Using the following data, calculate the U-value for the external wall of a house, built in the 1970s: External plaster thickness 16 mm Block outer leaf thickness 100 mm Cavity (un-insulated) width 100 mm Block inner leaf thickness 100 mm Internal plaster thickness 13 mm Thermal data of external wall : Conductivity of plaster (k) 0.430 W/m °C Conductivity of blockwork (k) 1.440 W/m °C Resistance of external surface (R) 0.048 m² °C/W Resistance of cavity (R) 0.170 m² °C/W Resistance of internal surface (R) 0.122 m² °C/W Using the following data, calculate the cost of the heat lost annually through the un-insulated external wall: Area of external wall 145 m² Average internal temperature 18 °C Average external temperature 5 °C U-value of wall as calculated at (a) above Heating period 10 hours per day for 42 weeks per annum Cost of oil 68 pence per litre Calorific value of oil 37350 kj per litre 1000 Watts = 1kj per second - Leaving Cert Construction Studies - Question 5 - 2007

Now, using the U-value calculated above, the heat loss can be calculated with the following formula:

$\text{Heat Loss} = U \times \text{Area} \times (T_{inside} - T_{outside})$

Substituting the values:

• Area = 145 m²
• $T_{inside} = 18 °C$
• $T_{outside} = 5 °C$:

$\text{Heat Loss} = 1.858 \times 145 \times (18 - 5)$

Calculating: $\text{Heat Loss} = 1.858 \times 145 \times 13 = 3482.25 \text{ Watts}$

For annual heat loss over the heating period:

• Heating period = 10 hours/day for 42 weeks = 10 x 7 x 42 = 2940 hours

Total heat loss in Joules: $\text{Total Heat Loss} = 3482.25 \times 2940 = 10227309 Joules$

To calculate the cost of heating:

1. First, convert the heat loss to litres of oil required:

• Calorific Value of 1 Litre Oil = 37350 KJ, therefore: $\text{Litres Required} = \frac{10227309}{37350} \approx 273.74 \text{ Litres}$

2. Now, calculating the cost:

• Cost per litre = £0.68
• Total cost: $\text{Cost} = 273.74 \times 0.68 \approx £186.94$

One proposed method to insulate the external walls involves cavity wall insulation. This can be achieved by:

• Blowing or injecting insulation material (e.g., Polystyrene Bead or Rockwool) into the cavity, ensuring it is adequately spaced.
• This method is effective, maintaining moisture control and minimizing heat loss, thus meeting Building Regulations for thermal performance.

Sketch: Consider a diagram showing the application of insulation with clear labels indicating the cavity, insulation, and outer brick layer.