U-values

The amount of heat energy lost from a house will depend on a number of things:

If it is very cold outside and very hot inside more heat energy will be lost than if there is only a small difference. Of course, if the temperature outside is the same as the temperature inside then no heat energy will be lost at all!

So the amount of heat energy lost will depend on the TEMPERATURE DIFFERENCE.

If we double the area of a wall then we double the amount of heat energy, passing through the wall at any time.

So the heat energy lost will depend on the AREA,

The amount of energy lost will depend on what the surfaces are made of and how thick they are. So that we can work out heat losses from houses, tables of values are given for different building materials. These are called U VALUES.

The U value tells you how much heat energy will pass through a 1 m² piece of the material in 1 second if there is a 1°C temperature difference between the two sides of the material.

Here are some U-Values for building materials:

BUILDING MATERIAL	U-Value / (W m^{-2 o}C^-1)
Roof Uninsulated roof Insulated room Wall Cavity wall Cavity filled wall Floor without carpets with carpets	2.0 1.0 1.0 0.5 1.0 0.3
Windows and doors: Single glazed Double glazed	5.0 2.5

EXAMPLE: Calculatie the heat energy lost per second from a poorly insulated house when the temperature inside the house is maintained at 18^oC by the central heating system and the temperature outside is only 2^oC. The dimensions are as follows:

area of uninsulated roof =150 m²

area of walls = 300 m²

area of single-glazed windows and doors = 50 m²

floor area (uncarpeted) = 150 m²

Temperature difference (DT) = temperature inside - temperature outside

= 18 - 2 = 16°C

Heat energy lost per second = U-value (W/m²°C) x area (m²) x DT (°C)

Heat energy lost per second through:

the roof = 2 x 150 x 16 = 4800 W

the walls = 1 x 300 x 16 = 4800 W

the windows and doors = 5 x 50 x 16 = 4000 W

the floor = 1 x 150 x 16 = 2400 W

Total heat energy lost per second through the whole house

= 4800 + 4800 + 4000+ 2400

= 16000 W

= 16 kW

Therefore to keep the house 16°C warmer than the outside would require a constant 16 kW output from the heating system.

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Q1. Use the table of U values above to calculate the heat energy lost per second through a 1m² single glazed window.

a) when the temperature outside is 0°C and inside is 20°C.

b) when the temperature outside is 10°C and inside is 20°C.

Q2. Use the table of U-values to calculate the heat energy lost per second from this well insulated house when the temperature difference = (temperature inside - temperature outside) = 18 - 2 = 16^OC .

area of insulated roof = 150 m²
area of cavity filled walls = 300 m²
area of double-glazed windows and doors = 50 m²
floor area (carpeted) = 150 m²