Flat roofs and balconies

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7.1.7Thermal insulation and vapour control

Thermal insulation, vapour control and ventilation shall ensure satisfactory performance, and prevent the formation of condensation which could adversely affect the construction.

Insulation should be:

  • bonded or mechanically fixed in accordance with the manufacturer’s recommendations; where mechanically fixed, it should be installed using fixings of sufficient length to ensure adequate penetration into the supporting structure
  • kept dry and installed in quantities which can be quickly covered if it rains (to aid bonding and to avoid trapping moisture).

Cold flat roofs are difficult to detail correctly but, where used, they should be in accordance with BS 5250 and have:

  • an effective vapour control layer at ceiling level
  • an unobstructed 50mm ventilation space above the insulation
  • adequate cross ventilation (openings at both ends of each joist void).

Composite decks should:

  • have two beads of sealant along each board joint at the foil underface (to maintain the integrity of the vapour control layer).

Insulation for inverted roofs should be:

  • extruded polystyrene (XPS), extruded polystyrene with a cementitious surface or suitably assessed in accordance with Technical Requirement R3
  • suitable for external use
  • suitable for the weight of the ballast and able to withstand anticipated traffic
  • protected by a geo-textile layer to prevent fines from reaching the membrane surface.

Vapour control layers should be provided to warm roofs, and:

  • include at least one layer of bitumen roofing membrane (S2P3) below the insulation, fully bonded or mechanically fixed to the structural deck as appropriate, or a suitable self-adhesive or torch-on membrane.
  • sealed at laps to the waterproofing, at the perimeter and at each penetration, e.g. at outlets and pipes (where the roof is a single-ply system, the vapour control is generally not sealed to the waterproofing).
  • where a mechanically fixed system is used, the vapour control layer should consist of suitable polyethylene sheet sealed at all laps.