Healthy Buildings- Air Plenums

The role of underfloor air plenums in ‘healthy buildings’


We spend around 90 percent of our working life indoors, where pollutant levels are often higher than those outside or at home. It follows that the quality of indoor air in offices is a key consideration in new commercial construction projects worldwide.
A healthy building is designed to improve and protect occupant wellbeing. Healthy building design doesn’t just look at minimising negative impacts on the environment, but instead looks to maximise the positive health and wellbeing impacts that buildings can have on occupants.
Underfloor air plenums
In its simplest form, an underfloor air plenum is the void between the floor structure of the building and a raised access floor, which can be adapted in order to distribute conditioned air to the spaces above. Underfloor air plenums are frequently used in office buildings, particularly highly-reconfigurable and open plan offices with raised access floors, but why?
There are a number of advantages for architects and specifiers when choosing a raised access floor incorporating an underfloor air plenum:

  • Improved ventilation efficiency and indoor air quality by delivering fresh supply air at floor level or near the occupant
  • Reduced energy use with cooling energy savings of up to 20%
  • Significantly reduced life-cycle building costs due to reduced expenses associated with occupant ‘churn’ or changing interiors
  • Reduced floor-to-floor height in new construction by reducing the overall height of the service plenum
  • Improved productivity and health of occupants
Air plenum diagram final

Example Access Flooring System

1.  Penetrations through the access floor, walls and subfloor including:
1a.  Cable bundles and cable trays
1b.  Pipes
1c.  Fire/plenum barriers
2.  Cable trunking – must be internally sealed within the void
3.  Masonry work - incomplete or poorly jointed walls will result in greater air leakage
4.  Risers need to be properly sealed for plenums/ducting
5.  Plasterboard on studs at board edges and the ends below the raised floor level
6.  Gaps between compartment barriers, top of raised access floor and sub floor respectively
7.  Gaps between any curtain walling/glazing

The importance of controlling air leakage
BSRIA, who provide specialist services in construction and building services, recently updated their performance guidelines, with revised upper limits on the leakage of underfloor plenums that are used for the supply of air ventilation. BSRIA Document Number 65/2016 revised criteria are as follows:

  • Raised access floor leakage criterion is 1litre/s/m2 @50Pa.
  • Plenum (void) criterion recommended is 0.7litre/s/m2 @50Pa.

Air plenum integrity is one of the most important aspects of designing, constructing and maintaining an underfloor air delivery (UFAD) system. It is vital that the entire design and construction team does their part to ensure the underfloor plenum is sealed properly. As BSRIA themselves suggest, “Air leakage from floor plenums can be an energy efficiency issue and could result in floor-mounted diffusers not operating correctly, which could cause discomfort to occupants.”
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