Underfloor air distribution, usually called UFAD, delivers conditioned air through a pressurized plenum under a raised access floor and into the space through floor diffusers. Air rises through the occupied zone, picks up heat and contaminants, and is returned at or near the ceiling. Most UFAD systems are used in offices, though they also appear in some institutional and educational buildings.
UFAD can provide good comfort, flexibility for changing layouts and some energy benefits. It also has a reputation for problems when the details are not handled carefully. This guide covers how the system works, what drives its performance, and how to avoid the common pitfalls.
How UFAD works
A raised access floor, typically 300 to 450 mm (12 to 18 in.) deep, creates a plenum above the structural slab. An air handler supplies air into this plenum, either through ducts that discharge into the plenum or directly through floor openings near the shaft. The plenum is kept at a low positive pressure, typically around 12 to 25 Pa (0.05 to 0.1 in. w.c.), and air leaves through floor diffusers.
Unlike overhead mixing systems, UFAD aims for partial stratification. Supply air at a moderate temperature fills the lower part of the room, and warmer air collects above the occupied zone, where it is returned. The occupied zone is conditioned, but the upper part of the room is allowed to run warmer.
UFAD sits between fully mixed overhead systems and displacement ventilation. Floor diffusers introduce air with more velocity than displacement outlets, so there is more mixing near the floor, but heat from lights and other overhead sources is still largely kept above the occupied zone. See displacement ventilation in commercial spaces for the related approach.
Benefits
- Layout flexibility: floor diffusers can be relocated when furniture or partitions move, and the raised floor carries power and data cabling.
- Higher supply air temperature: supply air at around 16 to 18 C (61 to 64 F) instead of 13 C (55 F) extends economizer hours and can improve chiller efficiency.
- Lower fan pressure: the plenum replaces much of the supply ductwork.
- Potential ventilation benefit: with good stratification, the air in the breathing zone can be fresher than in a fully mixed space. ASHRAE Standard 62.1 assigns zone air distribution effectiveness values depending on supply method and stratification.
- Individual control: occupants can sometimes adjust nearby diffusers.
Risks and common problems
| Problem | Cause | Mitigation |
|---|---|---|
| Plenum air leakage | Gaps at floor panel joints, walls, columns, cable cutouts and shafts | Specify leakage targets, seal the plenum, test it before floor finishes |
| Thermal decay | Supply air warms as it travels through the plenum, gaining heat from the slab and the floor | Limit plenum run lengths, use ducts or air highways to far zones |
| Perimeter zone difficulty | High heating and cooling loads near glass | Use dedicated perimeter units, fan-powered terminals or perimeter-specific solutions |
| Cold feet complaints | Supply air too cold or diffusers too close to seated occupants | Keep supply temperature in the recommended range; locate diffusers away from desks |
| Condensation in plenum | Humid air contacting a cold slab | Control supply dew point; manage slab temperature during startup |
| Poor stratification | Too much airflow or too many high-throw diffusers | Select diffusers and airflow for the intended stratification |
Plenum leakage
Leakage is the most frequent cause of disappointing UFAD performance. Air escaping through floor joints, wall bases and service penetrations bypasses the diffusers. Some leakage into the occupied space still does useful work, but leakage into walls, shafts, cores and adjacent floors is wasted.
Sealing must be designed and specified, not left to chance. Typical measures include gasketed floor panels, sealed perimeter details, sealed column and wall penetrations, and fire-stopping at shafts. Leakage testing of the plenum before carpet goes down lets defects be fixed while they are still accessible.
Thermal decay
Air moving through the plenum picks up heat from the structural slab (which has absorbed heat from the space below or from the previous day) and from the floor panels above. Over long plenum runs, supply air can warm by several degrees, leaving distant zones short of cooling. Solutions include ducting air closer to remote zones, limiting the maximum free path through the plenum, and accounting for decay in the load calculation.
Key design parameters
- Supply air temperature: commonly 16 to 18 C (61 to 64 F) at the diffuser. Colder air risks discomfort at ankle level.
- Plenum pressure: typically 12 to 25 Pa (0.05 to 0.1 in. w.c.). Higher pressure increases leakage.
- Floor height: enough for air, cabling and any ducts, typically 300 to 450 mm (12 to 18 in.).
- Diffuser type: swirl diffusers for interior zones, linear grilles at the perimeter, with or without variable-volume dampers.
- Return location: at or near ceiling height to take advantage of stratification.
- Thermostat location: within the occupied zone at about 1.1 to 1.5 m (43 to 60 in.) height, not near diffuser jets.
Load calculation differences
UFAD load calculations differ from mixed systems because part of the heat gain stays in the upper zone and goes directly to the return. ASHRAE-sponsored research and the ASHRAE UFAD design guide provide methods that split loads between the occupied zone, the upper zone and the plenum. Using a standard mixed-air load calculation without these adjustments can mis-size airflow. Energy modeling software with UFAD capabilities, such as EnergyPlus, can account for stratification and plenum heat transfer; see building energy modeling for HVAC design.
Perimeter zones in Canadian climates
In much of Canada, perimeter heat loss in winter is significant. Floor diffusers can deliver warm air, but warm air rising from the floor along glazing is effective only if the envelope is good. Common approaches include:
- Fan-powered terminal units in the plenum with heating coils, feeding perimeter linear grilles.
- Perimeter hydronic radiation or radiant panels with UFAD serving interior zones.
- Separate perimeter air handlers in large buildings.
High-performance glazing reduces perimeter loads and makes UFAD perimeter design easier.
Commissioning considerations
UFAD systems benefit from careful commissioning:
- Plenum leakage testing before finishes.
- Verification of plenum pressure control across each zone.
- Temperature stratification measurements at several heights.
- Diffuser airflow checks and confirmation of perimeter unit operation.
- Seasonal testing in both heating and cooling.
See our building commissioning service for the overall process.
Frequently asked questions
Does UFAD save energy?
It can, mainly through extended economizer operation, warmer chilled water and lower fan pressure. Savings depend heavily on plenum leakage, thermal decay and perimeter design, and are best evaluated with an energy model.
Is UFAD suitable for retrofit?
Usually only when a raised floor is already present or floor-to-floor height allows one. It is mainly a new construction or major renovation option.
Key takeaways
- UFAD supplies air through a pressurized raised floor plenum and relies on partial stratification.
- Warmer supply air and lower fan pressure can improve energy performance.
- Plenum leakage and thermal decay are the main risks and must be designed out.
- Perimeter zones in Canadian climates usually need dedicated solutions.
- Use UFAD-specific load and energy methods, and commission with leakage and stratification tests.
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