Walls
• Apply knowledge of thermal mass and
insulation with passive design strategies to
reduce the energy needed by active systems
Thermal Mass
• Material’s resistance to change in temperature
as heat is added or removed
• Objects with high thermal mass absorb and
retain heat
• Important to good passive solar heating
– Especially in areas with large swings in temp from
day to night
Thermal Mass
• With high thermal mass objects absorb and
retain heat
– Slowing the rate at which a space is heated by the
sun, and the rate the space loses heat when the
sun is gone
• Virtually no effect in steady-state heat flow
– When temps are relatively constant on each side
of the material
Density
• Dense materials usually store more heat
• Is the mass of a material per unit volume
– Imperial system pounds per cubic ft
– SI system kilograms per cubic meter
• For a fixed volume of material, greater density
will permit storage of more heat
Specific Heat
• High specific heat requires lots of energy to
change the temp
• Measure of the amount of heat required to
raise the temp of a given mass of material by
1
– Imperial – Btu/lb F
– SI – kJ/kg K
• One gram of water requires one calorie of
heat energy to rise one degree C in temp
• Water has a high heat capacity
Specific Heat
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Material Heat capacity J/(g•K)
Brick 0.84
Concrete 0.88
Granite 0.79
Gypsum 1.09
Soil 0.80
Wood 1.2 - 2.3
Water 4.2
Thermal Capacity (Thermal Mass)
• Density x Specific Heat = How much heat can
be stored per unit volume
• Indicator of the ability of a material to store
heat per unit volume
• Greater the thermal capacity the more heat it
can store in a given volume per degree of
increase
• Higher capacity can (not always) reduce heat
flow from the outside to the inside
environment by storing heat in the material
Thermal Lag (Time Lag)
• With a high thermal mass it can take hours for
heat to flow from one side of the envelope to
the other
• Slowing of the flow of heat – Thermal Lag
• Measured as the difference between peak
temp on the outside surface of a building
element and the peak temp on the inside
surface
Thermal Lag
• Some materials such as glass have very little
thermal lag
• Others such as double brick or rammed earth
walls could be 8-9 hours
• As the sun rises it heats the envelope, once
this envelope is saturated heat will flow to the
inside
Designing with Thermal Mass
• Common arch implementations
– Concrete floor slabs
– Water containers
– Interior masonry walls, back of a chimney
• Most useful in areas with large swings in temp
from day to night
• It may not prevent energy from flowing in or
out of the building as insulation would, but it
can slow heat flow enough to help with
people’s comfort
Hot/Cold Climates
• In climates that are constantly hot or cold the
effects can be detrimental
• All surfaces of the mass will tend towards the
avg daily temp
• If this temp is above/below the comfort range
it will result in more occupant discomfort due
to unwanted radiant gains/losses
Hot/Cold Climates
• Warm tropical and equatorial climates
buildings are typically open and lightweight
• Cold regions buildings are highly insulated
with little exposed thermal mass, even if used
for structural reasons
Thermal Mass for Solar Gain
• Often important to direct solar gain passive
design
• High thermal mass materials conduct a
significant proportion of incoming thermal
energy into the material
• Instead of only a few mm of wall heating up 510 the entire wall heats up only 1-2
• The material re-radiates the heat at a lower
temp but for a longer period of time
Thermal Mass for Solar Gain
• This helps the occupants stay more comfortable
for a longer duration
• When night time temps drop the energy stored in
the walls/floor re-radiates it back out
• Larger the area of thermal mass getting sunlight,
the more heat it receives, the faster it can heat
up and the more heat it can store
Thermal Conductivity with Thermal
Mass
• Insulation can be very helpful in keeping direct
solar gain in the building and not being
conducted into the ground or outside air
• Hot climates it can be beneficial for external
finishes to have low thermal mass and low
conductivity, this increases the effectiveness
of insulation
Thermal Conductivity
• Thermal lag from mass can greatly reduce the
need for insulation in the envelope, especially
in climates will large temp swings (day to
night)
• Combining thermal mass and insulation can
avoid unwanted temp swings indoors, but still
allow solar heat gain/radiative cooling
Thermal Conductivity
• Thermally conductive materials can be
desirable inside the space
• They quickly transfer heat built up away from
a surface struck by sunlight, deeper into the
material, which stores and evenly distributes
the heat within the space
• In less conductive materials the surface will
heat up more where the light strikes, creating
a hotspot there
Thermal Conductivity
• Thick concrete floor will conduct heat and
store it pretty evenly
• A wood floor will not distribute heat well
because although it has a good thermal mass,
it does not conduct heat well
Thermal Conductivity
• Careful when covering thermal mass with
materials such as carpet, cork, wall boards or
other insulating materials
• Isolate the mass from the solar energy you may
be trying to collect
• Ceramic floor tiles or brick might be a better
choice for covering a direct gain slab
Rules of Thumb for Design with
Thermal Mass
• Choose the right amount of mass, determined
by how much heat energy the space requires
(based on climate, orientation, and
surroundings). Increase in comfort and
performance with increase in thermal mass
• Large surface area of thermal mass with
sufficient solar exposure. A rule of thumb is a
mass surface-to-glass area ratio of 6:1
Rules of Thumb for Design with
Thermal Mass
• In direct gain storage, thin mass is more effective
than thick mass, the most effective thickness in
masonry materials is the first 100mm. Beyond
150 are unhelpful as heat is carried away from
the surface and lost. For wood the most effective
thickness is the first 25mm
• Insulating the thermal storage from exterior
climate from ext. climate conditions. In some
climates direct heat gain from sunlight on the
envelop or heat loss to the ground are beneficial
Rules of Thumb for Design with
Thermal Mass
• Locate as much thermal mass in direct
sunlight (heated by radiation) as possible.
Located outside of direct sunlight (heated by
air convection) is also important for overall
performance
• Thermal mass storage is as much as 4x as
effective when the mass is heated directly by
the sun and is subject to convective heating
from warmed air, compared to being only
heated by convection
Rules of Thumb for Design with
Thermal Mass
• Locating thermal mass in interior partitions is
more effective than external walls. Assuming
they both have equal solar access
• Most effective internal storage wall masses
are those located between two direct gain
spaces
• Thermal mass can be combined with glazing
to form “Trombe walls”
Phase Change Materials
• Relatively new class of materials which add
thermal mass without adding weight or bulk
• May replace standard wall board or be an
additional layer in the walls/floors
• Relatively rare
• Store heat by using the materials change of
phase, usually from a solid to liquid and back
• 100 calories of energy to heat a gram of water
from 0C to 100C. It takes 539 to turn a gram of
water at 100C into a gram of steam at 100C
• When the steam is turned back into water the
heat energy is released
Phase Change Materials
• Because of the large amounts of energy
needed for phase changes these materials
increase their thermal mass without adding
weight or size
• Most phase change materials use waxes or
salts that go from solid to liquid
• Some have pouches of materials like bubble
wrape
• Most have micro capsules mixed with normal
materials