Building Services Engineering 5th Edition Handbook

Condensation in buildings 253
The vapour pressure and corresponding dew-point temperature can now be calculated for
each numbered point through the wall:
p s3 = 1318.09 N GN s
50 × 0.013
m2 kg
× 109 N
1GN × 88.7 kg
10 9 m 2 s
= (1318.09 − 57.7) N/m 2
= 1260.4 Pa
t dp3 = 1 (
0.0684 × ln p
) s3 ◦C
600.245
= 1 ( ) 1260.4 ◦
0.0684 × ln C
600.245
= 10.85 ◦ C
p s4 = 1260.4 − 30 × 0.1 × 88.7 Pa
=994.3 Pa
t dp4 = 1 ( ) 994.3 ◦
0.0684 × ln C
600.245
= 7.38 ◦ C
p s5 = 994.3 − 6 × 0.04 × 88.7 Pa
= 973 Pa
t dp5 = 1 ( ) 973 ◦
0.0684 × ln C
600.245
= 7.06 ◦ C
p s7 = 973 − 40 × 0.105 × 88.7 ◦ C
= 600.5 ◦ C
t dp7 = 1 ( ) 600.5 ◦
0.0684 × ln C
600.245
= 0.006 ◦ C shows small calculation inacuracy
= 0 ◦ C the significant value
The dew-point temperature gradient ends with the input data and is superimposed upon a scale
drawing of the thermally induced gradient shown in Fig. 10.4.
Owing to the high thermal resistance and very low vapour resistance of the mineral fibre
slabs fixed in the cavity, under the design conditions as stated the material temperature drops
below the moist air dew-point temperature midway through its thickness. Interstitial condensation
will occur within the mineral fibre, air space and external brickwork. Ventilation of the
remaining wall cavity allows evaporative removal of the droplets. Variation of internal and external
air conditions will limit the duration of such temperature gradients. Periods of condensation
will be very intermittent. Solar heat gains to the external brickwork will raise the temperature
of the structure and help to reduce condensation periods. The walls most at risk are those
always shaded from direct sunlight and having reduced wind exposure due to the proximity of
nearby buildings.