimately connected to space moisture content.  Equilibrium moisture content in woods maintained between 6% and 14% corresponds to relative room humidity between [40% and 60%]+/-10% at dry bulb temperatures between 70F and 80F (21C and 27C). Ergo when moisture is controlled both in the wood and space, the differences in vapour pressure between the two is minimized which mitigates the risk of wood swelling or shrinking.

Figure 6. Maintaining dimensional stability in woods.

8. To prevent condensation on hydrophobic materials (glass, aluminum)

The dew point on a surfaces is a function of its temperature and the moisture content in the air immediately adjacent to the surface. With window and door glass and frames exposed to cooler temperatures there is wetting potential even with moderate humidity. The options are to lower room moisture and/or reduce the glass and frame conductivity. Again the physicians advise against ultra low humidity so in colder climates it’s important to work with the indoor environmental engineer to design the proper combination of space moisture (towards lower RH) and glazing systems (towards higher R value) in concert with the window treatments and HVAC systems (those that promote drying) so at design conditions there will be no wetting on these inherently cold surfaces (see #6 above).

Figure 7. Window type performance at various humidities and outdoor  temperatures. Source: THERM5.2/Windows5.2 NFRC Simulation Manual, Fenestration Heat Transfer Basics, Condensation Resistance

9. To prevent condensation in hydrophilic materials (drywall)

As above, except moisture diffuses into and through the material, causing increasing levels of dampness as it encounters cooler conditions.

Figure 8. Vapour pressure differentials drive moisture down a gradient into and through vapour permeable materials. When moisture reaches the dew point on a surface, condensation will occur.

10. To preserve moisture sensitive artifacts / collectibles / musical instruments etc.

There is no shortage of study in preserving moisture sensitive items such as photographs, literature, musical instruments, paintings, wood carvings and furniture. Damage or material stress can occur frequently when these items are transferred from one environment to another having a different relative humidity; or when moisture levels are left uncontrolled. Since not all artifacts use the same preservation approach it is important once again to work with the indoor environmental engineer to ensure the space conditions are suitable for the items of concern.

Figure 9. Preservation of photographs based on recommended values from the Image Permanence Institute. Note the lower RH range.

So there you have it, ten very good reasons why you should be controlling your humidity between [35% and 55%] +/- 5%. When you do this in conjunction with elevated air speeds from fans, you can in many cases reduce or eliminate traditional and energy intensive A/C systems; or at the very least optimize its' design and operation. It also opens up the use of radiant cooling systems known for their comfort and efficiency.

1. Moisture mind meld is the tag by Kristof Irwin from Positive Energy

Additional Reading:

Thermal Comfort: A 40 grit perspective for consumers
Built to code: What does it mean for consumer thermal comfort?
Thermal Comfort: A Condition of Mind

Humidity: Effects on the Environment and Occupants



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