U Values 2015 Leaving Cert Construction

**Questions and Solutions from SEC**

Using the following data, calculate the U-value of the external wall of a new dwelling house. The wall is of concrete block construction, with a 250 mm cavity. The cavity is a full-fill insulated cavity, with polystyrene bead insulation.

Specification of external wall:

External render thickness 16 mm

Concrete block outer leaf thickness 100 mm

Cavity width 250 mm Concrete block inner leaf thickness 100 mm

Internal plaster thickness 12 mm

Thermal data of external wall:

Resistance of external surface (R) 0.048 m2 °C/W

Conductivity of external render (k) 0.720 W/m °C

Conductivity of concrete blocks (k) 1.440 W/m °C

Conductivity of polystyrene bead (k) 0.031 W/m °C

Conductivity of internal plaster (k) 0.220 W/m °C

Resistance of internal surface (R) 0.122 m2 °C/W

Using the U-value obtained at 5(a) and the data given below, calculate the cost of heat lost annually through the external walls of each of the following:

• the new house - as specified at 5(a) above and

• an existing house that complies with current Building Regulations, having a U-value of 0.21 W/m2 °C for the walls.

Data for both houses:

Area of external wall of each house 150 m2

Average internal temperature 20 °C

Average external temperature 6 °C

Heating period 8 hours per day, every day for 35 weeks per annum

Cost of oil 95 cent per litre

Calorific value of oil 37350 kJ per litre

1000 watts 1kJ per second.