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

• 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 5-10 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