Undulating concrete ceilings at London’s Portcullis House help cool the interior by taking advantage of thermal mass cooling.
“The most consistently depressing thing in U.S. buildings, almost without exception, is the ceilings.”
Mike Taylor doesn't mean any offense by his blunt observation. It's just that the British architect has seen far too many buildings in the States that fail to maximize the full potential of ceilings, either aesthetically or structurally.
Taylor's firm, London-based Hopkins Architects, is known for designing spaces with wildly undulating concrete ceilings that serve as much more than mere architectural expression. When used in conjunction with underfloor air distribution (UFAD), the exposed concrete ceilings provide passive thermal cooling or heating for the interior space, helping to reduce overall energy costs.
“We like this notion that one element of the building can provide multiple functions,” says Taylor, explaining how exposed concrete can serve as the building's structure, finished ceiling surface, light reflector, and thermal mass source for cooling and heating. “We'd rather take the structural element, spend a bit more time, money, and effort on that, and use it as the final finish. You have to put structure in the building anyway—why not get more out of it?”
The firm's design approach for what Taylor calls exposed overhead thermal mass takes advantage of off-peak electricity rates. During summer evening hours, when the building is unoccupied, the UFAD system kicks on and slowly introduces cool air into the interior space, gradually cooling the massive concrete ceilings. This cool energy, in turn, is passively radiated downward into the interior space during the day.
The overhead thermal mass approach can work for both open-plan schemes and enclosed office applications, as shown here at Portcullis House, New Parliamentary Building in London.
“The great thing about locating the thermal mass in the ceilings is that, unlike the floor or the walls, the ceilings are not going to get covered up with furniture, carpet, and people,” says Taylor. “Even if the building owner installs partitions in the space, the thermal energy will still radiate down on the occupants.”
Taylor says that because it takes so long for the energy to transmit through the concrete slab, no more than one inch of exposed concrete is actually cooled and warmed during the average daily cycle. An inch may not sound like much, but energy studies show that even the slightest amount of overhead thermal cooling can have a significant impact on energy costs.
For instance, at Yale University's Kroon Hall, which is set to open in April, energy models show that, during peak summer use, cooling the facility's exposed concrete ceilings at night will result in a 1°C drop in room temperate during daytime occupancy.
“Roughly speaking, that is about 10 watts per square meter of cooling, or about 15% of summer demand just from running the air displacement system at night and cooling that concrete,” says Taylor, whose firm designed the facility in collaboration with Arup and Atelier ten, both based in London, and Centerbrook Architects, Centerbrook, Conn. “With night cooling you may have to run your system longer, but overall you are paying much lower costs and operating at lower speeds. That's a huge benefit.”
Hopkins Architects first employed its exposed overhead thermal mass design approach in the early '90s at the Inland Revenue Centre in Nottingham, U.K. There, a series of precast floor/ceiling panels span the width of the workspace and are supported by prefabricated brick piers that double as the exterior walls of the building. Taylor says that the undulating form of the precast concrete ceiling panels not only “makes for very nice architecture,” but also increases the ceiling surface area and helps to focus the cool energy by “enveloping the occupants.”
Inland Revenue Centre in Nottingham, U.K.
Inland Revenue Centre in Nottingham, U.K.
While overhead thermal mass is nearly always beneficial, nighttime cooling is not ideal for all building types, says Taylor. “You really want buildings limited to daytime occupancy, such as offices and schools,” he says, not multifamily residential or hospitals. The same holds for buildings that have dramatic fluctuations in heat loads and high air-exchange rates, such as arenas and laboratories.
Taylor says careful attention must be paid to the location of light fittings and sprinklers, which are routed from above and fixed through the slab, especially if they're going to be cast in the concrete. “You have to make sure to coordinate the placement of any fire compartmentalization in the floor void, as well as the acoustics between the rooms and elements like air displacement nozzles,” says Taylor
As for payback, Taylor says the difference in cost is really the difference between using a raised floor versus a traditional dropped ceiling, plus an upgrade for the concrete detailing. “The payback comes from reduced energy consumption, and that depends on the building use, the local climate, and energy costs,” says Taylor.