Ventilation of rooms

Habitable rooms

  • A window with an opening equivalent to 1/20th of the floor area, and
  • Background ventilation (also called trickle- or night-vents) of 8000mm², which can be closed to prevent draughts.

Bathrooms or shower rooms (with or without a WC)

  • Mechanical extract fan of 15 litres per second capacity, and
  • Either:
    background ventilation of 4000mm², and an opening window (no minimum size), or
    the extract fan has a 15 minutes overrun, controlled by the light switch or a humidistat.

Sanitary accomodation (i.e., a WC)

  • Either:
    a window with an opening equivalent to 1/20th of the floor area, and background ventilation of 4000mm², or
    mechanical extract fan of 6 litres per second capacity with a 15 minutes' overrun, controlled by the light switch or a humidistat.

If the new storey contains a kitchen or a utility room, or if there is an open-flued [i.e. not room-sealed] appliance in any room, then you should consult Building Control for further advice.

Ventilation of roofs

The introduction of a room into a previously open roof space will restrict the flow of natural ventilation through the roof, and could cause condensation within the space between the insulation and the roof covering. The existing ventilation provision, if any, at the eaves will have to be increased, with a path for the air to flow over the insulation and out through the top of the roof slope. 

Eaves ventilation

  • A continuous 25mm wide opening or the equivalent area is required to the length of the eaves.

Cross ventilation

  • A 50mm clear air space is required between the insulation and the roofing felt.

  • The depth of the rafters may need to be increased to allow for this space plus the depth of the insulation; this can be done by adding battens to the underside of the joists.

  • If roof trimmers or hips restrict the passage of air from the eaves to the ridge, a series of 25mm holes may be drilled into these members to allow a continuous airflow. It is necessary to ensure that the location and size of these holes will not impair the strength of these members.

Ventilation at the ridge or high level

  • A continuous 5mm wide opening or the equivalent area is required to the length of the ridge.

  • Where the ceiling (or the insulation) is continued up to the ridge (as in first diagram), ventilation will have to be provided at the ridge. The ridge tiles can be replaced or altered with a vented dry-ridge.

  • If there is a flat ceiling at some distance from the underside of the ridge (as in second diagram), then the void created over this ceiling can be vented. The simplest way to achieve this is by using vent-tiles. The spacing of vent-tiles varies with each type, and the manufacturer's recommendations should be followed. The vent-tiles must ventilate the void area and not lower down the roof slope.

roof ventilation loft conversions


roof ventilation loft conversions 2

Vapour control layer

Vapour control layers can reduce the amount of moisture reaching the void and the insulation (but not eliminate it completely, hence the need for ventilation).

  • The walls and sloping ceiling should be lined with either:
    a 500 gauge (150 micron) polythene sheet fixed between the lining and the insulation, or
    a foil-backed plasterboard.

Breather membranes

As an alterative to roof ventilation the tile underlay can be replaced with a breathable roofing membrane. These will need to be installed as per the manufacturer's instructions.

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