Further Research
Section 1
Information
1. How heat is lost
All forms of energy used in a house eventually become heat. This heat can escape as the result of one or more of three processes:
- Heat conduction
- Heat convection
- Heat radiation
The amount of heat lost will depend upon the thermal conductivity of the materials to be used to build the house. Total heat loss can be calculated for any building.
Definitions
a) Heat conduction
This is the transfer of heat through a medium. In houses it is the main process by which heat passes through solid barriers such as walls, ceilings, floors and windows.
The rate of heat conduction depends on:
- the thickness of the barrier
- its area
- the material from which the barrier is made
- the temperature on either side of the barrier
Heat flows from the warm to the cold side.
b) Heat convection
The movement of warm air from warm places, to be replaced by cooler air.
Warm air rises because it has a lower density than cold air.
Air itself is a good insulator if it is still. But warm air tends to rise. This is the basis of most heating systems for houses. It also increases the amount of heat likely to escape through walls, ceilings and windows.
Draughts and winds tend to cause greater heat loss.
c) Heat radiation
Heat radiation is electromagnetic radiation. It differs from light or radio waves only in the wavelength that can be transferred from a hot surface to a cooler one when there is nothing to conduct it and when there is no convection. Radiation is a significant cause of heat loss through windows.
Radiation allows heat to be transferred through a vacuum. Dull, black surfaces are good radiators: shiny metallic surfaces are poor radiators.
d) Heat loss
Heat loss is the rate at which heat passes through a material. This depends on:
- its coefficient of thermal conductivity
- its thickness
- its area
- the material from which it is made
- the difference in temperature between the sides of the material
Different building materials have different thermal conductivities.
Coefficients of thermal conductivities of building materials
| Asphalt |
1.2 |
Glass wool |
0.04 |
| Breeze blocks |
0.5 |
Hardboard |
0.13 |
| Bricks |
0.84 |
Linoleum |
0.22 |
| Cardboard |
0.1 |
Metals: |
|
| Carpet |
0.05 |
aluminium |
160.0 |
|
|
copper |
200.0 |
|
|
steel |
50.0 |
| Concrete |
1.8 |
Plasterboard |
0.16 |
| Corkboard |
0.4 |
Polystyrene |
0.03 |
| Felt (roofs) |
0.2 |
PVC (floor) |
0.4 |
| Glass |
1.05 |
Plastic sheet |
0.2 |
|
|
Stone |
1.75 |
| Air |
0.026 |
Thatch |
0.08 |
| Water |
0.6 |
Wood:soft |
0.13 |
|
|
|
0.15 |
2. How to calculate total heat loss
The rate at which a building loses heat depends on a combination of conduction, convection and radiation. This overall heat loss measured in watts per square metre per degree Celsius is called its U value. The smaller the U value the smaller the heat loss.
U values of main building structures
Structure
| Walls: |
|
| Solid brick wall (22 cm thick) |
2.2 |
| Brick cavity wall + plaster (26 cm thick) |
1.4 |
| Cavity wall: brick outer, block inner (26 cm thick) |
0.93 |
| Cavity wall with insulation in cavity |
0.69 |
| Concrete 20 cm thick |
2.9 |
| Timber frame with weatherboard + fibre in cavity |
0.61 |
|
Roof: |
|
| Concrete with asphalt |
3.1 |
| Tiles on roofing felt - plasterboard ceilings |
0.45 |
| Tiles on roofing felt - plasterboard ceilings + 10 cm of fibreglass insulation |
0.31 |
|
Floors: |
|
|
Concrete |
0.45 |
| Concrete: with carpet |
0.26 |
| Suspended wooden floor |
0.6 |
| Suspended wooden floor with carpet |
0.45 |
|
Glazing: |
|
| Single layer |
4.0 |
| Double with 12 mm air span |
2.4 |
The total heat loss through a solid brick wall 10m2 in area when the temperature inside the house is 20°C and that outside 10°C can be calculated using the formula:
Heat loss = Area x U value x difference in temperature
= 10m2 x 2.2 x (20-10) watts
= 220 watts
So to keep the temperature inside at 20°C requires an input of 220 watts.
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