HPAC Magazine

The logic of moisture management

February 1, 2016 | By Robert Bean

Condensation is not the dark side of chilled water and radiant cooling systems.

There is no shortage of articles describing the risks and jeopardies of using chilled water and radiant cooling systems, and how these systems cause condensation. It seems that mysterious aliens with a hidden agenda handed out scripts to industry authors instructing them to repeat in a James Earl Jones voice, “Luke, be wary of the radiant cooling systems – it shall condense and rain upon the galaxy.” To this I reply: What are the facts?

Here is a serving of universal logic for critics of chilled water and radiant cooling systems: 100 per cent of all condensation problems in buildings conditioned exclusively with refrigerated air, did not have a chilled water radiant cooling system to blame. We do not hear Darth Vader voices from the “refrigerated air only” camp. Only bad designers and bad installers can be held accountable for sweating refrigerated air-based systems because good designers and good installers would never let moisture become a problem.

Good designers and installers spend hours evaluating moisture loads and assembling building and HVAC components properly so sweating does not occur. But apparently with chilled water and radiant cooling the good logic of moisture management gets tossed to the far corners of the galaxy because evidently only bad designers and bad installers are permitted to work on these systems. That is definitely dark side thinking.

It is time to stop holding radiant cooling and chilled water systems to some unreasonable double standard and talk about the real problem, which is moisture. Moisture is the root of all that is good and bad in the universe. It is an equal opportunity offender and does not discriminate between HVAC system types. Statistically one is far more likely to find moisture problems in refrigerated air systems than chilled water radiant systems due to the installation ratio of air over water.

Here are six reasons why condensation with chilled water and radiant cooling panels is a straw man argument; regardless of HVAC system type, moisture must be managed for biological concerns, hydrolysis, dimensional stability of hygroscopic materials, and preservation of materials, respiratory comfort, and thermal comfort. Condensation becomes a moot point if you control moisture for these reasons (setting aside discussions around pipe insulation, infiltration, and so on). Let’s look at each of these conditions.

1. Biologicals

Without moisture control, numerous biological risks develop which support the growth of bacteria, viruses, fungi (moulds) and mites. According to a ASHRAE Transaction, Human Exposure to Humidity in Occupied Buildingsi and the ASHRAE Handbooks, humidity at less than 30 per cent or more than 60 per cent can introduce higher multiple microbial risk factors. As noted in the Environmental Protection Agency (EPA) Indoor Air Plus program, “You have to control humidity to below 60 per cent RH.” You will also find support from the medical community on this range. Stephanie H. Taylor, M.D. 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 (NBL), Univ. CA, Berkeley who said, “Moisture content may, indeed, be the most important environmental factor influencing the survival of airborne microbes.”

Taylor goes on to say, “Maintaining the relative humidity of hospital indoor air between 40 per cent and 60 per cent can significantly decrease healthcare associated infections.”ii  In addition to ASHRAE, the EPA and NBL this position is supported by numerous authoritative organizations including the Canadian Centre for Occupational Health & Safety, Health Canada and ACCA and The Indoor Environment & Energy Efficiency Association.

2. Hydrolysis

Hydrolysis is a water-based reaction that is used to break down certain chemicals. Studies by Dr. Richard L. Corsi, University of Texas (Austin), show paint emissions (specifically HC-O-O) are affected by rising relative humidity.iii Matthews et al, noted that changing the indoor conditions from 68F (20C) and 30 per cent relative humidity (RH) to 79F (26C) and 60 per cent RH would result in two to fourfold increases in formaldehyde concentration for the same air change rate. Hodgson et al, stated in its study on the topic, “This suggests that indoor humidity has a substantial impact on formaldehyde emission rates and concentrations.”iv

3. Dimensional Stability of Hygroscopic Materials

When hygroscopic materials such as wood are operated in an uncontrolled environment their moisture content can fluctuate. Such changes lead to dimensional instability due to shrinking and swelling. Both Canada Mortgage and Housing Corporation’s Wood Frame Envelopes Best Practice Guidev and Forest Products Laboratory’s (U.S. Department of Agriculture/Forest Services) Wood Handbookvi provide for the ideal “in service” wood moisture content as between six per cent and 14 per cent. This ideal in-service range corresponds to relative humidity between 40 per cent+/-10 per cent and 60 per cent+/-10 per cent at temperatures typical for space heating and cooling.

4. Preservation of Materials

According to the Image Permanence Institute (IPI), a university-based non-profit research laboratory devoted to preservation research, at approximately 73F dry bulb, “no risk” conditions exists for chemical decay of organic materials between 25 per cent RH and 35 per cent RH for a dew point condition between 45F and 59F.vii The American Museum of Natural History regarding preservation and temperature and relative humidity says this; “Different types of collections have substantially different relative humidity requirements…Specimens with metal components may benefit from RH levels that are as low as possible. Organic artifacts require more moderate RH levels to prevent desiccation or embrittlement. Most specimens benefit from RH levels that are moderate and stable to prevent physical damage that can be caused by wide climatic shifts. Generally, recommendations for museum environments are given as to [sic] 50 per cent while attempting to minimize dramatic swings to between 40-60 per cent, even if broad seasonal trends are hard to avoid.”viii

5. Respiratory Comfort

Research showing the effects of high and low humidity on respiration discomfort supports the humidity ranges above. In one study the least amount of people dissatisfied (PD) at 10 per cent, corresponds to space conditions of 20 per cent to 60 per cent relative humidity for a temperature range between 68F (20C) and 78F (26C). Increases in discomfort were observed during increases in relative humidity at a given air temperature. For example, at 72F (22C) there is a 10 per cent increase in people dissatisfied going from approximately 40 per cent RH to 65 per cent RH; and an additional 10 per cent dissatisfaction going from 65 per cent RH to 80 per cent RH.ix,x

6. Thermal Comfort

Two authoritative documents addressing humidity and thermal comfort are ANSI/ASHRAE Standard 55 and ISO 7730. At the humidity conditions defined in points 1 through 5, indoor climate engineers will meet the requirements of both the ASHRAE and ISO Standards.


Now that you have 60 per cent RH as recommended maximum, do you know what the sea level dew point is at say a 75Fdb? It is 60F. What is the lowest temperature a radiant floor cooling system should operate at? It is 66F. That would be a six degree Fahrenheit safety margin – far more than required by good engineering practice.

If you have made it this far you should conclude that there is zero logic in stating unequivocally, “don’t use chilled water and radiant cooling systems because of moisture concerns.” It is a silly statement. You must provide moisture control regardless of the HVAC type for the six reasons outlined above.xiii

With chilled water systems the control of space moisture is done with a dedicated outdoor air system. Noted DOAS expert Stanley A. Mumma, Ph.D., P.E., Fellow ASHRAE, Professor Emeritus of Architectural Engineering, Penn State University states, “The DOAS approach effectively eliminates biological contaminants and inadequate ventilation. It also avoids building-wide distribution of indoor chemical contaminants.”xiv

The need for a hybrid with two systems, one for ventilation and one for cooling is actually a good thing. Indoor air quality and energy specialists will attest that dedicated ventilation with radiant cooling provide superior control and efficiency over dual duty air only systems. In commercial systems Rumsey and others have demonstrated these hybrid systems can also be installed for less cost.xv So hybrid systems can enable better air quality, better comfort, better efficiency and for some projects – a lower capital cost.

Controlling moisture is a fundamental principle. It enables the use of chilled water and radiant cooling systems without going off the deep end on condensation concerns. Condensation is not the dark side of chilled water and radiant cooling systems – moisture is. Take care of the moisture and you take care of the dark side.

Robert Bean, who is president of Indoor Climate Consultants Inc., is a Registered Engineering Technologist in building construction through the Association of Science and Engineering Technology Professionals of Alberta and a Professional Licensee in mechanical engineering through the Association of Professional Engineers, Geologists and Geophysicists of Alberta.


i Sterling E M, Arundel A, Sterling T D., 1985 CH-85-13 No 1, ASHRAE Transactions, 1985, Vol 91, Pt 1. 11p.

ii Taylor, S.H. (2014) Infectious Microorganisms Do Not Care About Your Existing Policies. Engineered Systems <http://www.esmagazine.com/articles/96849-infectious-microorganisms-do-not-care-about-your-existingpolicies> accessed Jan. 2016

iii Corsi, R.L., 2013. Relative humidity and paint emissions (HC-O-O). Building Energy & Reactivity Complex Interactions. Simple Solutions, IAQ 2013 – Environmental Health in Low Energy Buildings – Vancouver, BC, Canada October 17th, 2013

iv Hodgson, A.T., et al. 2004. Volatile Organic Compound Concentrations and Emission Rates Measured over One Year in a New Manufactured House. Lawrence Berkeley National Laboratory. <http://www.osti.gov/scitech/servlets/purl/838617> accessed Jan. 2016

v Wood Frame Envelopes: Best Practice Guide. Canada Mortgage and Housing Corporation. 1999. <http://www.naturallywood.com/sites/default/files/CMHC-Best-Practice-Guide-Wood-Frame-Envelopes.pdf > accessed Jan. 2016

vi Wood Handbook. Forest Products Laboratory. U.S. Department of Agriculture/Forest Services) http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100 accessed Jan. 2016

vii Image Permanence Institute. <https://www.imagepermanenceinstitute.org/environmental/research/preservation-metrics> accessed Jan 2016

viii Temperature and Relative Humidity (RH). American Museum of Natural History.<http://www.amnh.org/ourresearch/natural-science-collections-conservation/general-conservation/preventive-conservation/temperatureand-relative-humidity-rh >Accessed Jan. 2016

ix Toftum, J., Jørgensen, A.S., Fanger, P.O. 1998. Upper limits of air humidity for preventing warm respiratory discomfort, Energy and Buildings, Volume 28, Issue 1, August 1998

x Fang L, Clausen G, Fanger PO. Impact of temperature and humidity on the perception of indoor air quality. Indoor Air 1998; 8:80–90.

xi ANSI/ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy

xii ISO 7730 Ergonomics of the thermal environment — Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria.

xiii Setting aside discussions on duct and pipe insulation.

xiv http://www.healthyheating.com/DOAS/DOAS_Introduction.htm#.VpbI3PkrL9g

xv Sastry, G., Rumsey, P. 2014. VAV-vs- Radiant-Side-By-Side-Comparison, ASHRAE Journal, vol. 56, no. 5, May 2014. < https://www.ashrae.org/resources–publications/periodicals/ashrae-journal/features/vav-vs–radiant-sideby- side-comparison>

Additional resources can be found at www.hpacmag.com, search radiant cooling and condensation.



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