Building Services Engineering 5th Edition Handbook

Energy economics 51
3.0
2.0
Total cost
Costs (£)
Cost of thermal insulation
1.0
Fuel cost of lost heat
Economic thickness
of insulation
0
0.025 0.050 0.075 0.100 0.125 0.150
Insulation thickness (m)
2.3 Economic thickness of the wall insulation in Example 2.12.
EXAMPLE 2.13
The roof of a factory in Lancashire is insulated with expanded polystyrene (EPS) slab. The
locality has 3070 degree days for a base temperature of 18 ◦ C and a heating season of
235 days. The average internal air temperature is 20 ◦ C. The EPS costs £65 per cubic metre
fitted, interest is charged at 7% and the life expectancy of the insulation is 45 years.
Heat costs £8.50 per gigajoule. The thermal conductivity of EPS is 0.035 W/mK. Find the
economic insulation thickness.
The original thermal transmittance of the single sheet roofing is 6.0 W/m 2 K.
α = 7% interest per annum + 2% depreciation per annum
= 9%
C = £8.50
GJ
× 1GJ
10 3 MJ
= £8.50 × 10 −3 per MJ
[
]
0.035 × 9 × 65
1/2
U e =
8.64 × 8.5 × 10 −3 W/m 2 K
× (3070 + 235 × (20 − 18))
= 0.28 W/m 2 K