Thermal Mass and Thermal Lag
Thermal mass is used to store heat from the sun during the day and re-release it when it is required, to offset heat loss to colder night time temperatures. It effectively "evens out" day and night (diurnal) temperature variations.
- When used correctly, thermal mass can significantly increase comfort and reduce energy consumption.
- Thermal mass is essential for some climates or design solutions but can be a liability if used incorrectly.
- Adequate levels of exposed (ie. not covered with insulative materials such as carpet) internal thermal mass in combination with other passive design elements will ensure that temperatures remain comfortable all night (and successive sunless days). This is due to a property known as thermal lag.
Thermal lag is a term describing the amount of time taken for a material to absorb and then re-release heat, or for heat to be conducted through the material.
Thermal Lag times are influenced by:
- * Temperature differentials between each face.
- * Exposure to air movement and air speed.
- * Texture and coatings of surfaces.
- * Thickness of material.
- * Conductivity of material.
Rates of heat flow through materials are proportional to the temperature differential between each face.
External walls have significantly greater temperature differential than internal walls. The more extreme the climate, the greater the temperature difference.
In warmer temperate climates, external wall materials with a minimum time lag of 10 to 12 hours can effectively "even out" internal/external diurnal (day/night) temperature variations. In these climates, external walls with sufficient thermal mass moderate internal/external temperature variations to create comfort and eliminate the need for supplementary heating and cooling.
In cool temperate and hot climates (or where the time lag is less than 10-12 hours), external thermal mass walls require external insulation to slow the rate of heat transfer and moderate temperature differentials. In these climates, thermal mass moderates internal temperature variations to create comfort and reduce the need for heating and cooling energy.
GLASS TO MASS AND FLOOR RATIOS
- Optimum (solar exposed) glass to floor area ratios vary between climates and designs. This is due to varying diurnal ranges and the balance required between heating and cooling.
- Location and exposure of thermal mass to direct and reflected radiation is also an important factor.
- The useful thickness of thermal mass is the depth of material that can absorb and re-release heat during a day/night cycle. For most common building materials this is 100 to 150mm.
- An exception is when thermal mass is used to even out seasonal temperature variations occuring in earth covered buildings. Summer temperatures warm the building in winter and winter temperatures cool it in summer. In these applications, lag times of 180 days are required in combination with the stabilising effect of the earth's core temperature.
- A "rule of thumb" for best performance is: the exposed internal area of thermal mass in a room should be around 6 times the area of north facing glass with solar access.
- In mixed climates where heating and cooling needs are equally important the amount of thermal mass used should be proportional to diurnal range. Higher diurnal ranges require more mass, lower diurnal ranges require less.
- In heating climates with minor cooling requirements larger glass areas with solar access can be beneficial providing that heat loss through glazing is adequately minimised and passive shading optimised. This requires double glazing and close fitting heavy drapes with snug pelmets.
- Maximise externally insulated, internally exposed thermal mass. Edge insulation is desirable for earth coupled slabs, especially in colder areas. Earth coupling should be avoided where ground water action or temperatures can draw heat from slabs.
- In cooling climates with minor heating requirements thermal mass levels are dependent on diurnal range as above but, additionally, the cooling effect of earth coupling (where achievable) can provide significant benefits. Slab on ground construction is ideal provided that slabs areprotected from summer heating and contact with sun.
- In predominantly cooling climates solar exposed glass areas should be eliminated and thermal mass minimised. Some exceptions apply for advanced design solutions.
- Detailed analysis of glass to mass and floor area is complex and can be confusing. Detailed coverage appears in other publications. Refer to the additional key references at the end of this sheet.
HOW THERMAL MASS WORKS
- Thermal mass acts as a 'thermal battery'. During summer it absorbs heat, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm.
- Thermal mass is not a substitute for insulation. Thermal mass stores and re-radiates heat. Insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator.
- Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperature.
- Correct use of thermal mass can can delay heat flow through the building envelope by as much as 10 to 12 hours producing a warmer house at night in winter and a cooler house during the day in summer.
- A high mass building needs to gain or lose a large amount of energy to change its internal temperature, whereas a lightweight building requires only a small energy gain or loss.
Winter
Allow thermal mass to absorb heat during the day from direct sunlight or from radiant heaters. It will re-radiate this warmth back into the home throughout the night.
Summer
Allow cool night breezes and/or convection currents to pass over the thermal mass, drawing out all the stored energy. During the day protect thermal mass from excess summer sun with shading and insulation if required.
USING THERMAL MASS
Thermal mass is most appropriate in climates with a large diurnal temperature range. As a rule of thumb, diurnal ranges of less than 6°C are insufficient; 7°C to 10°C can be useful depending on climate; where they exceed 10°C, high mass construction is desirable. Exceptions to the rule occur in more extreme climates.
In cool or cold climates where supplementary heating is often used, houses will benefit from high mass construction regardless of diurnal range. (eg. Hobart 8.5°C). In tropical climates with diurnal range of 7-8 (eg. Cairns 8.2°C) high mass construction can cause thermal discomfort unless carefully designed, well shaded and insulated.
Always use thermal mass in conjunction with good passive design.
THERMAL MASS PROPERTIES
- High Density - The more dense the material (ie the less trapped air) the higher its thermal mass. For example, concrete has high thermal mass, AAC block has low thermal mass, and insulation has none.
- Good Thermal Conductivity - The material must allow heat to flow through it. For example, rubber is a poor conductor of heat, brick is good, reinforced concrete is better. But if conductivity is too high (eg. steel) energy is absorbed and given off too quickly to create the lag effect required for diurnal moderation.
- Low Reflectivity - Dark, matt or textured surfaces absorb and re-radiate more energy than light, smooth, reflective surfaces. (If there is considerable thermal mass in the walls, a more reflective floor will distribute heat to the walls).
- Will the savings in heating and cooling energy be greater than the embodied energy content over the life of the building? Can lower embodied materials such as water or recycled brick be used? In addition, poor design of thermal mass may result in increased heating and cooling energy use on top of the embodied energy content.
TYPICAL APPLICATIONS
- In rooms with good winter solar access it is useful to connect the thermal mass to the earth. The most common example is slab on ground construction. A less common example is earth-sheltered housing.
- A slab on ground is preferable to a suspended slab in most climates because it has greater thermal mass due to direct contact with the ground. Slab edges should be insulated in cool and cold climates. The whole slab must be insulated from earth contact in cold climates, see page 5. Consider termite proofing when designing slab edge insulation. Brick or compressed earth floors are also appropriate.
- Use surfaces such as quarry tiles or simply polish the concrete slab. Do not cover areas of the slab exposed to winter sun with carpet, cork, wood or other insulating materials. Use rugs instead.
- Masonry walls also provide good thermal mass. Recycled materials such as concrete, gravel or re-used bricks can be used.
Insulate masonry walls on the outside, for example reverse brick veneer construction. Masonry walls with cavity insulation and rammed earth walls also provide good thermal mass. (Note: rammed earth has a low insulation value and requires external insulation in cool and cold climates).
Introduce thermal mass within lightweight structures by using isolated masonry walls or lightweight steel-framed concrete floors. Always insulate the underside and exposed edges of suspended thermal mass floors.
Water can be used to provide thermal mass. Walls may be built from water-filled containers.
Internal or enclosed water features such as pools can also provide thermal mass but require good ventilation and must be capable of being isolated as evaporation can absorb heat in winter and create condensation problems year round.
WHERE TO LOCATE THERMAL MASS
The location of thermal mass within the building will have an enormous impact on its year round effectiveness and performance.
- As a rule of thumb the best place for thermal mass is inside the insulated building envelope. Insulation levels required will depend on the climate. A better insulated envelope will mean more effective thermal mass.
- Thermal mass must be left exposed internally to allow it to interact with the house interior. It must not be covered with thermally insulating materials such as carpet.
- To determine the best location for thermal mass you need to know if your greatest energy consumption is the result of summer cooling or winter heating.
- Heating Application: Locate thermal mass in areas that receive direct sunlight or radiant heat from heaters.
- Heating and cooling: Locate thermal mass inside the building on the ground floor for ideal summer and winter efficiency. The floor is usually the most economical place to locate heavy materials and earth coupling can provide additional thermal stabilisation.
- Locate thermal mass in north facing rooms which have: good solar access; exposure to cooling night breezes in summer and additional sources of heating or cooling (heaters or evaporative coolers).
- Locate additional thermal mass near the centre of the building, particularly if a heater or cooler is positioned there. Feature brick walls, slabs, large earth or water filled pots and water features can provide this.
- Cooling Application - Protect thermal mass from summer sun with shading and insulation if required. Allow cool night breezes and air currents to pass over the thermal mass, drawing out all the stored energy.
- Roof-mounted solar pool heating is relatively inexpensive and can be used in conjunction with hydronic heating systems or water storage containers to heat thermal mass in winter or (in reverse) to provide radiant cooling to night skies in summer. This method can resolve situations where direct solar access for passive heating is unachievable or where conventional thermal mass is inappropriate
WHERE NOT TO LOCATE THERMAL MASS
In brick veneer houses with tiled roofs the thermal mass materials are on the outside and the insulative materials are on the inside. The value of thermal mass is minimal in this form of construction.
- Avoid use in rooms and buildings with poor insulation from external temperature extremes and rooms with minimal exposure to winter sun or cooling summer breezes.
- Careful design is required if locating thermal wall on the upper levels of multi-storey housing in all but cold climates, especially if these are bedroom areas.
- Natural convection creates higher upstairs room temperatures and upper level thermal mass absorbs this energy. On hot nights upper level thermal mass can be slow to cool, causing discomfort. The reverse is true in winter.
RENOVATIONS AND ADDITIONS
Shade thermal mass from summer sun.
- Remove carpet or insulative covering from concrete slabs that have exposure to winter sun in climates where thermal mass is beneficial. Tile or polish the surface to facilitate heat gain.
- Roof-mounted solar pool heating is relatively inexpensive and can be used in conjunction with hydronic heating systems or water storage containers to heat thermal mass in winter or (in reverse), to provide cooling to night skies in summer. This method can resolve situations where direct solar access for passive heating is unachievable or where conventional thermal mass is inappropriate.
- If adding a room, design for passive solar access and use slab on ground or high mass walls. Insulate the existing surface of any existing thermal mass.
HERMAL MASS
Interaction between heat loss and heat gain through windows and the thermal mass of the building should be considered.
- High mass buildings require adequate north facing window areas to take full advantage of passive heating and cooling.
- Low mass buildings need windows with low U-values to minimise heat loss at night and on cloudy days, and to reduce heat flow into the building in hot weather. A low solar heat gain coefficient and shading also help to reduce summer heat gain.
- Where passive design principles are compromised because of site or design restrictions, the use of energy-efficient windows is an essential alternative method for achieving thermal comfort and energy efficiency.
AIR-TIGHTNESS
- The thermal performance of windows and doors is lowered if they are not airtight.
- Heat loss and gain occur by air infiltration through cracks in the window assembly.
- Well-made frames and seals around opening sashes are an important feature.
- Sealing between the wall and window frame at installation is equally important.
- Infiltration is measured in terms of the amount of air that passes through a unit area of window under given pressure conditions.
Air infiltration for a particular window can be found at the bottom of the WERS rating label. The lower this number the better.
LIGHT TRANSMITTANCE
- Good window design and location maximises natural lighting. Bright, naturally lit homes promote health and well-being and reduce the need for electric lighting.
- Natural light provides good colour rendition and skin tones and is preferred by most indoor plants.
- The visible transmittance (VT) of a window is a measure of the amount of visible light transmitted through the glass. The VT for a particular window can be found at the bottom of the WERS rating label.
- Choose glass that generally has a VT of at least 0.5 (50 percent) to preserve natural lighting. All of the generic window types in WERS meet this requirement.
- A high VT is generally desirable to maximise daylight and view but this must be balanced against the need to control solar gain and glare in hot climates.
- Windows with special multi layer films are available that can maximise VT while reducing solar gain.
- Diffuse lighting (as opposed to direct sunlight) is generally the best for providing good uniform illumination over a room and avoiding glare.
- Skylights are an excellent way to provide natural day lighting for a room, particularly in cooling climates where shading and other passive design elements can reduce light transmittance through windows. Conventional skylights can let in too much heat and light, but new designs (such as angular-selective skylights) can be a very efficient way to light a room.