Appendix 5E: Underfloor displacement ventilation and reinventing the speculative office building

Conventional commercial buildings use huge fans to blow chilled, dried, and partly fresh air vigorously down from ceiling grilles so that it mixes with the warm, stale air. The mixture is then sucked out again through other "return" grilles in another part of each room and ducted away to be partly discharged, partly remixed with fresh air, rechilled, and recirculated as new "supply" air. The big ducts carrying the supply and return air are hidden in a "drop ceiling"—typically acoustic tiles suspended on a steel frame at least a foot and a half below the lowest point of the structure for the floor above. The drop ceiling also contains recessed "troffers"—metal boxes whose fluorescent lamps shine downwards, usually through glare-reducing louvers. Since the carpet is laid directly over the floor slab, the drop ceiling is the only place to hide the power and telecommunications wiring. All the wiring is therefore hidden above the drop ceiling in cable trays and conduits that snake over or under, between and around, the ducts. Because the wiring, ducts, pipes, and other things hidden in the ceiling are installed at different times by different trades, they are usually layered, further increasing the required height.

Whenever the office is rearranged, electricians must climb up, open up the ceiling tiles, and move much of the wiring and often the lights. For major moves, sheetmetal-workers and mechanical engineers must also relocate the air grilles, readjust the ventilation and cooling controls, and sometimes even supplement or move the ducts to keep the air distribution from getting too much worse. These are extremely costly and disruptive operations. Major moves involving more than one floor may also require punching holes through the concrete slabs to connect cabling between floors. It's hard to make provision for this in advance, because you can't predict where it'll be needed and because the wiring has to snake around complicated ductwork. Yet it's a nuisance at best and potentially impossible at worst when a different tenant who occupies the floor above or below you would be disrupted by the hammer-drilling.

Underfloor ventilation was originally developed for computer rooms so their massive cabling could be hidden beneath the floor where it's easiest to get at. Lately, adapted to ordinary offices, underfloor ventilation has achieved the simplicity, refined design, volume production, and functional integration it needs to work better than the traditional drop-ceiling-and-ductwork arrangement, yet cost no more up front. A modern underfloor system lays the carpet (more commonly recyclable carpet tileChapter 5: Building Blocks) not on the floor slab but on a smooth, rugged, solid-feeling modular raised floor. Its height, one foot above the concrete floor slab, is sufficient with careful layout to carry all the supply air and wiring—both power and telecommunications. The wiring uses simple plug-in modules, can easily be rerouted without climbing any ladders, and is instantly accessible just by lifting a floor tile. The modularity and the separate paths taken by air and wiring also permit easy floor-to-floor connection via channels left open earlier when the slabs were poured. This flexibility alone is so valuable that in the first year of occupancy, it saved Owens-Corning $300 per worker per move. The company's "churn rate" was 120% per year; a typical range is 50–150% per year, meaning that the average worker moves his or her desk every 9–24 months. The raised-floor system cut Owens-Corning's operating cost by a staggering $1.35 per square foot per year—equivalent to about three-fourths of an average office building's total energy bill.

The air can be distributed simply by pressurizing the whole floor plenum. The more effective removal of pollutants reduces the required airflow. This is because the air now enters the room in a completely different way called "displacement ventilation"1: as mentioned in Chapter 5, it seeps gently up at each workstation, under the individual worker's direct control—a highly valued attribute. The local diffusers are very easy to relocate if the space is rearranged. The fresh air "floods" the entire room from the bottom up, with no "stagnation" (unventilated spots). Drafts are eliminated. As the air gains heat from people and equipment, it rises and smoothly pushes the dirty air up to the ceiling, where small return ducts, typically hidden near the walls, or a very shallow ceiling plenum remove it. Replacing stale with fresh and warm with cool air by displacing the one with the other, like the action of a piston, is far more efficient for both energy and air quality than mixing them together. That's why displacement ventilation in Scandinavia has a 25% market share in commercial and 50% in industrial space.

Displacement ventilation has further advantages that save even more airflow. Cooling is more effective because warm objects, like bodies or computers, induce a chimney-like "stack effect" of heated air that convects up alongside them, flowing smoothly upward to remove heat and contamination. This concentrates the cooling air where it's most needed, and as more people arrive in the morning and more equipment gets turned on, the stack-effect cooling automatically keeps pace. Because air drawn off the ceiling has lingered near or around the warm lights, the exhaust air leaving the building carries more heat away with it than exhaust air from an induction system that turbulently mixes the air, diluting warm with cool. The supply air enters the room at 65 rather than the usual 55°F—avoiding chilled ankles and saving chiller energy.2 The higher supply temperature also expands the hours of the year in which an economizer can draw in cool outside air and turn off the chiller. Air-handling friction drops by two-thirds, saving fan sizing, capital cost, and operating energy and virtually eliminating fan noise.

So far so good: the raised-floor system can cost the same, its displacement ventilation cools and cleans the air better, energy use and noise greatly decrease, and the accessibility of the wiring greatly reduces relocation costs and improves flexibility. But what else happens? First, since there is no longer any ductwork within the drop ceiling, and there may well be no drop ceiling at all, its 14–18" duct height and 1.5–2" wiring height are replaced by that of a 12" raised floor that combines all these functions and saves a vertical distance of 2–8" per floor. Second, the ceiling can be as simple as white paint, acoustic tile, or any other desired finish applied to the underside of the concrete floor-slab above. A Class A space will normally still have a drop ceiling, but it can come to barely below the bottom of the structure—perhaps 1.5–2" below to accommodate sprinkler heads and pipes sharing the same height layer as the pendant-light wiring—rather than at least 18" lower.

This in turn makes it possible to replace the troffer downlights in the former drop ceiling with direct/indirect pendant lighting fixtures suspended from the ceiling. By throwing most of their light upwards, the pendant fixtures create a light and airy feeling, far nicer than an oppressively dark ceiling punctuated by strips of harshly glaring downlights. The indirect light virtually eliminates veiling reflections and hence lets you see as well with about a third as much light. Indirect, ceiling-washing light is also easily mixed with natural light bounced upward by small lightshelves at the perimeter windows; those lightshelves shield perimeter spaces from direct glare, and even out the distribution of light in the space. The more such natural light sweeps across the ceiling, too, the more the electric lights can be automatically dimmed, saving energy, reducing cooling loads, and extending lamp and ballast lifetimes. And of course the indirect lighting eliminates another 6" of height previously needed to embed the downlight troffers in the dropped ceiling, but now used instead to raise the ceiling by 6" so as to distribute the light much more evenly.

Now we've added better, cooler, more attractive and effective lighting to the menu. Moreover, now that pendant fixtures are being competitively mass-produced—they were a costly specialty item only a few years ago—they're now competitively priced. But fewer of them are needed, too, because by throwing their light upwards rather than downwards, and onto a higher ceiling, they spread it more evenly over a greater area. Moreover, with good lighting and office-furniture design, including glass-topped partitions for private offices, the light can be distributed so widely that fixtures typically needn't be moved when partitions or furniture beneath them is relocated. That reduces fixture counts, hence capital costs, and saves even more moving costs.

This brings us back to the question of vertical height. Adding up the required heights of all the elements of a conventional office building, including nine-foot ceiling heights, leaves room for not quite six stories without making the building slightly more than 75 feet tall. But that height triggers the Uniform Building Code's high-rise provisions, which require special and quite costly arrangements for elevators, fire protection, and other elements. By saving just a few inches per floor, you can nicely fit six stories into the 75-foot low-rise height limit. This has enormous implications for the ratio of capital cost to net rentable space. Moreover, saving, say, 8-18" of floor-to-floor height (a comfortable maximum if the original design was reasonably thoughtful) on each of six stories saves a full four feet of height in the building. That saving not only means less weight to hold up in the air; it also reduces the area of building skin, which costs around $60–80 per square foot of exterior surface.

This is only the beginning of a strategy that reassembles in a new way the complex four-dimensional (space and time) jigsaw-puzzle of a modern office building. Similar tricks yield further synergies throughout the design. For example, making the structural bays (space between vertical support pillars) the right size can yield strong structures with thin members, saving mainly height, yet maintain very flexible furniture layout, capturing in structural design the advantages of the low-rise design. Now add to all this the concepts of the Chicago office-tower retrofit example: efficient air-conditioning equipment, which in turn feeds less of its own waste heat back into the building to be removed all over again; more efficient lights and office equipment, making all the HVAC equipment smaller; less need to deliver cool air into the space because less heat is being let in through the superwindows or generated by the lights and equipment, hence smaller airflow space in the raised the savings of energy and money do multiply!

To be sure, in a speculative building, built for the open market, you don't know who will occupy the building, so you can't influence in advance the efficiency of the lights and office equipment they'll install. You therefore may not be as able to make the costly air-conditioning and ventilation equipment smaller and cheaper to nearly the extent you could if you're building the structure to suit a specific tenant, whom you can educate in advance and with whom you can share the resulting savings in capital as well as operating cost.3 But even for a speculative office building, this synergy between air-distribution, structural, wiring, lighting, daylighting, superwindow, and office-furniture design can be spectacularly rewarding.

1 This need not be underfloor; some displacement systems use low wall-mounted ducts and diffusers.

2 Displacement ventilation may require more careful attention to dehumidification, but this too can be done extremely efficiently where required.

3 However, ways to influence even speculative tenants are described in Chapter 5/Building Blocks, and there are many technical ways to ensure plenty of flexibility to accommodate even the cooling needs of unexpectedly inefficient tenants.

To make comments or report problems with this site, please contact

© All rights reserved. Published by Rocky Mountain Institute.
2317 Snowmass Creek Road  |  Snowmass, CO 81654-9199  |  Ph: 970.927.3851


Powered by Intrcomm Technology's SMC