The external wall of a house built in the 1990s is of 100 mm concrete block construction with a 100 mm partially filled cavity, as shown - Leaving Cert Construction Studies - Question 5 - 2019

To calculate the U-value of the wall, we first need to find the total resistance (R) of each layer and then apply the formula for U-value.

1. Resistance Calculation:

   • External Render:
     • Thickness = 0.016 m,
     • Resistivity = 0.048 m² °C/W.
   • Concrete Block Outer Leaf:
     • Thickness = 0.1 m,
     • Conductivity = 1.440 W/m °C.
   • Clear Cavity:
     • Resistance = 0.170 m² °C/W.
   • Polystyrene Insulation:
     • Thickness = 0.1 m,
     • Conductivity = 0.037 W/m °C.
   • Concrete Block Inner Leaf:
     • Thickness = 0.1 m,
     • Conductivity = 1.440 W/m °C.
   • Internal Plaster:
     • Thickness = 0.012 m,
     • Conductivity = 6.250 W/m °C.
   • Internal Surface Resistance:
     • R = 0.012 m² °C/W.

2. Total Resistance Calculation:

   Using the resistances: $R_{total} = R_{external} + R_{outer} + R_{cavity} + R_{insulation} + R_{inner} + R_{plaster} + R_{internal}$

   • To find U-value: $U = \frac{1}{R_{total}}$
   • Calculating this gives us:

   $R_{total} = 0.048 + 0.069 + 0.170 + 2.703 + 0.069 + 0.005 + 0.012$

   • Conclusively, calculating yields: $U = 0.516 \, \frac{W}{m^2 \, °C}$