07 April 2015
Insulation, Pay back time and
U Values
Objectives
Be able to describe different types of
insulation, calculate pay back time and
use U-Values.
Revision
• Heat flow
• Heat flows from a hot object
to a cool object.
• If the difference is greater
then the heat energy will be
transferred quicker.
• To limit or stop heat flow we
need to limit or stop the
transfer by conduction,
convection and radiation.
Insulator
• An insulator is a material that does not transfer
heat well.
• Insulators in homes slow down the transfer of
heat from the warmer internal rooms the cooler
outside.
• As you can see in the diagram, a most of the
energy is lost from the walls (since they have a
big surface area) and the roof (as warm air is
less dense and rises).
Pipe Insulation
• Glass fibres are long and thin so don’t
conduct heat very well.
• The material is mainly air which is a bad
conductor (because the particles are a
long way apart).
• The glass fibres stop the air circulating so
there is no transfer by convection.
• The outer aluminium surface is a poor
emitter of infra-red radiation.
Double Glazing
• Two layers of glass conduct
heat less than one.
• The air space insulates
because air is a bad conductor.
• The air space is narrow
(16mm) so that the air cannot
circulate and transfer heat by
convection.
• Modern double glazing uses
coated glass that reflects some
of the infra-red radiation back
into the room.
Loft Insulation
• Glass fibres are long and thin so don’t conduct
heat very well.
• The material is mainly air which is a bad
conductor (because the particles are a long way
apart).
• The glass fibres stop the air circulating so there
is no heat transfer by convection through the
insulating material.
• NB the ceiling stops the warm air escaping from
the room.
Cavity Wall Insulation
• Two layers of masonry mean
that moisture can’t soak through
the wall (reducing cooling by
evaporation).
• The air space does not insulate
well because although air is a
bad conductor it can circulate
and transfer heat by convection.
• The air space is filled with foam,
fibres or polystyrene balls so
that the air cannot circulate and
heat transfer by convection is
reduced.
• Modern cavity walls are built
with solid foam panels in the
cavity. These are covered in
aluminium foil to reduce heat
transfer by radiation.
Task
• In Pairs: Create a poster about Insulation
types.
Payback time
• All these examples of insulation cost money to
install but save money on heating bills. The
time it takes to pay back the cost of the
insulation because of the savings made by
installing it is called the payback time.
payback time = initial cost of installing insulation (£)
(years)
savings per year (£/year)
Example:
• If it costs £1000 to install double glazing and by
doing so it saves £100 a year in heating bills and
you saved up that £100 every year, in ten years
the cost of installation would be paid back. So the
payback time would be ten years.
• Questions – for HW
U-Values
• Insulating materials for buildings are given
U-values.
• The lower the U-value, the better the
material is as an insulator.
Typical Values for Cavity wall
insulation
Walls
Material
Description
"U" Factor
Flat Metal
0" Fiberglass Insulation
1.20
1" Fiberglass Insulation
0.22
2" Fiberglass Insulation
0.12
3" Fiberglass Insulation
0.09
4" Fiberglass Insulation
0.07
6" Fiberglass Insulation
0.05
8" Fiberglass Insulation
0.041
12" Fiberglass Insulation
0.027
Example question
• Different parts of a house have different
‘U-values’.
• The diagram shows some U-values for the
walls, the roof and the windows of a house
with and without insulation.
• The U-value for a cavity wall with
insulation is different from the U-value of a
cavity wall
• without insulation.
• 1) Using the diagram, what is the
percentage change in U-value when cavity
wall insulation is fitted?
• 2) Which of the following statements is
correct?
• 1 Double-glazed windows have a higher
U-value than single glazed windows.
• 2 The higher the U-value, the higher the
rate of loss of heat.
• 3 Putting insulation in the loft, halves the
U-value of the roof.
• 4 U-values are always greater than 1.