Unit 317 Session 5 principles of heat loss and heat gain in dwel

​​The principles of heat loss and heat gain in dwellings

​So far in this unit we have looked at different types of heating system, open-vented, sealed systems and underfloor heating.

But why do we need heating at all?

Open Ended

Why do you think we need heating systems? Once we heat a home, why doesn't it stay warm?

​The principles of heat loss and heat gain in dwellings

​In this section, we will investigate the heat loss and heat gain of a building.

Both principles rely on the steady state transfer of heat through the fabric of the building itself.

Open Ended

What do you think we mean by the term 'Fabric of the building' ?

​The building fabric is the components and materials that the building itself is made of, such as the walls, floors, roof, windows and doors.

​Heat loss

Heat loss from a building is the reason why we need central heating systems in our homes, offices, shops and factories.
The heat the building loses will have to be replaced with more heat to maintain a comfortable temperature within the building itself.
The principles of heat loss are known as ‘steady state thermal characteristics’ of a building.

Heat loss from a building is measured in watts and occurs two ways:

● through the building fabric

● due to ventilation.

​Heat loss through the fabric of the building

The thermal transmittance of heat from a building to the outside is known as U-values.

U-values express, for the purposes of calculation, the rate of heat transfer through the building structure (its walls, floors, ceilings, roofs and windows) and because the construction of buildings varies so much due to different materials and construction methods, the U-values vary too.

This means that on occasion the U-value for a particular building will need to be calculated from scratch.

​The calculation of a U-value for, say, an outside

wall, is based upon the heat loss of each element or material used in the wall.

​Each element will have its own heat loss measured in watts per metre kelvin (W/mK) known as a K-value.

​The units used to express U-values are watts per m² kelvin (W/m²K).
This means that if a wall, for example, had a U-value of 1.0 W/m²K, for every degree of temperature difference between the air on the surface inside the wall and the air on the surface outside, 1 watt of heat would pass through any m².

​2⁰C

​20⁰C

​In the picture we see the inside temperature is 20⁰C and the outside temperature is 2⁰C, this means an 18⁰C temperature difference, therefore the wall would loose 18 W/m²K per m².
This means if the wall measured 10m² then 180 watts of heat would be lost through the wall.

Multiple Choice

What would the loss in W/m²K be if the 1m² wall in the picture had a 24⁰C internal temp and -2⁰C external temp?

• 24 Watts

• 22 Watts

• 26 Watts

• 48 Watts

Multiple Choice

What would the loss in W/m²K be if the 15m² wall in the picture had a 24⁰C internal temp and -2⁰C external temp?

• 240 Watts

• 320 Watts

• 260 Watts

• 390 Watts

​So, it follows that the smaller the U-value, the better the wall is at keeping the heat in.

Most materials have published K-values for the rate of thermal conductance through them and this is measured under specific conditions.

​K-values for the wall construction:

Brick = 0.77 W/mK

Cavity (Air) = 0.18 W/mK

Thermalite block = 0.11 W/mK

Mineral wool insulation = 0.38 W/mK

Plaster board = 0.25 W/mK

​Because each material is of a specific thickness (such as 102 mm brick), the thickness of the material must be divided by the K-value to obtain the R-value:

​Brick = 0.102 m ÷ 0.77 W/mK = 0.132 (R4)

Cavity = 0.050 m ÷ 0.18 W/mK = 0.277 (Ra)

Thermalite block = 0.100 m ÷ 0.11 W/mK = 0.909 (R3)

Mineral wool insulation = 0.050 m ÷ 0.038 W/mK = 1.315 (R2)

Plaster board = 0.0125 m ÷ 0.25 W/mK = 0.050 (R1)

​K-values for the wall construction:
Brick = 0.77 W/mK

Cavity (Air) = 0.18 W/mK

Thermalite block = 0.11 W/mK

Mineral wool insulation = 0.38 W/mK

Plaster board = 0.25 W/mK

​So:

​Brick = 0.102 m ÷ 0.77 W/mK = 0.132 (R4)

Cavity = 0.050 m ÷ 0.18 W/mK = 0.277 (Ra)

Thermalite block = 0.100 m ÷ 0.11 W/mK = 0.909 (R3)

Mineral wool insulation = 0.050 m ÷ 0.038 W/mK = 1.315 (R2)

Plaster board = 0.0125 m ÷ 0.25 W/mK = 0.050 (R1)

​K-values for the wall construction:
Brick = 0.77 W/mK

Cavity (Air) = 0.18 W/mK

Thermalite block = 0.11 W/mK

Mineral wool insulation = 0.38 W/mK

Plaster board = 0.25 W/mK

​Calculate the 'U' value of the external wall shown in the picture.

Open Ended

Calculate the 'U' value of the external wall shown in the picture.

​Calculate the 'U' value of the external wall shown in the picture.

​Rsi 0.130

R1 0.06

R2 1.97

R3 1.36

R4 0.42

R5 0.14

Rso 0.040

= 4.12

1 ÷ 4.12 = 0.24 W/mK

​Now try one yourself,
Take your time.

Open Ended

Calculate the 'U' value of the external wall shown in the picture.

​Rsi = 0.130

R1 = 1

R2 = 2.63

R3 = 1.59

R4 = 0.31

R5 = 0.13

Rso = 0.040

= 5.83

1 ÷ 5.83  = 0.17 W/mK