UFAD Technology Benefits
UFAD Technology Needs
Underfloor air distribution (UFAD) is a method of delivering space conditioning
in offices and other commercial buildings that is increasingly being considered
as a serious alternative to conventional ceiling-based air distribution systems
because of the significant benefits that it can provide. This technology uses
the open space (underfloor plenum) between the structural concrete slab and the
underside of a raised access floor system to deliver conditioned air directly
into the occupied zone of the building. Air can be delivered through a variety
of supply outlets located at floor level (most common), or as part of the
furniture and partitions. UFAD systems have several potential advantages over
traditional overhead systems, including improved thermal comfort, improved
indoor air quality, and reduced energy use. By combining a building's heating,
ventilating, and air-conditioning (HVAC) system with all major power, voice,
and data cabling into one easily accessible service plenum under the raised
floor, significant improvements can be realized in terms of increased
flexibility and reduced costs associated with reconfiguring building services.
These raised floor systems are particularly appropriate for office buildings
housing today's businesses with their typically extensive use of information
technologies and high churn rates.
Originally introduced in the 1950s in spaces having high heat loads (e.g.,
computer rooms, control centers, and laboratories), and subsequently introduced
in office buildings in the 1970s, UFAD systems have achieved considerable
acceptance in Europe, South Africa and Japan for more than a decade. However,
growth in North America has been relatively slow up until the last few years.
As with any new and unfamiliar technology, resistance to wider use is driven by
the perceived higher risk to designers and building owners, a lack of available
information in the form of standardized design guidelines, and in the case of
underfloor air, the perceived higher first costs of raised flooring. In
response to this need for more information, the Center for the Built
Environment (CBE) developed this web site to provide a complete and unbiased
description of underfloor air distribution and related technologies.
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For purposes of introducing the concept of an underfloor air distribution
system, it is instructive to identify how these systems differ from
conventional ceiling-based air distribution systems. Figures 1 and 2 show
schematic diagrams of an overhead system and an UFAD system, respectively, for
a cooling application in an open-plan office building. Some of the most
important advantages of underfloor systems over ceiling-based systems occur for
cooling conditions, which are required year-round in the vast majority of
interior office space in many parts of the United States.
Historically, the approach to HVAC design in commercial buildings has been to
supply conditioned air through extensive duct networks to an array of diffusers
spaced evenly in the ceiling. As shown in Figure 1, conditioned air is both
supplied and returned at ceiling level. Ceiling plenums are typically quite
large to accommodate the large supply ducts that must fit through them. Return
air is most commonly configured as an un-ducted ceiling plenum return. Often
referred to as mixing-type air distribution, conventional HVAC systems are
designed to promote complete mixing of supply air with room air, thereby
maintaining the entire volume of air in the space (floor-to-ceiling) at the
desired setpoint temperature and ensuring that an adequate supply of fresh
outside air is delivered to the building occupants. This control strategy
provides no opportunity to accommodate different thermal preferences among the
building occupants or to provide preferential ventilation in the occupied zone.
Figure 1. Conventional overhead air distribution system
With UFAD systems, conditioned air from the air handling unit (AHU) is ducted
into the underfloor plenum where it typically flows freely to the supply
outlets. Underfloor systems are generally configured to have a relatively large
number of smaller supply outlets, many in close proximity to the building
occupants, as compared to a conventional overhead system. Outlets may be floor
diffusers, as shown in Figure 2, or, particularly when part of a task/ambient
conditioning (TAC) system, desktop or partition outlets equipped with
individual control. If the outlets are adjustable, this arrangement provides an
opportunity for nearby occupants to have some amount of control over thermal
comfort conditions in their local environment. Air is returned from the room at
ceiling level (un-ducted plenum return is shown). This produces an overall
floor-to-ceiling air flow pattern that takes advantage of the natural buoyancy
produced by heat sources in the office and more efficiently removes heat loads
and contaminants from the space, particularly for cooling applications. In
contrast to the well-mixed room air conditions of the conventional overhead
system, stratification is actually encouraged above head height where increased
temperatures and higher levels of pollutants will not affect the occupants.
Figure 2. Underfloor air distribution (UFAD) system
Although not shown in Figure 2, there are three basic approaches to configuring
the supply-air side of an UFAD system:
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- pressurized underfloor plenum with a central air handler delivering air
through the plenum and into the space through passive grills/diffusers;
- zero-pressure plenum with air delivered to the space through local
fan-driven supply outlets in combination with the central air handler; and
- in some arrangements the supply air is ducted through the underfloor
plenum to the supply outlets, although in this last configuration certain
energy and cost benefits may be reduced compared to the first two
UFAD Technology Benefits
UFAD systems have several potential advantages over traditional ceiling-based
air distribution systems. Well-engineered systems can provide:
These advantages will be realized only if UFAD technology is appropriately
designed and applied.
- Improved thermal comfort. By allowing individual workers to have some
amount of control over their local thermal environment, individual comfort
preferences can be accommodated.
- Improved ventilation efficiency and indoor air quality. Some improvement
in indoor air quality can be achieved by delivering the fresh supply air
near the occupant at floor or desktop level, allowing an overall
floor-to-ceiling air flow pattern to more efficiently remove contaminants
from the occupied zone of the space.
- Reduced energy use. Energy use can be reduced through a variety of
strategies including controlled thermal stratification, higher supply air
temperatures, and reduced static pressures in the underfloor plenum.
- Reduced life cycle building costs. Raised access flooring provides maximum
flexibility and significantly lower costs associated with reconfiguring
- Reduced floor-to-floor height in new construction. UFAD systems can lead
to reduced overall service plenum heights compared to conventional overhead
systems. A single large overhead plenum to accommodate large supply ducts
(Figure 1) can be replaced with a smaller ceiling plenum for air return
combined with a lower height underfloor plenum for un-ducted air flow and
other building services (Figure 2).
- Improved occupant satisfaction and productivity. Research evidence is
mounting that occupant satisfaction and productivity can be increased by
giving individuals greater control over their local environment.
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UFAD Technology Needs
In spite of the advantages of UFAD systems, there exist some barriers (both
real and perceived) to widespread adoption of this technology. These are
summarized below along with links to additional information on this website.
Go to 'More Info' for a summary of ongoing research by CBE addressing UFAD
technology needs; and 'Design Guidelines' for more information related to
design, construction, and standards and codes.
- New and unfamiliar technology. There has been until recently a lack
of quantitative performance data and systematic design guidelines on this
- Perceived higher costs. The perceived higher cost of UFAD systems
is one of the main reasons that UFAD technology is not used more widely by
the industry today. There are several factors, however, that can make first
costs very competitive with life cycle costs significantly lower than
conventional overhead systems.
- Limited applicability to retrofit construction. The installation of
UFAD systems and the advantages that they offer are most easily achieved in
new construction. The feasibility of adding a raised floor may be restricted
in the renovation of many buildings having limited floor-to-floor heights.
Recent CBE research has found that low-height underfloor plenums (7 inches
[0.18 m] and lower) can provide uniform air flow performance, which may
improve the situation.
- Problems with applicable standards and codes. Since underfloor
technology is relatively new to the building industry, its characteristics
may require consideration of unfamiliar code requirements and, in fact, may
be in conflict with the provisions of some existing standards and codes.
- Limited availability of UFAD products. Although the situation is
beginning to change in recently years, in the U.S. there are only a few
manufacturers offering UFAD products.
- Cold feet and draft discomfort. Underfloor systems are perceived by
some to produce a cold floor, and because of the close proximity of supply
outlets to the occupants, the increased possibility of excessive draft.
These conditions are primarily indicative of a poorly designed or operated
- Problems with spillage and dirt entering underfloor plenum. Concern
is sometimes expressed about the increased probability of spillage and dirt
entering directly into the underfloor supply air stream, and therefore being
more widely distributed throughout the occupied space. Tests have shown,
however, that this is not the case.
- Condensation problems and dehumidification in UFAD systems. In
humid climates, outside air must be properly dehumidified before delivering
supply air to the underfloor plenum where condensation may occur on cool
structural slab surfaces. While humidity control of this sort is not
difficult, given the large surface area of the structural slab in the
underfloor plenum, it is important that it be done correctly.
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As awareness of the advantages of UFAD systems grows within the building
industry, more of these installations will be completed in the coming years. As
this occurs, it will be important to quantify the environmental and
productivity benefits using underfloor technology. Building owners, developers,
and other technology users need this kind of proof of performance
cost-effectiveness (reduced life-cycle building costs) to overcome the barrier
of higher first costs often associated with this and other intelligent building
technologies. CBE will be monitoring selected UFAD projects and posting their observations on this website.
'Case Studies' presents six project profiles and one case study report, as well
as a list of all UFAD building projects CBE is currently compiling
Efforts are now well underway in the following areas to provide more
information and support to the building industry on UFAD systems.
As this and other information becomes available, it is expected that greater
acceptance and well-designed applications of this promising and innovative
technology will occur.
- Design guidelines: In September 1999, CBE initiated a research
project funded by ASHRAE and CBE to develop a design guide on UFAD and
task/ambient conditioning (TAC) systems. The guide, entitled
"Underfloor Air Distribution (UFAD) Design Guide," is complete
(December 2003) and now available from ASHRAE.
- Standards: ASHRAE Standard 113-1990 (Method of Testing for Room Air
Diffusion) is currently being revised to include a new standardized test and
analysis method for evaluating the performance of UFAD and TAC systems. The
recently revised and approved-for-release ASHRAE Standard 55-2004 (Thermal
Environmental Conditions for Human Occupancy) will include some allowance
for changes in the perception of thermal comfort when individuals are given
some control over their local environmental conditions.
- Research: CBE and other institutions are involved in several ongoing
research projects related to UFAD technologies, as previously mentioned and
described in Current Research under 'More Info'.
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