A typical high rise office building with a single-skin, air-tight, fully transparent façade and all-air ventilation/air tempering systems poses some basic Grade 12 Physics questions. Why are we blowing warm air down from a ceiling air duct? Why are we transferring energy (hot or cold) with air when water has 3000 times the energy storage capacity?
We waste about 65% of the energy consumed by building systems simply moving air around. This means only 35% goes toward offsetting space heating, cooling and ventilation loads. To continue our line of questioning, why are we enclosing ourselves in sealed greenhouses completely isolated from the outdoor environment? Why don’t we protect our “transparent” building skins from excessive solar exposure when it’s sunny and from excessive heat losses when it’s cold?
Historically, energy has been extremely cheap in North America, and resources plentiful. That may be why North Americans consume two times the energy per capita as Western Europeans. The idea of the fully transparent, hermetically sealed building was a neat concept and designers prided themselves in coming up with a system that would work in absolutely any climate and in any building.
As they are wont to do, times are changing. Energy costs are rising dramatically. We want to be healthy and comfortable. In a nod to simpler inventions, we want to be able to open a window to let in the fresh air. We want to start working with the environment instead of simply reacting to it.
Material choice is an essential component to designing better buildings. Location, orientation, shape, skin, structure and materials form the foundation from which all other building design choices flow. It is, therefore, absolutely essential that structural, mechanical, electrical, and architectural designers collaborate to create a living, breathing complete building.
Buildings’ energy sources vary, but in terms of building systems, there is one package of technologies that, in combination with a heavy mass structure, result in the most energy-efficient, comfortable and healthy indoor environments being inhabited today. Borrowing heavily from Europe, where resources have long been scarce and expensive, heavy mass, well insulated and naturally ventilated buildings with radiant heating/cooling systems are becoming more popular for North American building projects. These are buildings that work with, rather than against, the environment and the laws of physics.
The Effect of Thermal Mass
In all but the most hot, humid climates the best way to build a healthy, comfortable indoor environment is to make effective use of the building’s mass to dampen its response to swings in outdoor temperatures. One of the best building construction materials for acting as a thermal storage medium is concrete.
Concrete will store, absorb or emit a large amount of thermal energy over a relatively long period of time. The thermal mass of concrete helps keep temperature swings to a minimum, so high mass systems can be considered self-regulating.
Radiant Slab Heating and Cooling
Radiant slab heating and cooling systems, sometimes called “’high-mass” radiant systems, take advantage of the thermal properties of concrete. Heating and cooling is achieved by circulating tempered water through networks of piping contained within a building’s structural elements. Heat is exchanged between the water and the building structure, typically through suspended concrete floor slabs. The structure transfers heat to or from the occupants. In short, the building itself is used to manage heating and cooling loads.
Radiant slab systems are more efficient than traditional all-air systems because water is a much more effective heat-carrying medium than air. Less energy is consumed pumping water through a radiant system than would be needed to push the amount of air required for the same thermal load.
Natural and Displacement Ventilation
The purpose of a ventilation system is to remove air-borne contaminants and maintain high indoor air quality.
In a displacement ventilation system, 100% filtered outdoor air enters the room at low level and velocity. Because the air is slightly cooler than the room temperature it spreads out and pools on the floor. Occupants heat the air around them causing it to rise towards the ceiling. Convection draws fresh air up, through the occupied zone towards an exhaust at the ceiling. The air stratifies in the room and never re-circulates, ensuring contaminant-free air ventilation. Humidity is controlled by tempering and de-humidifying the incoming air ventilation. In temperate climates such as Canada’s West Coast, displacement ventilation can often be achieved naturally by openable windows or through a stack effect.
Return to top The Hunt for Thermal Comfort
Traditional all-air HVAC systems perform combined space temperature and ventilation functions by moving large volumes of air and controlling the air temperature in the occupied space (through a thermostat).
However, human comfort perception is determined by the operative temperature which is a combined effect of air temperature and the average temperature of surfaces enclosing the space (Mean Radiant Temperature). In fact, the radiant temperature is the main component of our comfort perception. Consider as an example that you are sitting next to a cold window in a room whose thermostat reads 24 degrees Celsius. You will likely feel the cold radiation from the window and experience discomfort even though the air temperature is still 24 degrees.
Radiant heating and cooling systems provide control over surface temperatures within the occupied space and ensure evenly distributed resultant temperatures. Radiant slab systems function well with displacement ventilation because thermal comfort is addressed by a separate system.
The Benefits: Clearer than Concrete
Compared to a conventional all-air system:
There is a significant reduction in the volume of air circulating through the building with a radiant slab & displacement ventilation system. The size of the ventilation system and the amount of space it consumes is therefore reduced, allowing lower floor to floor heights and/or greater architectural freedom.
Radiant slab systems are simple systems that pose fewer and less complex maintenance problems than highly mechanized systems.
Displacement ventilation provides superior indoor air quality and thermal comfort, increases occupant productivity, while fewer fans and reduced ductwork make for quieter buildings.
How Do I Sign Up?
There is no secret to designing beautiful, energy efficient and healthy buildings. Hire a good architect to devise the beauty. Bring the whole team on board early in the project; this includes innovative structural, mechanical and electrical designers along with the owner and contractors to throw all their ideas on the table.
The best way to determine what results you can achieve before the building is constructed is to model it. There are advanced computer software programs available today, that are powerful enough to model the complete energy performance of a building. These programs take into account a building’s climate, materials (including mass), orientation, glazing, openable windows, occupant loads, radiant heat transfer and buoyancy-driven air flows. In this way, the most efficient and effective building can be designed, requiring the smallest mechanically driven building system.
By Christy Love, EIT and Vladimir Mikler, P.Eng. and Joseph Lyman of the Insulating Concrete Form Association
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