Modern construction has reduced the roof space and modern techniques have generally resulted in buildings which do not breathe adequately. Ventilation of the roof space improves living comfort by minimising heat build-up above ceilings and reducing the risk of condensation - causing damage to lining and roofing. This is especially so below tile roofs where aluminium foil sarking is used, and below metal deck roofs.

Ventilation

There is a strong need for a program of education in the building industry on the benefits of ventilating the ceiling and wall areas by way of:

• Soffit Vents
• Gable or Roof Vents
• Wall Vents
• Mechanical Ventilation

  Ventilation recommendation, requirements and advantages are illustrated in the Australian Standard 2589/The New Zealand Building Code (E3 & G4) BRANZ Bulletin 302 (Nov 92). Extracts from this publication are included in most plasterboard manufacturers specifications.

  Unfortunately the majority of new dwellings are constructed with little or no consideration given to ventilation requirements. Condensation and/or moisture can enter the roof or wall cavities in many ways.

  Many types of roofing allow considerable quantities of moisture into the ceiling area thus causing immeasurable damage to both timber framing and plasterboard. Failure to keep the roof space adequately vented can result in joint distortion (peaking etc), board sage, and nail popping.

  Gypsum Plasterboard will give many years of satisfactory performance under a wide range of climatic conditions. To ensure the long term performance of both lining material and paint finish, the roof space should be adequately ventilated.

  Condensation

  When air is cooled its ability to contain water vapour is reduced and any excess of water vapour condenses. In buildings it may condense on or in walls, floors and ceilings. In most cases this condensed water evaporates again, but sometimes it cannot evaporate and a wall, floor or ceiling may remain wet for days, or even months. This may lead to dimpling of nail heads, sagging of ceilings, rotting framing timbers and encouragement of mould growth.

  Many inter-related factors must be taken into account by the building designer if condensation is to be eliminated in the building structure. Such factors include orientation, temperature and relative humidity differentials, shading, ventilation, methods of heating and cooling, building techniques, use of materials of low or high thermal capacity, function of the building, and living habits of the occupants.

  The aim should be to always:

  • Keep materials and structures above the "dew point" temperature.
  • Keep air spaces well ventilated to allow re-evaporation.
  • Keep moist air away from cold surfaces (e.g. by using a vapour barrier, i.e. a thin membrane of low water vapour permeability).

  More information on condensation and its occurrence in houses may be found in Experimental Building Station publications: Notes on the Science of Building Nos 61 and 78. As a general rule, the advice given in the EBS Notes on the Science of Building No. 61 should be followed:

  "Locate the vapour barrier as close as possible to the surface which will normally be at the higher temperature at the time of the condensation hazard."

  The aluminium foil sarking on roofs should be installed in accordance with AS1904 or NZ S4222.

  Recommendations

  In cold or moderate climates unheated spaces above ceilings should be adequately ventilated as follows: Provide effective cross-ventilation for all spaces between roof and ceilings by screened louvres or other approved and acceptable means.

  The ratio of total net free ventilating area to area of ceiling should not be less than 1:150, but the ratio may be 1:300 provided that -

  1. A vapour barrier having a transmission rate not exceeding 1 perm (*) is installed on the warm side of the ceiling; or

  2. At least 50% of the required ventilation area is provided by ventilators located in the upper portion of the space to be ventilated (at least 90mm above eaves or cornice vents) with the balance of the required ventilation provided by eaves or cornice vents.

     Attic space which is accessible and suitable for future habitable rooms or walled off storage space shall have at least 50% of the required ventilating area located in the upper part of the ventilated space as near the high point of the roof as practical and above the probable level of any future ceilings.

     Note: In hot, humid climates, where houses may be airconditioned below the dew point of the outside atmosphere the wall and ceiling framing members and internal linings should be fully protected against condensation by an impermeable membrane separating them from the external moist air. The membrane should be thermally insulated to maintain it at a temperature above dew point. (* 1 perm = 85 x 10-v g/N.s)