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
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 2nd generation Residential Thermal Performance
computer simulation modelling tools do not deal adequately with the
thermodynamics of high mass, vegetated and green roof
Radiant Interactions of Mass with
Air Temperature Effects of Common Mass Materials
of Software Simulation tools
Embodied Energy Issues
Further Reading and
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
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
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
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 2nd generation thermal
modelling tools do not deal adequately with the thermodynamics of
vegetated high mass and green roof structures.
Radiant interactions of mass with perceived comfort
2.1 The role of
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.
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
FC Sheet (compressed)
Earth Wall (Adobe)
Compressed Earth Blocks
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.
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
T(env) = 2/3 Mean radiant surface temperature + 1/3 Air
Thermal mass influences bodily comfort by
providing heat source and heat sink surfaces to support the
radiative heat exchange comfort processes.
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
• 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
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
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.
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.
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
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
Double Brick (250)
Rammed Earth (250)
Compressed Earth Blocks (250)
Sandy Loam (1000)
Table 4: Time lag figures for various
(Baggs, SA, JC, DB., 1991) and (Think Brick,
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
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.
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
air/mass, glass/mass relationship
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.
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
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).
insulation relative to mass
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).
(% of overall building floor area)
Table 5: Recommended North
Window Area (for Passive Solar Homes and Small Commercial Buildings
(adapted from Baverstock and Paolino, 1986 and Ballinger et al,
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
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
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
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'
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
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
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.
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.
- 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;
- 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.
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
Mortensen, N., "The Natural Airconditioned
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,
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.