In this extract from the Loft Conversion Project Guide, by the Construction Products Association, we look at good practice techniques and information on the need to provide ventilation to the converted roof.

When carrying out a loft conversion, ventilation is a very important issue. Depending on the planned use of the converted loft, you will need to consider ventilation to either two or three locations. The first and most obvious location is ventilation to the converted roof itself; second, a habitable room; and third, the ventilation requirements if a bath/ shower or WC is to be installed.

The roof

To provide adequate ventilation to a roof space you first need to be aware of the roofing felt/ underlay that is currently in place, if at all, and its type and condition.

There are normally two types of underlay used: high resistance (HR) (which has a vapour resistance greater than 0.25 MN.s/g), or low resistance (LR) (which has a vapour resistance less than or equal to 0.25 MN.s/g).

Type HR underlay with a ventilated void beneath

To minimize the risk of interstitial condensation occurring, it is necessary to provide a well-sealed ceiling and to ensure that 50mm deep vented voids (at least 25mm deep at the centre of the drape) are maintained between the insulation and the underlay. It is important that these air paths remain unobstructed during the life of the building.

Ventilation openings should be provided to each void, at both high and low level, to allow free air movement through the gap between the insulation and the underlay. The following information applies for each of these locations:

  a. Low-level openings should be equivalent in area to a continuous opening of not less than 25mm x length at eaves. 
   b. High-level openings should be equivalent in area to a continuous opening of not less than 5mm x length at the ridge or hips.

Note: Where there is no cross communication between each roof slope, 5mm should be provided on both sides of the ridge.

Particular attention must be paid to potential restrictions at eaves, at changes in roof slope, at valleys and hips, and at changes in construction details where such a void may be difficult to achieve. Obstructions such as dormers, roof windows, compartment walls, fire barriers or changes in pitch create separate voids in the roof slope. Where this occurs the roof void should have additional ventilation openings:

  a. Immediately below the obstruction equivalent to 5mm x length of obstruction 
  b. Immediately above the obstruction equivalent to 25mm x length of obstruction.

Type LR underlay fully supported on the insulation (no void)

It is not necessary to provide a void beneath the underlay in roofs of this type, but a vapour control layer (VCL) should be provided below the insulation. Where joints are present in the VCL, they should have at least 100mm laps and be well sealed. A VCL is generally 500 gauge (120 micron) polyethylene sheet.

To minimize the risk of interstitial condensation occurring, it is necessary to provide a well-sealed ceiling and ensure that there is sufficient air movement between the underlay and the roof covering to allow moist air to migrate to the atmosphere. For air-open outer coverings no specific provision for batten-space ventilation is required, but for tight outer coverings batten-space ventilation should be provided. In this instance you should make available either:

  a. Ventilation openings to the batten space which are equivalent to a continuous slot 25mm wide in the eaves and 5mm wide at the ridge and 25mm at counter battens

or;

  b. Ventilation openings to the roof void or air void below the underlay at: 

1. Low-level equivalent in area to a continuous opening of not less than:
   i. 25mm x length at eaves for pitches of 15 degrees or less
   ii. 10mm x length at eaves for pitches of more than 15 degrees.

2. High-level equivalent in area to a continuous opening of 5mm in:

  i. Roofs where the pitch exceeds 35 degrees 
   ii. Roofs of any pitch with a span greater than 10m
   iii. Lean-to and mono-pitch roofs.

Note: Most traditional unsealed slating and tiling methods are sufficiently air-open.

Type LR underlay unsupported (small void)

There are many forms of roof where, for constructional reasons, there is a small void above the insulation. To minimize the risk of interstitial condensation occurring, it is necessary to provide a well-sealed ceiling and ensure that there is sufficient air movement between the underlay and the roof covering to allow moist air to migrate to the atmosphere. For air-open outer coverings no specific provision for batten-space ventilation is required, but for tight outer coverings batten-space ventilation should be provided (as outlined above for Type LR underlay fully supported on insulation with no void).

If the integrity of the roof and wall vapour control layer can be maintained, there is no need to provide ventilation in the void between the insulation and the LR membrane. If there is any doubt about the ability to provide and maintain an effectively sealed vapour control layer, ventilation openings should be provided to each void, at both high and low level, to allow free air movement through the gap between the insulation and the underlay.

  a. Low-level openings should be equivalent in area to a continuous opening of not less than 25mm x length at eaves. 
 
b. High-level openings should be equivalent in area to a continuous opening of not less than 5mm x length at the ridge or hips. Where there is no cross communication between each roof slope, 5mm should be provided on both sides of the ridge.

About this book

The Loft Conversion Project Guide is an easy-to-use guide to loft conversions and provides architects, building control, builders and DIY enthusiasts with up-to-date and reliable guidance on achieving compliance with the Building Regulations.

Download The Loft Conversion Project Guide from the Construction Products Association website
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