Thermal
Mass
Thermal mass can
moderate the temperature of occupied spaces, minimize the need for
mechanical cooling and reduce winter heating requirements, but to
do so it must be “coupled” to heat sources and some means
of distribution such as the air handling system. Although greater
mass in a building means that more heat can be stored, practical issues
limit the useful amount.
Providing dedicated
thermal mass is expensive with the possible exception of multiple
layers of gypsum board and high-density concrete topping on wood floor
systems. The most cost effective method usually is to take advantage
of thermal mass in the building structure.
In most commercial
buildings, poured concrete or cellular precast slabs and shear walls
are the largest thermal mass.
- There is little
performance increase beyond 4 in. thickness. A 3-in. slab provides
95% of the performance of a 4 -in. slab.
- High-density
concrete provides more thermal mass than low-density concrete. In
multi-unit wood building construction, the thin concrete layer often
placed on wood subfloors for fire retardancy and acoustic separation
can be made from high-density concrete.
- Improving airflow
across the surface is usually the most cost-effective means of using
thermal mass.
The amount of
heat transferred to and from walls and floor slabs can be increased
by maximizing the exposed surface area.
- The corrugated
profile of steel decking sections used in concrete composite floors
can increase surface area by 15% to 50%. Use coffered ceilings or
waffle slabs to increase surface area, but also provide smooth reflective
surfaces for daylight distribution.
- Expose the
underside of floor slabs to the inhabited spaces.
- Use operable
windows and HVAC diffusers that direct airflow toward massive elements
at night to lower their temperature.
Suspended ceilings
create an insulating barrier. If a suspended ceiling must be used
for acoustic control or to hide lights, ducts or wiring, provide “thermal
transparency” in the ceiling:
- Use perforated
or open-grid ceiling tile. Even 15% open area can allow significant
air circulation.
- Limit the area
of massive elements covered.
If mechanical
ventilation is used:
- Use ceiling-mounted
fans to assist air movement and mixing with air being introduced
through open windows.
- Pass air beneath
raised floors over an exposed floor slab. The cool air can then
be introduced to the room through floor-mounted diffusers to provide
displacement ventilation.
For passive solar
heating, either “direct” or “indirect” thermal
storage:
- Use concrete,
adobe, tile, brick, stone or masonry floors. Here, the thermal mass
must be exposed directly to winter sunlight.
- Use double
gypsum board throughout the spaces “thermally linked”
to south facing windows or clerestories. This diffuse thermal mass
approach depends on inter-reflected sunlight and convection currents
to transfer the solar heat gain to the wall surfaces.
Cautions
- Exposed hard
surfaces reflect sound. Pay attention to noise distribution.
- Additional
thermal mass, such as high-density concrete topping, will alter
structural and seismic loads. Consult with structural engineer.
- If night ventilation
is automatically controlled, use simple systems. If manually controlled,
make them foolproof.
- If cooling
loads are low, thermal mass benefits are limited.
