Thermal mass
What is thermal mass? |
Thermal mass in
summer | Thermal mass
in winter | How
is thermal mass measured? | Does thermal mass have any
disadvantages? | How long is
the CO2 payback for thermal mass?
What is thermal mass?
Thermal mass is a term that describes the ability of a material
to store heat; something many construction materials can do to a
greater or lesser extent. But, to be useful in the built
environment, they must also be able to absorb and release heat at a
rate roughly in step with a building’s daily heating and cooling
cycle. Concrete and masonry products do this well and, being dense
materials, can also store a lot of heat. Timber absorbs heat too
slowly to offer much effective thermal mass, and steel conducts
heat too rapidly to be in synch with a building’s natural heat
flows over the day.
Thermal mass in summer
On warm summer days, walls and floors with thermal mass will
steadily absorb heat at their surface, conducting it inwardly, and
storing it until exposed to the cooler air of the evening/night. At
this point, heat will begin to migrate back to the surface and be
released. In this way, heat moves in a wave-like motion alternately
being absorbed and released in response to the change in day and
night-time conditions. This ability to respond naturally to
changing conditions helps stabilise the internal temperature and
provides a largely self-regulating environment, reducing the risk
of overheating and the need for mechanical cooling.
Thermal mass in winter
The ability of heavyweight buildings to stay cooler during the
summer by soaking up heat on warm days is fairly well understood.
Perhaps less well understood is that this daily cycle of absorbing
and releasing heat continues on a year-round basis and can reduce
the energy needed to keep a heavyweight building warm during the
heating season. This works through the ability of thermal mass to
capture and recycle heat gains from south facing windows, along
with those produced by lighting, people and appliances. As the
temperature drops overnight, this is slowly released back into the
building, helping keep it warm and reducing the need for
supplementary heating. Whilst lightweight buildings are also
capable of doing this, the extent to which the heat gains can be
utilised increases with the level of thermal mass; something that
is now recognised in SAP which is compliance tool for
Part L1A of the Building Regulations which deals
with new dwellings. For a more detailed overview of thermal mass
and how it can be used, see:
Thermal Mass Explained, and for an example of how it can
enhance SAP /Part L1A compliance see:
Zero Carbon Performance - cost-effective concrete and masonry
homes.
How is thermal mass measured?
Part L of the Building Regulations and its associated compliance
tools (SAP & SBEM) account for thermal mass using k-values
(kJ/m2K), which provide a gauge of thermal capacity per square
meter of floor or wall. A lightweight wall might have a k-value of
around 10 kJ/m2K, whilst for a heavyweight wall it can be up to 230
kJ/m2K. Some generic k-values for various types of construction are
published in Table 1e of SAP 2009, whilst more comprehensive values
for concrete and masonry constructions can be found in:
Thermal Performance: Part L1A. Alternatively, bespoke k-values
and other thermal mass related information can also be calculated
using a free
Thermal Properties Calculator
Describing a material or construction as having high, medium or
low thermal mass gives a useful indication of its ability to store
heat, as does its k-values. But, in order to know how effective it
will be in practice, there are a couple of other important factors
that need to be taken into account. These are firstly the length of
time available to get heat in and out of the material, which is
typically assumed to be 24 hours (i.e. heating during the day and
cooling at night), and secondly, the rate of heat flow at its
surface i.e. through carpet, plasterboard, tiles etc. These factors
are both accounted for in admittance values, which provide a simple
means of assessing the approximate in-use heat absorption
performance of walls and floors etc. For more information see:
Thermal Mass Explained
Admittance and k-values relate to the absorption of heat at the
inner surface of a wall and floor, which is the most significant
function of thermal mass in building design. There is however
another thermal mass related property called decrement which has a
bearing on summertime performance. Decrement describes the way in
which the density, heat capacity and thermal conductivity of an
external wall (for example), can slow the passage of heat from one
side to the other (decrement delay), and also reduce those gains as
they pass through it (decrement factor). For more information see:
Thermal Mass Explained and for a comprehensive range of
decrement values for concrete and masonry construction see:
Thermal Performance: Part L1A
Does thermal mass have any disadvantages?
In summer, thermal mass is only beneficial if nighttime ventilation
(or some other means) can be used to cool it down. Local issues
such as, pollution and security concerns can sometimes make this
impracticable, although there are often ways to overcome these
problems. In winter, older heavyweight buildings with comparatively
low levels of insulation and poor airtightness often required a
relatively long pre-heat period to warm up the fabric, resulting in
a slightly more fuel being used than in a similar lightweight
building. However, the greatly improved standard of fabric
performance in new build means this is no longer the problem it
once was, as the fabric retains much more of its warmth during
periods when the heating is off. In practice, the ability of
thermal mass to reduce the cooling load in many building types,
particularly offices, is far more significant than the preheat
issue.
Nevertheless, in some types of intermittently occupied
buildings, for example a weekend holiday cottage, thermally
lightweight construction may still be the best option where heating
is concerned, as it will enable a more rapid warm up period. For
new mainstream housing, the preheat issue is negligible, and as we
move towards the 2016 target for new homes to be zero carbon, the
increasing standard of insulation and airtightness ensures the
passive benefits of thermal mass during the heating season are of
more significance.
How long is the CO2 payback for thermal
mass?
This very much depends on the building type and the way in which
its thermal mass is used. For commercial buildings such as offices,
it should first be noted that the embodied CO
2 in a
typical office design is more or less the same regardless of
whether it has a steel or concrete frame; a conclusion reached by
studies undertaken by both the steel and concrete sectors. So, for
many buildings it’s a moot question. In the case of a very
heavyweight building, the additional concrete can result in a
slightly higher level of embodied CO2, but the operational savings
afforded by the thermal mass will typically offset this in a matter
of months rather than years.
For housing the situation is slightly different: The embodied
CO2 in a typical masonry home is about 4% higher than an equivalent
timber frame home. Studies by Arup and the NHBC Foundation both
arrived at this figure. The Arup study went on to look at
operational impacts and found that the passive benefits of thermal
mass during the heating seasons resulted in CO2 savings that offset
the figure of 4% in around 11 years.
In respect of summertime performance, the CO2 payback for
housing is less tangible as domestic air conditioning is not
particularly common, although this may change as the effects of our
warming climate are felt in the future. The Arup study considered
the impact of climate change and found that lightweight homes are
at greater risk of frequent overheating in the coming years, even
when enhanced shading and ventilation are used to mitigate higher
temperatures. At this point, the use of air conditioning may become
necessary, resulting in increased annual CO2 emissions compared to
heavyweight home which was shown to delay the occurrence of
frequent overheating by a further 20 to 60 years depending on the
level of thermal mass.
For further details of the Arup study please download the full
report by
clicking here.
For more information on the embodied CO2 in steel and concrete
frame building see:
Concrete Structures 10