1. Insulation, Pay back time and U Values
06 April 2017
Insulation, Pay back time and U Values
Objectives
Be able to describe different types of insulation, calculate pay back time and use U-Values.

2. 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.

3. 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).

4. 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.

5. 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.

6. 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.

7. 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.

8. Task
In Pairs: Create a poster about Insulation types.

9. 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)

10. 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.

11. Questions – for HW

12. U-Values Insulating materials for buildings are given U-values.
The lower the U-value, the better the material is as an insulator.

13. 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

14. 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.