Building Services Engineering 5th Edition Handbook

78 Heat loss calculations
operation in the room. Indoor scaffolding will be needed. Care must be taken not to lay
the insulation on top of luminaires and electrical cables, to avoid the overheating of lamps
and wiring.
Q = 1
R si
W
m 2 K × A m2 × (t 1 − t 2 ) K
= 1 × (19 − 17.6) W
0.1
= 14 W
14 W = U W
m 2 K × 1m2 × (19 − 5) K
U = 14
19 − 5 W/m2 K
= 1 W/m 2 K
R n = 1
0.25
m 2 K
W
= 4m 2 W/K
resistance of the existing roof = 1 m 2 K
1 W
= 1m 2 K/W
insulation resistance = (4 − 1) m 2 K/W
= 3m 2 K/W
glass fibre thickness l mm = 3 m2 K
W
= 120 mm
× 0.04 W mK × 103 mm
1mm
Glass fibre thickness to be used will be 150 mm.
R n = 1 1 + 0.15
0.04
= 4.75 m 2 K/W
U n = 1 W
4.75 m 2 K
= 0.21 W/m 2 K
m 2 K
W
The new thermal transmittance is below the desired value of 0.25 W/m 2 K and is suitable.
Check that the new U value is correct:
R n = 0.1 + 0.012
0.06 + 0.15 0.015
+ 0.17 +
0.04 0.38 + 0.019
0.5 + 0.025
0.96 + 0.05 m2 K
W
= 4.73 m 2 K/W