FIGURE 1. An example of an UFAD system (Courtesy of the Center for the Built Environment).

As a writer, I like to think that I have a league of admirers out there, like the tortured fans of Stephen King, waiting for my next missive. With that in mind, I was sure many of you had been waiting for me to write about underfloor air distribution systems (UFADs) before you ventured out and did one yourself. This would explain the lack of universal acceptance in the North American market for UFAD to date.

On the other hand, I could be as irrelevant as my wife suggests, and the reason UFAD had not gone mainstream wasn’t because I hadn’t sounded the call, but rather because the information out there on UFAD may be downright muddled and confusing.

In an effort to clarify my perspective, I consulted my fan mail from the last few years, and I was surprised to learn that in fact I had never received any fan mail. This sobering discovery led me to the conclusion that a lack of industry clarity was likely at fault and I, in fact, wasn’t to blame.

So, in an effort to solve both problems (increase my fan mail base and solve the UFAD conundrum) I chose to study the topic anyway and share my findings with all.

Now I think you will agree that there is nothing quite as pretentious as an article with a title that forces you to consult a dictionary. But in an effort to make the UFAD UFO title gimmick work, I had to come up with a subtitle using the letters U, F, and O that conveyed the true essence of my study: That there is a great deal of confusing information out there both pro and con on UFAD.

The “O” word that I settled on was obfuscation, which WordNet® 2.0 defines as:

1: confusion resulting from failure to understand [syn: bewilderment, puzzlement, befuddlement, mystification, bafflement, bemusement] 2: the activity of obscuring people’s understanding, leaving them baffled or bewildered [syn: mystification].

What struck me about the definition was that I believe that the first meaning speaks to the state of the average HVAC practitioner when it comes to UFAD, while the second explanation brings to mind the overt and covert acts of many in our field to sway our collective opinions.

Us vs. Them

Over the span of 2003 and 2005, no fewer than seven feature articles were dedicated to UFAD in the four primary HVAC journals: Engineered Systems, ASHRAE Journal, Consulting-Specifying Engineer, and HPAC Engineering. In all cases, UFAD was discussed in either a neutral or relatively positive light. Two were case studies1,2; one was an objective roundtable discussion3; and the rest were design guides4,5,6,7 either written by or referencing the work of the Center for the Built Environment at the University of California, Berkeley (CBE) (

In the opposite corner during the same period, the Testing Adjusting and Balancing Bureau (TABB) issued two TABB Talk newsletters that included three articles8,9,10 on UFAD. Their tone ranged from academically skeptical (Woods) to downright antagonistic (Webster). In each article the work, of the Building Diagnostic Research Institute (BDRI) ( and the National Energy Management Institute (NEMI)( was cited.

I believe that it is relevant to note that NEMI is the research arm of the Sheet Metal Workers’ International Association (SMWIA) and the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA); two groups who benefit from the installation of ductwork. And, the CBE is funded, in part, by an assortment of equipment and systems manufacturers with a vested interest in the success of UFAD (raised floor producers for example).

I do not want to imply, nor do I believe, that there are nefarious intentions out there. Still, where someone is coming from can almost always enlighten you on where they are trying to take you. And in the case of UFAD, the most vocal and eloquent authorities on the topic are coming from two very different places.

As we dive into the obfuscations, I would like to point out that neither the CBE, the BDRI, nor the NEMI use the blatant generalizations that I present in this article. These groups are always fair in my estimation, and strive to point out that no system is perfect.

The problems come from what people do with the information provided by the two groups, and our natural tendency to oversimplify what we do not completely understand. The old adage that a little knowledge is a dangerous thing applies in spades here.

But now, with the disclaimers out of the way, let’s dive into the muck.

FIGURE 2. An example of an UFAD system (Courtesy of the Building Diagnostic Research Institute).

Con: UFAD Systems Are Uncomfortable

Almost always, the first argument made against UFAD is the cold foot hypothesis. It seems intuitive that colder air at the floor will always lead to cold feet and uncomfortable drafts. But to say that all UFAD systems cause such discomfort is as myopic as stating all conventional overhead (COH) VAV systems have problems with dumping at low flow. Improper selection and location of diffusers is at the root of both problems.

First, the air in a UFAD system is delivered at or around 65°F (18°C). So relative to the 55° (13°C) we are used to dealing with (e.g., dumping), it is not as big a problem. But it can be if the diffusers are placed incorrectly. That is why it is recommended to provide a 3- to 6-ft diameter clearance zone at each diffuser or terminal unit.

This concern also relates to another common assumption that all UFAD systems are displacement ventilation (DV) systems, and that in turn you have a fully stratified environment from floor to ceiling. In fact, DV systems should only be used in cooling-only applications and are not the norm for most UFAD systems.

In UFAD, the space is partially stratified, but fully mixed in the occupied zone. In turn, the temperature at the floor is not as cold as you might have thought. This is accomplished using swirl diffusers. The name elicits visions of billowing skirts, but in fact, the velocities are relatively low. However, if the diffuser is placed too close to a workstation, it will lead to discomfort.

Pro: UFAD Systems Use Less Energy

Theoretically, in the right climate and under the right conditions, a UFAD system may use less energy than a COH system. Conventional wisdom is that you use less energy because of a reduction in fan hp due to less pressure drop; an extended airside economizer window because of the higher supply air temperature; and a reduction in space and plant load due to stratification and higher average space temperature.

Where do we start? The assumption that you use less fan energy requires that you also assume you have a less restrictive distribution system, or that the space requires no more and possibly less air than a COH system, or both. And, one has to believe all the air being delivered gets to where you want it.

Well, not all UFAD systems are unducted. And a strong argument exists to consider ducting (see discussion below) so the system may or may not have less static pressure. Also, because the supply air temperature is higher than with a conventional system you may actually have to deliver more air, not less. Of course, the stratification and higher average temperature are working in your favor, but the point is it isn’t a gimme.

Which leads us to leakage. One of the best arguments against unducted UFAD systems is their propensity to leak around the raised floor tiles, through openings in between spaces, and even up into walls. Proponents of UFAD will counter that ducted systems leak too, but honestly, that’s a weak comeback.

The reality is when you use the space below the floor as a supply plenum, you have just increased the surface for leakage exponentially. Further, you are now depending on trades beyond Division 15 to be part of your air distribution system quality control team. Of course, details exist and precedents have been set across the country where building teams have worked together. But by and large, leakage is a common complaint and a common contributor to higher energy use than originally anticipated.

Regarding the economizer window, if you are in a part of the country where airside economizers are of marginal value (almost everywhere, in this author’s opinion), or if you have to dehumidify (don’t we all?), the potential for savings is minimal. And speaking of humidity, that leads us to another argued con.

Con: UFAD Systems Cannot Handle Latent Loads And Contribute To Mold Growth

I find this argument startling. I am asked from time to time to do a little forensic engineering, and more than once I have come across mold and mildew problems either created by or exacerbated by the HVAC design. None of these systems has employed UFAD. But what they all shared in common was a shoddy design: the product of the designer’s dehumidification and building pressurization ignorance.

I believe that the knee jerk reaction to UFAD is you can’t dehumidify with 65° (18°C) air. Yes, you can’t dehumidify with a 65° (18°C) wetbulb. But you can sure as shootin’ dehumidify with a 65° drybulb if the dewpoint is sufficiently low. And the same holds true for a COH at 55° (13°C) drybulb. There will always be situations where you need to drive the dewpoint down lower to deal with a high internal latent load or a tight space constraint.

But the counter to this argument is usually something like “Aha, UFAD flunky, you then must require simultaneous heating and cooling more often than not, which violates all that is holy … and ASHRAE 90.1!” At that point, if I were you, I would pull out my psych chart (don’t you carry yours with you at all times?) and walk them through a return air bypass scheme or a runaround heat pipe scenario, or I would extol the virtues of heat recovery.

The mold argument stems from the reality that if any surface in the plenum drops below the dewpoint of the supply air (the slab for example), there will be condensate and a potential for mold growth. This isn’t a problem unique to UFAD, but it is made worse in an unducted system given the large surface area of the structural slab and the sheer size of the potential problem. The solution is the same though for any system: Provide sufficiently dry air and insulate appropriately.

Pro: UFAD Guarantees Leed® Points

I cringe when I pick up an architectural journal and see a discussion of some LEED® project written by an interior designer or some other equally unqualified purveyor of HVAC wisdom. Since many LEED buildings employ UFAD, and to the uneducated it is so obviously different than what they are used to, it is often cited as one of, if not the biggest, contributors to the low anticipated energy usage.

Of course, the increased performance is probably due to a higher performance chiller, intelligent controls strategies, or any combination of various progressive tactics. But the only thing that architect can see or understand is that darned raised floor.

Under Version 2.1, the Increase Ventilation Effectiveness credit under the Indoor Environmental Quality section of LEED was often cited as the reason for going with UFAD. But a properly designed COH system and a typical UFAD system employing a partially stratified approach will both earn a ventilation effectiveness (Ez) of 1.0 per ASHRAE 62-2004, Table 6-2.

Only a true displacement system with full stratification meets the criteria that garners the holy grail of an Ez greater than 1. But note that things change depending on whether you are in a heating or cooling mode. Ez equals 1.0 for ceiling supply/ceiling return or floor supply/ceiling return of cool air. But Ez falls to 0.8 for the COH system if you are heating, and 0.7 for the floor system.

That means if you are heating with a floor supply/ceiling return UFAD design, then you would actually be required to bring in more outdoor air during the heating season than with the COH! Ouch! That can’t help.

Where Is ASHRAE?

In my efforts to glean the truth about UFAD, I found myself wondering where ASHRAE came down on the topic. Obviously, ASHRAE does not endorse systems, but it does implicitly sanction their use and development by issuing appropriate design guides and printing related peer-reviewed articles in the ASHRAE Journal.

With that in mind, you might charge ASHRAE with a sort of guilt by association due to the fact that the Underfloor Air Distribution (UFAD) Design Guide, recently commissioned by ASHRAE, was written in large measure by Fred Baumann of the CBE. Also, the ASHRAE GreenGuide proposes that a DV system, via UFAD or otherwise, is a viable green strategy.

However, it bears noting that the much touted Advanced Energy Design Guide for Small Office Buildings, makes no mention of UFAD, and in fact, it could be argued that it excludes UFAD (the unducted plenum version at least) by recommending fully ducted and pressure tested supply and return air systems (strategies HV9 and HV11 respectively).

And last, while not the official line of ASHRAE, or the Ottawa Valley Chapter, or his employer, Soheil Rastan offers one of the best essays I have come across questioning UFAD and unducted systems in particular. I highly recommend giving it a read at the Ottawa chapter’s website.12

In Conclusion

In the end, UFAD really isn’t that different from any other type of system, be it airside or waterside. You have to approach it carefully and understand the unique challenges. But when you understand the problems, the solutions become obvious.

While I have focused on the exaggerations and generalizations, there are some issues that are truly unique to UFAD, and I have tried to identify them in the sidebar. I do not propose that I have listed all related issues, but I believe these are in the top tier.

My conclusions are not that different from the NEMI, BDRI, and the CBE: Good design equals good systems. In fact, look at Figures 1 and 2 from the CBE and NEMI/BDRI respectively. Although the BDRI example is specific to the GSA and offers more explicit direction regarding building pressurization and ventilation, the UFAD element is not that different. One does not preclude the other.

So why use my suggestions instead of others? Well, first of all, don’t. Use them in conjunction with the other articles and guides I have referenced. But perhaps my perspective as a designer vs. that of a researcher or a manufacturer can help you triangulate to a position you and your clients can be comfortable with. In the end, I just hope that in advancing the discussion I have helped advance the science.

And if you think I have, you can send your fan mail to the address listed below.


ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality.

American Society of Heating, Refrigerating and Air-Conditioning Engineers, ASHRAE GreenGuide, Atlanta: 2003.

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Advanced Energy Design Guide for Small Office Buildings, Atlanta: 2004.

Baumann, F. Underfloor Air Distribution (UFAD) Design Guide, American Society of Heating, Refrigerating and Air-Conditioning Engineers., Atlanta: 2003.

Baumann, F. “Letters/Readers Feed Back – The Real Facts on UFAD,” Consulting-Specifying Engineer, 2004: 37(1): 7-8 and 73.

Sidebar: UFAD FAQ

Many issues that are raised when discussing UFAD actually apply to COH systems as well. Often these supposed benefits or detriments are simply about good or bad design regardless of whether it’s over your head or under your feet. Others are truly unique to UFAD and should be considered accordingly.

Common Challenges/Opportunities
  • Ventilation efficiency: Both UFAD and COH systems can meet the criteria for ventilation effectiveness. Only pure displacement ventilation systems provide an advantage and only in a cooling application.
  • Energy use: There are well designed and installed UFAD and COH systems, as well as bad. Energy efficiency is not unique to either, it’s just that different strategies need to be employed. The life-cycle cost argument is similar.
  • LEED®: Both systems can be part of an effective LEED strategy.
  • Comfort. A well thought out system can avoid cold feet and drafts, or in the case of COH, dumping. Conversely, a bad design can cause a problem either way.
  • Floor-to-floor heights: This is probably a better question for the architect. Some argue that only having to accommodate one plenum reduces floor-to-floor. Others suggest that since the ceiling has to accommodate sprinklers, lights, etc., regardless, that unless you nix the ceiling altogether (not common), you have added height by adding a second plenum.
  • First cost: Depending on the comfort level of the local design and construction community regarding UFAD, and the level of systems integration achieved in the design (i.e. plenum used efficiently, floor-to-floor optimized, etc.) UFAD can be either a boon or a bust.
  • Latent cooling and dehumidification: Differing paths to the same conclusion: Dry air, controlled building pressure, and an intelligent application of insulation.

Challenges Unique To UFAD
  • Unfamiliarity: Even with decades of use in Europe and advances here at home, there is no getting around the fact that there is not a significant track record in the North American market. This relates to contractors, their bids, and quality control. Operators must learn different strategies and overcome their own paradigms. And code officials will often be creating policy on the fly. With limited precedent comes limited confidence.
  • Leakage: If you commit to an unducted system, you are in fact committing to one huge duct: the floor plenum. Strategies are being developed, but lessons are still being learned. The penalty for leakage is lack of comfort, lack of control, and higher energy use.
  • The dirty plenum perception: While some argue that the velocities are not great enough to carry dirt, the conventional wisdom is that the floor and especially the space beneath it are dirtier than a duct. Whether you solve this with ductwork, filtration, or a heart-to-heart conversation, the perceived threat must be addressed with those concerned.
  • Negative press: No matter how enthusiastic you may be, it is very likely that someone along the way is going to throw a flag because of something that they have heard or read. Be prepared to counter broad attacks with specific solutions.

Opportunities Unique To UFAD
  • Improved IAQ in the breathing zone: While there are those who question the validity of this claim, the preponderance of the evidence suggests that supplying fresh air low and returning it high carries contaminates away from the breathing zone, hence increasing IAQ.
  • Occupant satisfaction: If properly applied, having the conditioned air where you need it, and providing some level of personal accessible control, like a diffuser incorporated into their workstation furniture or simply an adjustable diffuser nearby, increases the relative and perceived comfort of the enduser.
  • Stratification: Especially in taller spaces, accounting for an increase in the average space temperature and being able to handle the load from the lights and roof at the plant instead of at the space, does present more energy saving opportunities than with a fully mixed system.
  • Advancing the science of HVAC: While none of us wants to be a pioneer at the expense of a client’s comfort and satisfaction, each UFAD project will offer more information and broaden our community’s design options. Just like VAV before it, UFAD will eventually become a common design alternative. You can wait until it’s safe to go in the water, or you can jump in and start swimming with the underfloor sharks.