UK Building Regulations
for Ventilation and 
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  Ventilation Energy and Environmental Technology
                                     from VEETECH Ltd.              Updated 11th November  2009

Building Airtightness - Is Tight Right for British Homes?
Mechanical Ventilation Case Studies point to an 
Energy and Indoor Air Quality Disaster for Homes 

Condensation - Bathroom Ventilation

The Problem

Bathrooms create water vapour problems which, if left unattended, will result in condensation and mould growth. Moisture can also diffuse throughout the rest of the dwelling hence adding to condensation risk elsewhere.

Mitigation strategies must consider the bathroom as its own microclimate which undergoes a regular cycle of moisture emission and retention. The Figure below encapsulates the problem. In this example a 15 minute shower, in a sealed bathroom, results in a relative humidity of above 70% for many hours. A temporary dip is achieved by opening a window for 15 minutes but humidity quickly rises again after the window is closed. The reason is easy to understand. Once the shower is completed, residual moisture remains on  surfaces in the bathroom and on towels used for drying. The towels may then be placed on a heated rail or over a radiator. Evaporation from the wet surfaces serve to keep the relative humidity high while condensation occurs on cooler surfaces. With no source of moisture escape, this process will continue. A short airing allows moisture in the air to escape but, after the airing period,  new evaporation from the still wet surfaces and towels causes the relative humidity to rise again.

The Solution

The solution is to use humidity controlled extract ventilation so that controlled ventilation will take place when the humidity rises. The amount of ventilation needed can be reduced by wiping up as much retained surface moisture as possible, provided that the collected moisture is removed from the bathroom (and dwelling) completely. The collected moisture must not be left where it can be re-evaporated. Equally, towels should also be removed and either dried outside or dried in an extracting clothes dryer.

It would also be desirable to 'seal' the bathroom from the remainder of the dwelling by using draught strips around the door and ensuring that the bathroom door is closed at all times. This will not only prevent the escape of moisture into the dwelling but will avoid winter heatloss caused by bathroom ventilation. The extractor would need to be provided with makeup air which could come from a small window vent.  A further effective solution would be to use a wall mounted ventilation heat recovery unit since this would both extract the moist air and supply the make up air while recovering much of the lost heat.