Controlled Ventilation, Relative Humidity and Health

It is generally agreed that the three most important factors connected with condensation in dwellings are the amount of moisture being produced by the inhabitants, the temperature of the air, and the ventilation rate. It can be argued that these are under the control of the occupant. However, in reality, the ventilation rate is not. It is not only very difficult to control the ventilation rate by opening windows (even if the optimum ventilation rate is known), but also, in many instances, occupants are reluctant to open windows for heat loss and security reasons.

Studies have shown that the ventilation rate can have a major impact on the relative humidity (r.h.) of the air in the dwelling. If the ventilation rate falls much below ¾ air changes per hour (acs/hr) then the r.h. will shoot up, and if it rises above 1½ acs/hr, the r.h. will increase steadily. The JB Condensation Control Units are positive input ventilation units designed to provide controlled ventilation within this optimum band.

Condensation should not be a problem from a health and comfort point of view if the average r.h. level in a dwelling is kept below 70%.

Human Comfort The Chartered Institute of Building Service Engineers recommends that the r.h. of the air in dwellings should be in the range 40-70%.

Mould Growth I quote from B.R.E. Digest 297: “mould spores can germinate at relative humidities as low as 80-85%. If the r.h. is over 70% for long periods mould will spread” and also from B.R.E. Digest 139: “Mould growth ceases below 70% r.h.”

Asthma & Rhinitis Linked to Allergy of House Dust Mite G.W. Brundrett in “Criteria for Moisture Control” 1990 writes “The link between the house dust mite and the common problems of asthma and rhinitis has only recently been made … Both asthma and rhinitis are now associated with an allergic response to the house dust mites and their metabolites … The mites are more numerous in damp houses … The most appropriate solution is to introduce a regular, thorough, dust cleaning programme … and, finally, by maintaining a room environment below 70% r.h. ...”.

Radon Gas Radon gas is a naturally occurring radioactive gas which seeps into properties from the ground and can cause lung cancer. Hundreds of thousands of homes in the UK are affected. The Department of the Environment, Transport and the Regions (DETR) recommends positive input ventilation as "the system both dilutes the radon to acceptable levels ( below the action level of 200 Bq m³) and stops some of it getting in". For more information on this subject consult the following websites: the DETR at www.environment.detr.gov.uk/radioactivity/radon, the National Radiological Protection Board at http://www.nrpb.org.uk/ and the Building Research Establishment at www.bre.co.uk/radon