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Part J Heat Producing Appliances

Appendix B - Assessing Air Permeability of older dwellings in relation to permanent ventilation requirements


General Guidance

The minimum requirements for permanent ventilation for certain appliances depend on knowledge of the air tightness of the dwelling where they are to be installed. Dwellings built after 2008 are likely to have evidence of the air tightness either through an individual air permeability test certificate or through representative testing of the same design of dwelling on the same housing development.

Retrofitting measures that would reduce air permeability

Older houses are unlikely to have been tested but are unlikely to achieve an air permeability of less than 5.0 m³ / (hr.m²) at 50 Pa unless the building fabric has been upgraded. These would include all or most of the following measures:

  • Full double (or triple) glazing;

  • Secondary glazing fitted;

  • Effective closures on trickle vents and other controllable ventilation devices;

  • All external doors with integral draught seals and letter box seals;

  • Internal and external sealing around doors and window frames;

  • Filled cavity or solid walls;

  • External insulation fitted;

  • Internal insulation plastered or with airtightness membrane fitted;

  • Impermeable overlay and edge sealing of suspended ground floors;

  • Careful sealing at junctions between building elements such as between walls and floors or ceilings;

  • Careful sealing around loft hatch;

  • Careful sealing around chimney or flue penetrations;

  • Careful sealing around internal soil pipe;

  • Careful sealing around domestic water and heating pipes passing into externally ventilated spaces;

  • Careful sealing of all service penetrations in the building fabric (electricity, gas, water, drainage, phone, TV aerial, etc.);

  • Internal warning pipe for WC;

  • All cable channels for light switches and power sockets sealed;

  • All cable entry for lighting and ceiling roses sealed. Recessed lighting should not penetrate ceilings below attic spaces.

Failure to implement even a few of these measures will typically mean that the overall air permeability will probably exceed 5.0 m³ / (hr.m²) at 50 Pa. However individual rooms in some older houses with solid walls and solid floors can be inherently air-tight when fitted with modern glazing.

The situation may therefore need to be assessed with respect both to the overall dwelling and to the individual room where the appliance is to be fitted. If in doubt then assume that the air permeability is lower than 5.0 m³ / (hr.m²) at 50 Pa and fit the
appropriate permanent ventilation or seek specialist advice.

Further information on sources of air leakage can be found in GPG224 Improving airtightness in dwellings.