Environmental and Social Aspects of Domestic Bathing - Ethical Engineering for International Development and Environmental Sustainability - page 205

Water Vapour For water at 40 °C, the heat of vaporisation is 2.40 kJ/g (Kaye and
Laby 1973 , p 225) or 0.67 kWh/l. Without moisture in the form of fi ne droplets, the
humidity problem would be easily dealt with. At ambient temperature of 21 °C, the
mass of water vapour in 1 m 3 of saturated air is 18 g (Kaye and Laby 1973 , p 27), so
the excess water if the normal relative humidity were 60% is 7 g. In a small bath-
room, volume 10 m 3 (this fi gure is estimated to be typical in the UK; in other pros-
perous countries, bathrooms are usually bigger), the energy embodied in the excess
vapour is 0.05 kWh. This shows that water vapour itself is not a major energy term,
although it is important for health and for the longevity of buildings and fabrics, on
which more later.
Condensation Only a small fraction of the dampness produced in bathing is water
vapour. Presumably because the production of droplets and the condensation of
water on cold surfaces are hard to quantify in a usefully realistic way, either by
theory or experiments, they receive little attention compared with the great deal of
attention accorded to embodied energy. A bather who is not energy conscious may
produce a wet bathroom and a large amount of sopping towels. The towels might
hold 1 kg of water (estimated by simulating profl igate bathing and weighing the
towels dry and wet). Drying these in a tumble drier uses 0.67 kWh. This is a large
amount, equivalent to using an 8 kW electric shower for 5 min. And, as discussed in
the following main section, the carbon dioxide emission per unit of energy is high
for mains electricity. I estimate that a frugal user can - by using less water and a
slightly lower temperature, spending a minute or so mopping up afterwards and dry-
ing self by shaking and rubbing the body using a small towel (washcloth sized) and
fi nally a medium-sized towel - produce less dampness by an order of magnitude.
Condensation on the Bathroom Walls A small bathroom 2 × 2 × 2.5 m 3 has a total
surface area of 28 m 2 . If the average thickness of the condensation were 0.1 mm
(a rough estimate taken by inspection following a typical shower in winter without
fan extraction), the volume of condensation would be 2.8 l. The energy needed to
evaporate this would be 1.9 kWh. In fact one does not, or should not, evaporate so
much dampness in the dwelling but rather extract it.
Extracting Moisture The DIY World ( undated ) has useful advice about bathroom
condensation and recommends an extractor fan suffi ciently powerful to extract the
moisture immediately from the air before it condenses on cold surfaces.
Are you plagued by condensation in your home? By running a bath or by having a shower,
the bathroom produces high levels of moisture in a very short period of time, which will
quickly condense unless conditions are such so as to prevent it. To create an environment
that prevents this from occurring, an extractor fan capable of extracting fi fteen to twenty air
changes per hour is essential (The DIY World undated ).
(An air change is the extraction of a volume of air equal to the volume of the
room.) The same source also makes the valid but rarely mentioned point that 'Due
to the potentially large volume of moisture that could be produced by any member
of the family bathing or using the shower, consideration may not always be given to
condensation as being a problem. For this reason, the bathroom can be a diffi cult