Relative Humidity in Dwellings

Key facts

• Funding Body/Client: The Department for Communities and the Local Government (DCLG)
• Project Partners: N/A
• Total Project Value: N/A
• UCL/IEDE Project Value Share: N/A
• Duration: 2005-2007

As part of the Relative Humidity in Dwellings project (BD2515), UCL was asked to evaluate the current moisture criteria and guidelines in the current ‘Approved Document F – Ventilation’ (ADF) and suggest relevant changes to ADF if necessary.

Part I evaluated the current moisture criterion i.e. that there should be no visible mould growth on external walls.  ADF also states: “For the purpose of this Approved Document, the moisture criterion will be met if the relative humidity (RH) in a room does not exceed 70% for more than 2 hours in any 12 hour period, and does not exceed 90% for more than 1 hour in any 12 hour period during the heating season”. Our conclusions relating to this statement were as follows:

• A number of interpretations of the paragraph are possible
• The literal interpretation is that if these criteria are exceeded once in a heating season then the system has failed to demonstrate compliance.
• The literature does not support this interpretation
• Our field and laboratory work has shown that a significant proportion of buildings are likely to fail this interpretation even though many will not have mould growth.
• This reading of the guidelines may not have been the original intention when the words were drawn up but no other interpretation is offered – for example, no indication of the period (i.e. number of repeating days) over which such conditions may be tolerated is given in ADF.

Therefore, our overall conclusion from Part I of the work was that the current guidance needs amendment although this is not to say that following the system guidance given in ADF will lead to over-ventilation.

Part II of the work investigated options for the revised criteria. Our suggested new ADF moisture criteria are:

There should be no visible mould growth on external walls.  For the purpose of this Approved Document, the moisture criterion will be met if the average relative humidity (RH) in a room is less than the following during the heating season:

Daily average 90%

Weekly average 79%

Monthly average 70%

These criteria are based on the guidelines suggested by International Energy Agency (IEA) Annex 14.  IEA Annex 14 provides criteria for surface RH values to avoid mould growth. In order to move from air RH to surface RH we suggest a simple modifying 10% (noted in BS5250) be applied. This 10% factor is also utilised implicitly in the current criteria.

It is our contention that these new criteria are soundly based and have less potential for misinterpretation than the current criteria. Other approaches for the construction of the guidelines are certainly possible but the suggested modifications to ADF represent what our work has assessed as being the most suitable compromise between a reasonable representation of the physical processes involved and the need for understandable and applicable criteria. However, it should be clearly stated that there is still some uncertainty with regards to the exact, optimal Annex 14 criteria and associated 10% modifying factor.   Finally, this project was tasked with addressing the issue of mould.  However, House Dust Mite (HDM) populations - based on work undertaken at UCL and elsewhere - seem to tolerate lower RH levels than required for mould growth.  Thus, even if one successfully controls for mould one may not be controlling effectively for HDM.