Question

The inside temperature of a wall in a dwelling is 15°C. If the air in the room is at 21°C, what is the maximum relative humidity, in percent, the air can have before condensation occurs on the wall?

Answer 1

Given:

Wall's inside temperature, $T_{wa} = 15^{\circ}C$

Room air temperature,  $T_{air} = 21^{\circ}C$

Solution:

To calculate percentage max relative humidity, we make use of steam table for saturated pressure of wall and air:

From steam table:

At $T_{wa} = 15^{\circ}C$:

$P_{wa(sat)} = 1.7057 kPa$

At  $T_{air} = 21^{\circ}C$:

$P_{air(sat)} = 2.487 kPa$

Now,

$% Relative Humidity (RH)_{max} = \frac{P_{wa(sat)}}{P_{air(sat)}} \times 100$

$(RH)_{max} = \frac{1.7057}{2.487}\times 100$

(RH)_{max} = 68.58%

Therefore, max Relative Humidity of the air before the occurrence of condensation in wall is 68.85%