Thermal Mass & its Role in Building Comfort and Energy Efficiency

D. Baggs: Technical Director, Ecospecifier

This Technical Guide seeks to explain thermal mass, its benefits and limitations to enhance an understanding of mass in common building materials and its design implications and strategies for use in buildings.

It looks at comparative performances between a variety of building materials, including:

• Concrete block
• Cavity brick and brick veneer
• Autoclaved Aerated Concrete
• Polystyrene faced systems
• Concrete
• Stone
• Earth

As well as considers benefits of mass as well as potential urban heat island and embodied energy vs operational savings issues and looks at high mass strategies and benefits of earth integrated and green roof buildings.

Some guidance is also provided in relation to where the 2^nd generation Residential Thermal Performance computer simulation modelling tools do not deal adequately with the thermodynamics of high mass, vegetated and green roof  structures.

Part 1:

Radiant Interactions of Mass with Perceived Comfort

Part 2:

Internal Air Temperature Effects of Common Mass Materials

Part 3:

Design Considerations

Part 4:

Urban Heat Island Effects

The 'Green Roof' Revolution

Part 5:

Limitations of Software Simulation tools

Embodied Energy Issues

Conclusion

Further Reading and References

For further guidance on the topics relating to the comparitive performance of cavity brick and brick veneer see the Enhanced Content links below:

Think Brick Australia, 2006, Energy Efficiency and the Environment: The Case for Clay Brick,

Sugo, H.O., Page, A.W., Moghtaderi, B., 2004, CPBI Research Paper 18: A Comparitive Study of the Thermal Performance of Cavity and Brick Veneer Construction, The University of Newcastle

Sugo, H.O., Page, A.W., Moghtaderi, B., 2005, CPBI Research Paper 19: The Study of Heat Flows in Masonry Walls in a Thermal Test Building Incorporating a Window, The University of Newcastle

1.0   Overview

Thermal mass elements in buildings assists in the reduction of energy consumed in heating and cooling in  most climate zones, can significantly reduce the ecological impacts of burning fossil fuels due to energy production, as well as reduce costs, improve comfort and reduce or eliminate the need for air conditioning.

Introducing thermal mass into light weight structures is the only way to cool a structure down once the external temperatures exceed comfort levels and ventilation fail to provide comfort. Likewise, using thermal mass in lightweight structures is the only way winter daytime temperatures can be stored to keep buildings warm in winter evenings without introducing external energy sources. This is sometimes known as 'Passive Solar Design'.

However, with the introduction of second generation National Home Energy Rating Scheme (NatHERS) software tools such as AccuRate, First Rate and BERS Pro, the sometimes seemingly rigid precepts of passive solar design become more like guiding principles to stimulate design to enable ideas to be tested, simulated more accurately optimised and finessed.

This Technical Guide seeks to explain thermal mass, its benefits and limitations to enhance an understanding of mass and its uses in common buildings and provide guidance in areas where even the sophisticated 2^nd generation thermal modelling tools do not deal adequately with the thermodynamics of vegetated high mass and green roof  structures.

2.0       Radiant interactions of mass with perceived comfort

2.1  The role of mass

While the 'greenhouse' effect can trap large amounts of heat during the day, this heat can also be quickly lost unless materials with 'thermal mass' (which act as a storage medium for the heat) are used.

Thermal mass influences comfort by radiant exchanges with the skin. In fact radiant exchange with mass surfaces is singularly the most efficient way of maintaining comfort compared with an other technique as the body is more that twice as sensitive to radiant losses and gains than all other pathways combined (conduction, convection, respiration, evaporation) and more than four times as sensitive than any other single pathway (see 2.3 below).

In summer, the thermal mass effect is the most important of only three commonly possible means of 'actively' cooling a structure without the use of externally sourced energy or fuel. The others are ventilation and evaporation. As noted above, in many climatic conditions, thermal mass effects are the only means of cooling a building.

In winter, mass storage effects can create comfort by harnessing large amounts of excess solar energy to initially store and later reradiate low level radiant heat via a 'thermal flywheel' effect. In doing so, thermal comfort of occupants can be maintained while achieving energy savings of up to 100% in heating and cooling loads (in some building types). This effect is dependant on the amount of mass introduced and the balance and composition of a number of key building parameters such as glass area and protection, insulation, orientation, ventilation and building usage.

2.2    Heat storage and mass

Thermal mass is the ability of a material to store heat. Materials suitable for thermal mass are heavy (or dense) materials with the ability to store large amounts of heat energy to provide warmth in winter and coolth (the opposite of warmth) in summer, within a relatively small volume (see Table 1).

Material	Density (Kg/m3)	Specific heat (kJ/kg.K)	Volumetric heat capacity Thermal mass (kJ/m3.K)
Water	1000	4.186	4186
Concrete	2240	0.920	2060
AAC	500	1.100	550
Brick	1700	0.920	1360
Stone (Sandstone)	2000	0.900	1800
FC Sheet (compressed)	1700	0.900	1530
Earth Wall (Adobe)	1550	0.837	1300
Rammed Earth	2000	0.837	1673
Compressed Earth Blocks	2080	0.837	1740

Table 1.  Density, specific heat and thermal mass of a range of materials

Note: Figures are based on a number of sources and include estimations and interpolations.

2.3    Thermal comfort

Thermal comfort exists when a body's heat loss equals its heat gain or vice versa.

The body exchanges:

• 62% of this heat via radiation,
• 15% by evaporation,
• 10% by convection,
• 10% by respiration and
• 3% by conduction.

Hence, when considering normal comfort conditions inside a building the radiative effect of surrounding surfaces is at least as important as air temperature. Relatively small changes in mean radiant temperature have a far greater effect than similar changes in air temperatures (Ballinger 1992). This gives rise to the importance of recognising the overall Environmental Temperature [T(env)], as opposed to just the dry bulb temperature.

T(env) =  2/3 Mean radiant surface temperature + 1/3 Air temperature

Thermal mass influences bodily comfort by providing heat source and heat sink surfaces to support the radiative heat exchange comfort processes.

3.0   Internal Air Temperature effects of common mass materials

Thermal mass affects the temperature within a building by stabilising internal temperatures in three ways:

•     stabilising internal temperatures by providing heat source and heat sink surfaces for radiative, conductive and convective heat exchange processes;

• providing a time-lag in the equalisation of external and internal temperatures; and

• providing a temperature reduction across an external wall (the decrement factor).

3.1    Stabilisation of Internal Temperatures

When heat enters a space directly by penetration of sunlight, lighting, equipment losses or heating, the temperature rise will be in inverse relationship to the accessible volume of thermal mass. Therefore, the indoor temperature will rise almost immediately if there is little thermal mass in the room. Figure 1 uses an example of a simple box 1150 x 1530 x 1570 mm, with a single window 660 x 1010 mm to demonstrate the effect of thermal mass on internal air temperature using a variety of materials. Figure 2 compares several different building sized test modules monitored for actual thermal performance. Both of these diagrams represent unventilated spaces.

In situations of high thermal resistance and low levels of thermal mass, rapid heating and cooling will occur. Conversely, low levels of thermal resistance and high thermal mass significantly reduce the necessity for heating and cooling (Givoni 1981, p 144). This effect is also demonstrated in Figure 3, which also compares two houses, one of lightweight construction (brick veneer, uninsulated walls, timber floor) and the other of heavyweight construction (double brick, insulated, slab on ground).

3.2     Management of Insulation and Shading

The mass effect can be enhanced with the use of insulation external to the mass and insulation of the glazed areas. In summer, thermal mass can be used in conjunction with passive solar design strategies to inhibit heat build up. In areas where there is a significant diurnal temperature range, night ventilation will cool the building by convection, leaving it ready to absorb the heat build up during the next day. Table 2 describes user shading and ventilation strategies for summer thermal comfort.

	Summer		Winter
Device	day	night	day	night
Windows, doors	closed	open	closed	closed
Blinds (external)	closed	open	open	closed
Curtains (internal)	closed	open	open	closed

Table 2: User control of shading and ventilation devices

In winter, thermal mass will increase thermal comfort in a building when solar heat stored in the materials during the day is released as internal temperatures fall. Heat loss from the building after the sun has left the glazing can be prevented by drawing insulating curtains over the windows, or by using double glazing permanently in place. Table 2 describes user shading and ventilation strategies for winter thermal comfort.

3.3    Time Lag

The effect of using heat generated during the day to warm at night in winter and vice versa in summer is known as the 'thermal flywheel' effect. The effectiveness of the flywheel depends on the time lag introduced to a building by an external wall or other boundary element. As can be seen from Figure 3, time 'lag' is the time delay between external maximum or minimum temperatures and internal maximum or minimum temperatures respectively.

In Figure 5 below, it can be seen that in a brick veneer test building the time lag is 5 hours, whereas, in the un-insulated cavity brick construction it is 7 hours.

Time lag is determined mostly by the rate of heat transfer through an element or material. For most low-rise buildings in temperate climates, massive external walls over 220kg/m2 can, according to the BCA, be used in many climates without the need for external insulation.

Figure 6 below, shows the heat flux through a western uninsulated cavity brick western wall and shows the heat flux in and out of the wall due to lag. The are below the 'zero' level is the heat loss and it can be seen it occurs after approximately 7.00pm in the evening. While 700-900 W/m² fall on the wall, only 5-6W/m² actually manage to penetrate the wall by early evening, with the rest being lost to the outside once the heat flow reverses (Sugo, Page & Moghataderi, 2004). In highly rated low energy buildings, cavity insulation may still be required to further reduce even this small heat flow.

Table 4 below shows time lag figures for a variety of building materials. In cool climates where significant heating is necessary and adequate solar gains cannot be relied to keep overnight temperatures stable, insulation of exposed external walls is warranted.

In earth covered and earth-integrated buildings where floor levels can be 4-5 metres 'below' ground, the time lag effect can be manipulated so that instead of a diurnal time lag (where day heats night in winter and night cools day in summer), a seasonal time lag can be experienced where summer temperatures warm the occupants in winter and winter temperatures cool them in summer.

Material (thickness in mm)	Time lag (hours)
Insulated Brick Veneer	5.0
Concrete (250)	6.9
Double Brick (250)	7.0
AAC (200)	7.0
Adobe (250)	9.2
Rammed Earth (250)	10.3
Compressed Earth Blocks (250)	10.5
Sandy Loam (1000)	30 da

Table 4: Time lag figures for various materials
(Baggs, SA, JC, DB.,  1991) and (Think Brick, 2006).

4.0   Design Considerations

4.1    Thermal mass and building type

The most obvious energy efficiency benefits from the use of thermal mass accrue in low-rise buildings. Thermal mass, when used together with passive solar design techniques and natural ventilation, can effectively eliminate the requirement for air conditioning. Temperatures in such buildings can 'free-run' - that is, they do not need to be modified artificially except for small amounts of heating on extremely cold days or during extended overcast periods. Buildings in this category include single houses, medium density residential, low-rise commercial buildings and some small scale educational and industrial buildings. The internal conditions in such buildings are dominated by the influence of the climate.

In other building types including medium and high-rise commercial and educational structures, the internal conditions are generally dominated by the heat loads generated within the building envelope and these are typically be cooled by other means, including air conditioning.

4.2    Thermal mass and air conditioning loads

Mass can be extremely useful in structures which require air conditioning. This can be achieved by night-time cooling of the mass to reduce the day-time heat loads in summer, or to even out the heat loadings in winter and eliminate the need for cooling during winter. Baverstock (1994) has shown that mass used in this way can provide 27% of the overall building cooling benefits and 38% of the overall building heating benefits. This was in an energy efficient commercial building in Perth that was shown to be 69% more efficient than average non-low energy buildings and 79% more efficient than the recommended upper-range figures published by the Property Council of Australia for Perth. For more detailed information, refer to the case study CAS 2: Solar Information Centre, Perth.

Leading edge commercial buildings use a variety of ways of engaging thermal mass cooling or pre cooling of spaces to minimise the amount of cooling needed. Examples of these systems include eliminating ceilings and exposing the underside of floor slabs, using access floors over exposed floor slabs as plenums to deliver conditioned air (precooling the slabs using night time ventilation allows late start up and/or early shut down of chillers).

4.3    The air/mass, glass/mass relationship

Air/mass

Baverstock (1994) also confirms previous authors' claims that energy savings of approximately 70-90% are achievable in commercial air conditioned buildings by using energy efficiency techniques in combination with thermal mass. He notes that for the thermal mass to work effectively at reducing air conditioning loads, the thermal mass had to be 'accessible' to the air mass within the building. Accessible spaces include exposed walls and partitions, ceilings and tiled concreted slab floors. He also found that the required relationship between area of mass and the gross floor area of the building was 0.6 or greater where the thermal mass was 150 mm thick structural concrete (2400 kg/m³, 2040 kJ/m³ storage capacity). While Baverstock used floor area as the reference, the functional relationships are really between the internal volume of air in a room or building, and the surface area and heat storage capacity of the mass that is accessible to that air.

Glass/mass

There are also optimum ratios for the combination of windows and thermal mass. In winter, mass needs heating to enable comfort conditions to be maintained. This heat will ideally come from solar gain provided by north-facing or other windows with appropriate summer shading. Table 5 shows the recommended area of north-facing windows for each capital city, based on the minimum thermal mass volumes shown in the previous table. The source of this data (Baverstock and Paolino 1986) also provides recommended window areas for walls with other aspects.

The inter-relationships between climatic conditions, thermal mass storage requirements and window areas can be seen in Figure 7 below. In general it can be seen that the more extreme the daily maxima and minima, the higher the thermal mass requirements, and the more window area is needed to warm the mass in winter.

4.4    Locating mass in a building

The most economical and effective means of locating mass within a building is to place it where it can have contact with dry earth, e.g. a slab-on-ground. As well as providing the thermal mass of the slab, this has the effect of earth-coupling the air mass inside the home to the infinite thermal mass of the ground beneath it.

This coupling with the earth provides the occupants with resultant indoor temperatures which are far more stable than externally. In summer, if the slab is not insulated with carpet, a slab-on-ground provides a building with the potential for significant heat losses to the earth. In winter, a slab-on ground not only provides the potential for storage of daily solar heat gain from north windows but, even during overcast days, also provides significant stored heat from the earth below the slab. Figure 8 shows examples of design formats that will provide this effect (in order of increasing benefit).

4.5    Locating insulation relative to mass

Concrete slab-on-ground

Except in the most extreme climates, a concrete slab-on-ground does not require insulation on the complete underside, as this negates the effect of earth coupling. In extremely hot or cold climates polystyrene sheet foam insulation along the edge of the concrete slab and extending as a 'skirt' for 1.2 m around the perimeter is preferred (Watanabe 1986).

Location	North Window Area (% of overall building floor area)
Hobart	17.0
Sydney	15.5
Perth	14.5
Adelaide	14.0
Melbourne	13.5
Canberra	12.5
Brisbane	9.5
Darwin	4.7

Table 5: Recommended North Window Area (for Passive Solar Homes and Small Commercial Buildings or Spaces)
(adapted from Baverstock and Paolino, 1986 and Ballinger et al, 1992)

Suspended concrete

Insulation is not required on the underside of suspended concrete slabs supported by fully enclosed walls. However, as the air space under the slab provides indirect 'earth-coupling', it should be isolated from the outside air by insulating the walls. Suspended concrete slabs with the external surface exposed to the elements will require insulation located on the outside surface (i.e. underside of floor slab and top side of roof slab).

External walls

As noted above, current BCA 'Deemed to Satisfy' provisions require minimum levels of added insulation for all these wall types under 200kg/m² and are a good indicator of desirable insulation levels for different wall construction in different climates.

In the case of the various kinds of earth walls such as adobe, rammed earth and compressed earth blocks, with their time lags of 10-11⁺ hours, their apparent performance is enhanced if they are either left unsealed or finished with a 'breathable' paint. This is to take advantage of the thermal energy which can also be absorbed or released by the processes of latent heat of water vaporisation or condensation.

These processes are believed to provide earth walls with actual thermal performance characteristics which exceed the theoretical or calculated thermal performance. Research being undertaken at the Cra' Terre' Institute of France has indicated that favourable winter thermal performances seem to rely heavily on this yet to be fully quantified effect. Nonetheless in cold temperate and cold climates the thermal performance of even earth walls can necessitate the introduction of insulation if inadequate solar heat cannot be stored from daytime gains.

5.0   Urban Heat Island effects

One thermal mass effect that is having significant deleterious impacts in cities around the world including Australia is know as the Urban Heat Island (UHI) effect. This is a function of high mass surfaces in the city such as concrete roofs, roads and to a lesser extend walls, absorbing daytime heat and re-radiating it at night. This reradiated heat forms a heat 'halo' around the city and subjects all residents to significantly hotter night time conditions.

Melbourne University recently measured average summer nightime temperatures 3.4-4.5 degrees Celsius above the long term mean and on still nights increases of 10 degrees Celsius were experienced. Similar studies in Brisbane, Tokyo and many other cities worldwide have confirmed this phenomena is common to all cities to some extent. This will have obvious impacts in decreased comfort and increase air conditioning loads. It also has serious consequences for increased global warming and climate change if left unchecked. It can be improved in various ways including shading of walls, increasing the reflectivity with light colours and employing different treatments on roofs

6.0   The 'Green Roof' revolution

Urban heat islands have become such a phenomenon worldwide that vegetated green roofs or conventional buildings such of offices now account for 20% of the new roofs in Germany, Tokyo has mandated them on all new commercial buildings over 1000m² and massive projects are appearing throughout the US such as a 40,000m² green roof on the Ford Motor Company's Detroit manufacturing plant.

Green roofs provide significant energy savings via insulation, thermal mass, evaporation and evapotranspiration pathways as well as major water & visual quality outcomes.

They are being promoted for use in Singapore for greening highrise buildings as in the 'Handbook on Skyrise Greening' 2002 and in Australia also and were recently the subject of a book by the NSW Government Architect Chris Johnson entitled 'Greening Sydney' 2003.

Green roofs are one form of earth integrated buildings that are the subject of the Author's 'Australian Green Roofs and Earth Covered Buildings' (see References).  All of these publications set out the benefits of earth integration and show that thermal mass effects combined with evapotranspiration and vegetative shade create thermal comfort, energy efficiency and other synergistic benefits well beyond the potential of ordinary structures (see references for further benefits).

The Australian Green Roofs Infrastructure network was formed in Brisbane in March 2007 along with the  launch of the 'Green roofs for Healthy Australian Cities' association to promote the implementation of this approach which is starting to sweep the world.

7.0   Limitations of Software tools

The second generation NatHERS tools such as AccuRate, First Rate and BERS Pro are significant improvements over first generation tools and they are effective at providing simulations of ordinary buildings. However they still have limitations that impact on their ability to accurately simulate (or in some cases simulate at all) certain building types, features and climates such as:

• Passive/Active add on systems;
• Inner city areas impacted by Urban Heat Island Effects that are not included in the temperature data (normally taken at airports away from inner urban areas);
• Earth wall  structures where the full thermal effects of these structures is still not fully understood and practical experience indicates that conventional understanding is inadequate to explain the full thermal effects of earth structures;
• Vegetated green roofs and earth covered buildings where software that simulates shade cover, evapotranspiration and moisture dependent thermal diffusivity of soil are required.

More sophisticated commercial modelling tools such as DOE2, TAS, DesignBuilder/EnergyPlus and IES Virtual Environment are required to undertake these types of assessments in combination with specialised vegetated earth temperature simulation programs such as TCAL (see References).

8.0   Embodied Energy issues

There is a tendency to design high mass buildings with much reinforced concrete and clay fired bricks which tend to be high in embodied energy. This is particularly the case with deep earth covered buildings where in the past deep section insitu concrete has been the preferred structure.

However, in todays greenhouse gas emission (GGE) aware world, the GGE of the energy embodied in the structure needs to be compared against the savings due from the building's energy efficiency and the 'payback' analysed as to whether it is 'worthwhile'. In light of recent published targets where globally, scientific models are showing a need to reduce emissions by up to 60% by 2030 to keep global temperature increases to only 1 degree, does a structure that takes 50 years to payback its GGE of construction indicate that the initiative should not proceed even if the structure lasts say, 500 years?

Given the imminence of global warming, probably not, so structures that use low embodied energy materials to provide access to mass need to be employed wherever possible e.g. any technique that provides access to the free mass of the earth by coupling either directly or indirectly as in Fig. 5 sketches 2-8 are easily made GGE efficient. Structures represented  in sketches 9-11 need to be carefully considered to ensure low GGE profile structures and systems are used to minimise the 'payback' period. Obviously impacts such as urban heat island effect minimisation need to be taken into account in this process.

9.0   Conclusion

Mass is an essential aspect of energy efficient design. However, mass alone will not create a thermally comfortable building. The inclusion of thermal mass has to form part of an integrated approach to the thermal design of buildings, incorporating correct orientation, appropriate areas and treatment of windows, insulation (when required), site integration considerations, appropriate use of natural and/or mechanical ventilation and appropriate back-up heating or cooling sources. It is important that designers consider energy efficiency techniques such as thermal mass as an inherent part of design and architectural expression and that they be included conceptually from the outset.

Further Reading

• BDP Environment Design Guide: GEN 12; GEN 13; DES 2; DES 4, DES 20; DES 22; DES 23; TEC 2; TEC 6; PRO 2; PRO 7; PRO 8; PRO 12.
• Hastings, SR, 1999, Solar Air Systems, Built Examples International Energy Agency, James and James, London.
• Sunaga, N, 1999, 'A Bioclimatic Design for Tropical Climate', PLEA 99 Sustaining the Future. Energy - Ecology - Architecture, Vol 2, Passive and Low Energy Architecture Conference, PLEA International with University of Queensland, Brisbane.
• Zeiher, L, 1996, The Ecology of Architecture, Whitney Library of Design, New York.

References

AccuRate; Building Simulation Software, www.hearne.com.au

AIA Research Corporation May 1976, Solar Dwelling Design Concepts, Washington, DC.

Baggs, SA, Baggs, JC and Baggs, DW, 1991, Australian Earth-Covered Buildings, Dual Harmony Publishers, 1300 88 55 78.

Baggs, D.W. and Mortensen, N., Thermal Mass in Building Design, Environment Design Guide Building Design Professions, Design Note 4, Melbourne 2006.

Ballinger, J, et al, 1992, Energy Efficient Australian Housing, 2nd Edition, AGPS, Canberra.

Baverstock, G and Paolino, S, 1986, Low Energy Buildings in Australia, Graphic Systems, Western Australia. (08) 9386-3888

BERS Pro, Building Simulation Software, www.solarlogic.com.au

Givoni, B, 1981, Man, Climate and Architecture, Van Nostrand Reinhold, New York.

IES Virtual Environment, Building Simulation Software, www.iesve.com

Mortensen, N., "The Natural Airconditioned House".http://www.dab.uts.edu.au/ebrf/index.html

National Parks and National University of Singapore, Handbook on Skyrise Greening  in Singapore, Singapore, 2002

Sugo, H.O, Page, A.W., & Moghtaderi, B., A Comparitive Study of the Thermal Perfromance of Cavity and Brick Veneer Construction, CBPI Research Paper 18, Clay Brick and Paver Institute, 2004

Think Brick, Energy Efficiency in the Environment: The Case for Clay Bricks, Baulkham Hills, 2006

TCAL, Earth Temperature Simulation Software; Natural Integrated Living Pty Ltd, Brisbane, 1300 88 55 78

TAS, Building Simulation Software, www.edsl.net

Watanabe, T, et al, 1986, 'Case study on thermal insulation systems for the earth-contact floor', in Report on the International Symposium on Earth Architecture, Kyushu, Japan.

 

All websites last accessed on 9/4/13.