The primary source of space heating is usually warm air ducted to a space from a central air handler. However, this isn’t always the only source of heat to a room. The ability of building mass to store thermal energy and to radiate it into the occupied spaces has been discussed.
This is not often a principal heat source, but it can have a marked effect upon occupant comfort.
It’s best to design the opaque surfaces in a space to not vary much from the average room temperature. It’s generally accepted that the average temperature of these surfaces should remain within 5°F of the room temperature, and in the worst winter weather, the coldest surface should not vary more than 25°F from the room temperature. A greater deviation has an adverse effect upon the occupants’ comfort.
These circumstances are not usually a problem in interior zones unless there’s a wide variation in internal heat generation. Examples of this might be a kiln in a crafts shop, high-intensity lights in a hospital operating theater, a large classroom filled with many people for hours on end, or the assembly floor in a factory with welding and other heat- producing operations taking place. Thermal mass is an effective way to control radiant comfort under such circumstances: excess heat is absorbed and distributed evenly over a large surface, then slowly re-radiated into the space at a temperature near ambient.
Exterior zones, to include rooms with a roof load as well as those with wall exposure to the outside, can also benefit from strategically located thermal mass. It’s best if this mass is located internal to the envelope insulation so that its radiant temperature will be closer to the rooms’. This is especially important in rooms with windows since the glass surface usually has a temperature near the outside ambient, making it necessary for the balance of the surfaces in the room to have a temperature very close to the room ambient, so that the overall average is within 5°F of the air temperature.
Careful use of sunlight incident to the windows offers a convenient solution to the need for sufficient temperature of the room mass. Blinds properly tilted, or a wide reflective windowsill can be used to direct the light up to the ceiling. This not only projects the daylight far into the room and simulates a luminous ceiling that emits a diffuse light to the work plane below, but also warms the ceiling as the non-visual portion of the radiation is absorbed. Thus the ceiling is able to serve as a radiant surface.
Unfortunately, ceilings are the least massive of any building component, often comprised of acoustic tiles on a suspended T-bar grid. The balance of the ceiling space is used to run ductwork, piping, and conduit, so it’s helpful to have a suspended ceiling that is easily disassembled to access the mechanical and electrical systems. For rooms that have daylighting available much of the time and which are to be fitted with the controls to make practical use of the light of day, it adds little to the project to augment the mass of the ceiling targeted by the light with sheetrock panels, masonry, or other building materials.
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Excerpt from Energy Conservation Projects.