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Fundamentals of indoor environmental quality / thermal comfort and air quality solutions using radiant based HVAC

Luke, Condensation is not the Dark Side of Chilled Water and Radiant Cooling Systems.
Copyright 2016, Robert Bean, R.E.T., P.L.(Eng.) All world rights reserved. Originally for HPAC Canada Magazine.

Ok in this sequel of defending the universe of chilled water systems we need to take a deep breath…ala Obi-Wan Kenobi, "must - use - The Force." Let’s start off by stating the obvious, there is no shortage of articles describing the risks of using chilled water and radiant cooling system and how these systems cause condensation. It’s like those with a hidden agenda from a Star Wars plot 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 whisper in my famously subdued Yoda voice – WTF (i.e., Where’s the Facts)?!!  

For cynics of chilled water and radiant cooling system here is a serving of universal logic; 100% of all condensation problems in buildings conditioned exclusively with refrigerated air, did not have a chilled water radiant cooling system to blame. That would be all, as in every one of them with moisture problems could not be tied back to a chilled water system. Stay with me on this...

I’m going to get to the DNA of the problem shortly but let me first ask a rhetorical question; why do we not hear those Darth Vader voices from the “refrigerated air only” camp draw our attention to an equivalent amount of cynicism to the well-known moisture problems on ducts, insulation, grills, registers, steel decking, and drywall and T-bar ceilings? Standing as judge and jury over chilled water system while failing to give equal time to the plethora of microbial and building material problems associated with refrigerated air based systems seems to be a common characteristic amongst the Rebel Forces. So follow along, it gets better. You see 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. Why you ask? Well good designers and installers spend hours evaluating moisture loads and assembling building and HVAC components properly so sweating doesn’t 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 (wait for it) evidently only bad designers and bad installers are permitted to work on these systems. That’s dark side thinkin’ don’t ya think? 

It’s time for the brain washing to stop - 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 much to the anguish of the Rebel Forces, moisture holds no discrimination for HVAC system types. Just for effect, consider statistically one is far more likely to find moisture problems in refrigerated air systems than chilled water radiant systems simply due to the installation ratio of air over water so put your light sabres away and let’s get real about the real issue. Ok…deep breath (again)…must return to a state of peace…Namaste. 

Here are six reasons why condensation with chilled water and radiant cooling panels is a straw man argument so read carefully; 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. Aside from discussions around pipe insulation and building tightness, if you control for these six things then condensation becomes a moot point. It also begs to ask, if designers and installers of all HVAC system types are not focusing on these far more important elements, what exactly are their systems based on? Let’s look at each of these conditions: 

1. Biological Concerns

Without moisture control numerous biological risk develop which support the growth of bacteria, viruses, fungi (moulds/molds) and mites. According to a ASHRAE Transaction, “Human Exposure to Humidity in Occupied Buildings”[i] and the ASHRAE Handbooks, humidity at less than 30% or more than 60% 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% RH." You’ll 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% and 60% can significantly decrease healthcare associated infections.”[ii]  In addition to ASHRAE, the EPA, and NBL this position is supported by numerous authoritative organization including the Canadian Centre for Occupational Health & Safety, Health Canada and ACCA, The Indoor Environment & Energy Efficiency Association.

Figure 1. Controlling moisture for biologicals.


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% relative humidity (RH) to 79F (26C) and 60% RH would result in two to fourfold increases in formaldehyde concentration for the same air change rate. Hodgson et al, stated in their study on the topic, "This suggests that indoor humidity has a substantial impact on formaldehyde emission rates and concentrations." [iv]

Figure 2. Controlling moisture for hydrolysis.

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 Guide [v]and Forest Products Laboratory’s (U.S. Department of Agriculture/Forest Services) Wood Handbook[vi] provide for the ideal "in service" wood moisture content as between 6% and 14%. This ideal in-service range corresponds to relative humidity between 40%+/-10% and 60%+/-10% at temperatures typical for space heating and cooling.

Figure 3a. Controlling moisture for dimensional stability.

Figure 3b. Controlling moisture for dimensional stability.

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% RH and 35% 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 50% while attempting to minimize dramatic swings to between 40-60%, even if broad seasonal trends are hard to avoid."[viii]

Figure 4. Controlling moisture for preservation of artifacts.

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%, corresponds to space conditions of 20% to 60% 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% increase in people dissatisfied going from approximately 40% RH to 65% RH; and an additional 10% dissatisfaction going from 65% RH to 80% RH.[ix], [x]

Figure 5. Controlling moisture for respiratory comfort.

6. Thermal Comfort

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

Figure 6. Controlling moisture for thermal comfort.

Final Thoughts

So now that you have 60% RH as recommended maximum in your intellect do you know what the sea level dew point is at say a 75Fdb? It is 60F. Do you know what the lowest you should go for say a radiant floor cooling system should be? It is 66F. That would be a 6F safety margin far more than required by good engineering practice. Condensation on the floor, ya right – give me a break! For those developing additional arguments in your heads see the bibliography and additional resources.

Alright, if you’ve made it this far and if you have any of Princess Leia’s DNA in you, then you should be concluding there is zero logic in stating unequivocally, "don't use chilled water and radiant cooling systems because of moisture concerns". It’s a silly statement. You must provide moisture control regardless of the HVAC type for the six very good reasons outlined above.[xiii] With chilled water systems the control of space moisture is done with a dedicated outdoor air system (DOAS). 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]

For those of you whose brains are heading in the direction of, “yes but then you need a hybrid with two system, one for ventilation and one for cooling”. This is actually a good thing as 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. If mechanical designers, builders and HVAC installers don’t focus on this fact, then they shouldn’t be doing cooling systems.

So Luke, 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.

End of story.


[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 <>   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. <>  accessed Jan. 2016

[v] Wood Frame Envelopes: Best Practice Guide. Canada Mortgage and Housing Corporation. 1999. < > accessed Jan. 2016

[vi] Wood Handbook. Forest Products Laboratory. U.S. Department of Agriculture/Forest Services)  accessed Jan. 2016

[ix] Toftum, J., Jrgensen, 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.

[xv] Sastry, G., Rumsey, P. 2014.  VAV-vs- Radiant-Side-By-Side-Comparison, ASHRAE Journal, vol. 56, no. 5, May 2014. <>  

Additional Resources:

Bean, R. Part 2 Together Forever HPAC Magazine, March, 2012. <>

Bean, R. 2013. Trs Bien for large scale radiant cooling. HPAC Magazine Canada, Sept. 2012. < >

Bean, R. 2013. Radiant Cooling for Sceptics: How to do radiant cooling in high humidity geographies

Bean, R. 2014. Why And How To Do Radiant Cooling. HPAC Magazine Canada, Feb. 2014. <>    



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



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