Pitched roofs

7.2.15Ventilation, vapour control and insulation

Roofs shall have adequate precautions against condensation and cold. Issues to be taken into account include:

  1. ventilation, vapour control and insulation
  2. dormer construction
  3. pipework.

Ventilation, vapour control and insulation

To provide adequate ventilation and to avoid condensation in roof voids, pitched roofs that have insulation at ceiling level should be ventilated to the outside air:

  • Ventilation openings should prevent the entry of birds, etc. (fabrications with 3mm-10mm openings are acceptable)
  • Ventilation paths should remain clear, i.e. not blocked by insulation or the structure
  • A spacer in the eaves should be used to allow insulation to be installed over and beyond the wall plate to minimise the cold bridge without blocking the ventilation path (the spacer should be of sufficient length to maintain ventilation above the insulation)
  • Where proprietary eaves ventilators are used, they should be fixed in accordance with the manufacturer’s instructions.

Ridge or high-level ventilation equivalent to a continuous opening of 5mm should be provided at the highest point of each roof slope in accordance with BS 5250 in the following situations:

  • Unventilated cold roofs have insulation placed over a horizontal ceiling and a vapour-permeable underlay (type LR) is used.
  • Vapour permeable underlays are used on sloping roofs with areas covered by non-permeable materials (e.g. flat roofed areas of mansard roofs).
  • The roof is covered with high water vapour resistant (type HR) underlay and the pitch exceeds 35° or the span exceeds 10m (this is in addition to eaves ventilation).

Where high water vapour-resistant (type HR) underlay (e.g. types 1F/5U felts) is used, eaves ventilation should be provided on opposite sides of the roof to permit cross ventilation, and:

  • where the roof pitch is 15°or more, ventilation equivalent to a 10mm slot running the full length of the eaves should be provided
  • where the ceiling follows the slope of a roof, regardless of pitch, or where a cold roof has a pitch less than 15°, ventilation equivalent to a 25mm slot running the full length of the eaves should be provided (a nominal clearance of 50mm should be maintained between the insulation and the roof underlay)
  • for mono-pitched roofs, cross ventilation should be in accordance with BS 5250 and have ventilation equivalent of a continuous high-level 5mm slot, in addition to eaves ventilation.

To reduce moist air entering the roof space:

  • gaps should be sealed where services pass through the ceiling
  • where used, downlighters should be specified and sealed to limit air leakage.

Vapour control layers should be provided in accordance with the design, and where required should be:

  • placed on the warm side of insulation
  • used in roof constructions where the ceiling board is fixed to the rafters.

Where the ceiling below a cold pitched roof includes a vapour control layer, the design should ensure adequate ventilation is provided to the habitable areas to prevent condensation problems in the home.

Access hatches to cold roof voids should have:

  • an air leakage rate not more than 1 M3/h at a pressure of 2 Pa when tested to BS EN 13141-1, or
  • a push-up cover with a minimum weight of 5.5 kg and compress a closed cell seal or ‘o-ring’ between the cover and frame (clamps may also be required to ensure that the cover compresses the seal).

The thermal performance of the access hatch should contribute to the overall thermal performance of the ceiling or wall in which the hatch is located, and avoid cold bridging.

Proprietary hatches should be fitted and sealed to the surrounding construction in accordance with the manufacturer’s instructions.

Insulation should be of sufficient thickness to meet the requirements of Building Regulations, and laid over the whole loft and wall plate.

Table 8: Suitable materials for roof insulation

Mineral fibre mats BS EN 13162
Blown mineral fibreBS 5803-2
Blown cellulose fibreBS 5803-3
Proprietary productsTechnical Requirement R3

Dormer construction

Ventilation to dormers should be provided from eaves to eaves or from eaves to ridge.


To reduce the risk of freezing or condensation forming on pipework, the following precautions should be taken:

  • Where possible, water pipes should be below the main roof insulation
  • Water pipes should be insulated in accordance with Chapter 8.1 ‘Internal services’
  • Roof insulation should be placed above and around water tanks, but not below them
  • ‘Cold rising’ pipework above ceiling level should be insulated, even where it is below the main roof insulation.

In England and Wales, account should be taken of Accredited Construction Details.