More on methods of reducing heat transfer - thermal conductivity
e.g. in a house and other situations to minimise heat losses
an investigation of the insulating properties of materials
Doc Brown's Physics Revision
Suitable for GCSE/IGCSE Physics/Science courses or
Whatever 'heat system' we are dealing with, heat energy is
always lost. You can never get from an energy store, a 100%
efficient conversion to useful energy. Therefore, it is of great importance to minimise
heat losses and save money in the process! A good example is how to save money in the home or any
other building where heating systems of some form are used. This page also
describes a simple experiment to investigate the insulation effectiveness of
may need to be able to use your knowledge and understanding to
Compare ways in which energy is
transferred in and out of objects by heating and ways in which the rates of
these transfers can be varied.
Evaluate the design of everyday
appliances that transfer energy by heating, including economic
considerations eg reducing unwanted heat energy transfers - heat losses cost
Evaluate the effectiveness of different types of material used for insulation,
including thermal conductivity (eg U-values - a measure of the rate of heat
transfer) and economic factors including payback time.
Evaluate different materials according to their
specific heat capacities.
The heat specific heat capacity
in simple terms is how much energy (J) is needed to heat a specific mass (1
kg) by one degree oC.
Examples may have studied
include the use of water, which has a very high
specific heat capacity,
oil-filled radiators and
electric storage heaters containing concrete or bricks.
Reminder of particle theory: There
is always a net transfer of thermal energy from hot materials to colder ones
... conduction of thermal energy
through the bulk of a substance, where higher kinetic energy particles
either bump into (liquids or gases) or vibrate against, lower kinetic energy
particles, so that thermal energy is transferred.
... convection involves the bulk
movement of particles, the hotter higher KE particles in gases or liquids
space out more lowering the density of them and so will rise with respect to
the surrounding cooler fluid. These convection currents are effectively a
'buoyancy' current because the less dense warmer fluid is trying to float on
the cooler more dense fluid.
... infrared - thermal radiation - surface particles
of a material at a higher temperature will emit more infrared radiation than
a colder material surface. All material surfaces are constantly absorbing
and emitting infrared, but there will be net transfer of thermal radiation
from a hotter thermal energy store to a cooler one.
Applications of heat transfer science
All materials conduct heat to a
greater or lesser degree.
The thermal conductivity of a
material is a measure of how efficiently heat is transferred through a
material by conduction.
Materials like metals are very good
heat conductors and transfer thermal energy very quickly.
Materials like stone, brick, wood and
concrete etc. are poor heat conductors and have low thermal
conductivities - therefore can be used as good thermal insulation
Thermal conductivity data is particularly
important when considering the material required to fulfil a particular
application e.g. in heating systems when in one situation you might want
good insulation and in another rapid heat transfer.
Clothes and blankets trap pockets of
Air is a poor conductor so the
trapped air provides an effective layer of insulation.
Because of the porous nature of the
material, your body heat cannot be conveyed away by convection.
Heating and insulating
U-values measure how
effective a material is as an insulator.
What is the U-value of a
material? What does the U-value mean?
The U value of a material is related to
its thermal conductivity.
The U-value of a material gives
a numerical value of how efficient heat is transferred through a material.
Materials with high U-values are
relatively good (faster) conductors of heat (poorer insulators) and those with low
U-values are relatively good insulators (poorer-slower heat conductors)
The lower the U-value, the
better the material is as an insulator.
Conserving energy in the home
Methods of reducing the rate of heat
energy transfer reduce costs and also in there own small way contribute to
reducing global warming by using less fossil fuel based energy supplies.
What is effective? What is
cost effective? Not always the same! Payback time?
Loft insulation - cheap and
effective - a thick layer of fibreglass wool or similar material laid all over the loft floor
and reduces conduction and convection of heat lost through the roof -
payback time a few years.
Thick walls made from a good
insulating material with a low thermal conductivity.
The thicker the wall, the lower the rate of heat transfer, a slower
rate of cooling, better heat retention due to a lower thermal conduction
Good insulating materials (poor heat
conductors), irrespective of thickness, are brick, stone and breeze blocks
Cavity wall insulation is another
technique for heat retention -
insulating foam injected between two brick walls (inner wall and outer wall) - reduces
heat loss by conduction and convection across the walls - quite costly, payback time a
Air is a poor conductor so the trapped
air provides an effective layer of insulation and because of the porous
nature of the foam, heat cannot be conveyed away by convection.
Sometimes the gap is just left with air in, a sort of
brick/breeze block equivalent of double glazing windows, but still
heat loss mainly by conduction because air has a very low thermal
Hot water tank jacket - cheap
and effective - a jacket of lagging of a foam filled plastic cover reduces
conduction and radiation heat losses - quick payback time.
Double glazing of glass windows - expensive,
longer payback time - insulating air is trapped between the glass panes a
few mm apart, so
reducing heat losses mainly by conduction.
Draught-proofing - cheap and effective
draught excluders, a few years payback time - strips of foam or plastic around door
frames, thick curtains across the windows - all of these measures reduce
heat loss from the house mainly by convection i.e. warm air in rooms
brushing against cold surfaces like windows or warm air moving to colder
unused parts of the house.
Pipes - hot water pipes can be
covered in insulation to minimise heat losses by conduction and convection -
often foam which traps an insulating layer of air.
The most effective methods of
insulation give you the biggest annual savings of heat energy, but the most cost-effective
methods tend to be the cheapest. For double glazing and cavity wall
insulation you need to think long-term to get your money back.
A simple experiment to investigate
the insulating effectiveness of various materials.
How can we get a simple comparative
measure of how effective a material is in reducing the rate of heat
Below is illustrated a simple experiment
to get a comparative thermal conductivity value for a solid material.
The basic idea is to use a container e.g.
a copper calorimeter filled with a fixed amount of water and covered with a
lid to seal the system and minimise heat losses from the surface of the
The lid must have a hole in it to allow
the thermometer to be suspended in the bulk of the water.
The copper vessel, water, thermometer and
insulation constitutes the system and we are investigating heat loss
from this system.
The copper vessel is quite good to use
because its made of a good conductor just like an immersion cylinder in a
Therefore any decent insulating material should be quite effective in
reducing the rate of heat loss i.e. rate of temperature fall.
You can experiment with materials like
carpet, bubble wrap, cotton wool, polystyrene etc. which are poor heating
conducting non-metallic materials that also trap air to a greater or lesser
The hot water can be quickly produced in
an electric kettle and measure out a fixed amount eg 100 cm3 into the copper
It is important not only to keep the
water volume constant, but also the same starting temperature.
You take the initial temperature and then
allow the system to cool for eg 10-20 minutes, re-measure the temperature and
calculate how much the temperature has fallen.
If using a single temperature difference
measurement you must run the experiment for the same length of time.
The smaller the temperature fall the
better the insulating properties of the material - the lower its thermal
You can be more elaborate with the
experimental measurement by taking a series of readings every few minutes
and then plotting temperature versus time to produce cooling curve graphs.
The initial steepness of the graph line (a
negative gradient) gives you a measure
of the heat flow from the system.
The greater the temperature fall, or
the greater the rate of temperature fall, the greater the heat conductivity
of the material and the less effective the insulating capacity of the
By doing the experiment without an
insulating layer you get a sort of baseline value.
You then repeat the experiment with
different solid materials, and, although not always possible or convenient,
ideally each insulating layer should be of the same thickness.
You will have to poor the water away each
time and allow the container to cool down before repeating the experiment
with the same amount of water and start temperature.
You can attempt to cover all the surface
of the 'system' but at the very least wrap the insulating material around
the curved side of the copper container, even just this will give valid
You can also try out different
thicknesses of the same material.
In this experiment you can use sheets of polystyrene foam,
thick plastic sheeting (not expanded), sheets of newspaper, layer carpet, wool, cotton wool,
bubble wrap, felt, fibre glass etc.
i.e. anything you can conveniently wrap around the copper container!
You should find that materials trapping
air e.g. wool or foam will show smaller temperature falls, better heat
insulators than bulk materials like newspaper or thick plastic sheeting.
You should also find that the greater the
thickness of the material the better the heat retention, so the temperature
fall should be roughly inversely to the thickness.
Here are some typical relative thermal conductivity
values for a variety of materials.
aluminium 205, brass 109, cast iron 58,
steel alloys 16 to 43, stone 1.7, dense brick 1.3, concrete 0.4 to 1.7,
common brick 0.6 to 1.0, glass 0.8 to 0.9, water 0.58, lightweight concrete
0.1 to 0.3, non-expanded plastics 0.1 to 0.5, wood 0.14 to 0.19 (timber
dependent), insulating brick 0.15, balsa wood 0.05, wool/felt insulation
0.04 to 0.07, fibre glass 0.04, glass wool insulation 0.04, wool blankets
0.04, cotton wool insulation 0.03, plastic foams 0.02 to 0.03
I've included a wide variety of
materials, even though they obviously will not be used as heat insulators,
but a high thermal conductivity is important if you do want to transfer heat
energy efficiently e.g. a metal radiator.
====> the lower the thermal conductivity
value, the better the insulating effectiveness of the material
the wide difference between metals and
non-metals in thermal conductivity
the increased insulation effectiveness
when air is trapped e.g. glass and glass wool, bulk plastic and expanded
plastic eg plastic foams
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