NBS Technical Author, Charles Stirling, passes on his knowledge of vapour permeable membranes. Not forgetting their application in Scottish rigid sarking constructions gained in his experience north of the border.

Introduction

Roofing underlays are typically positioned over the roofing structure, i.e. rafters and sarking boarding, and below the slating or tiling. Traditionally, impermeable, reinforced bitumen membranes were specified as underlays. However, the introduction of lightweight, vapour permeable plastics membranes has caused some confusion; in particular in the need for any associated ventilation to roof voids.

BS 5534 Code of practice for slating and tiling indicates that a roofing underlay should:

  • Provide a barrier to minimize the wind uplift load acting on the slates or tiles
  • Provide a secondary barrier to the ingress of wind-driven snow and dust
  • Transport into the roof drainage system any moisture in the batten space
  • Provide temporary, i.e. less than three months, weather protection (including rain impact resistance) before the installation of the primary roof covering.

It additionally recognizes that there are two types of underlay:

  • High water vapour resistance (type HR) underlay that has a water vapour resistance in excess of 50 MNs/g, which effectively prevents the transfer of water vapour
  • Low water vapour resistance (type LR) underlay that has a water vapour resistance not more than 0.25 MNs/g, which allows the transfer of water vapour. These LR underlays are sometimes referred to as vapour permeable or breather underlays.

Traditional reinforced bitumen membranes have a high resistance to water vapour and the enclosed roof voids usually require to be ventilated to the outside to reduce the risk of condensation. This ventilation is typically provided by openings, with or without proprietary ventilators, provided at eaves and/ or ridge or within the plane of the inclined roof. However, with increasing thicknesses of insulation being introduced into the roof construction, it can become difficult to provide this ventilation.

Vapour permeable underlays

These are typically lightweight plastics membranes, manufactured from single ply or laminated spun bound polypropylene or polyethylene. They are susceptible to degradation where exposed to direct sunlight and should, typically, only be left unprotected for a maximum of three months. A more ultraviolet resistant membrane or tray should be specified at eaves details, extending into gutters.

The fibrous structure of vapour permeable underlays is sufficiently dense to prevent liquid water from penetrating; while allowing water vapour to diffuse. This vapour permeability reduces the need, in some instances, for the provision of additional passive roof space ventilation, provided manufacturer's guidance and recommendations given in any third party accreditation are followed.

As with traditional reinforced bitumen membranes; vapour permeable underlays can be laid over boarding or sarking (fully supported) or draped over rafters or counter battens (unsupported). When laid over a material which has a high resistance to water vapour, e.g. plywood, orientated strand board (OSB) or particleboard, a vapour permeable underlay will not reduce that high resistance. A vapour permeable underlay laid over open jointed softwood boarding may be considered as providing a low resistance.

When the underlay is laid over rigid boarding, counterbattens may be deemed necessary to prevent any penetrating moisture building up behind slating/ tiling battens by allowing free drainage down the roof slope. Counterbattens are typically not required with an unsupported underlay; where there is sufficient drape between rafters.

Condensation risk

In cold, unventilated roof constructions there is a risk of condensation forming on the underside of the vapour permeable underlay. This risk is highest: during periods of low temperature, i.e. below 0°C; when there is a high moisture loading with the occupied building, e.g. during the first heating season when the levels of entrapped or construction moisture are highest; and when the vapour control layer at ceiling level is not sufficiently effective. However, the levels of condensed moisture are typically low and will readily evaporate and diffuse safely through the membrane when ambient conditions reverse, i.e. the external temperature rises.

The following precautionary steps can reduce these risks:

  • Sufficient drying out time prior to hand-over: Construction moisture, arising from wet processes (plastering, in situ concrete flooring), and moisture entrapped while the building structure was exposed to the natural elements, requires to be effectively dried out and ventilated to the outside before the building is occupied
  • Effective vapour control at ceiling level: Vapour control layers, penetrations and hatches should be effectively sealed to provide a continuous barrier to the transfer of moist air from the occupied building into cold roof voids. Of particular risk are the ceilings above kitchens, bathrooms and shower rooms
  • Sealing of down lighters: Recessed light fittings which extend into cold roof voids should provide an effective seal against the transfer of moist air into roof voids. Down lighters should be installed with a suitable hood or capping sealed to the vapour control layer at ceiling level; this should be constructed from materials which are not affected by heat from the enclosed lamp. Horizontal, ceiling insulation should fully cover any hood/ capping to reduce the risk of thermal bridging
  • Effective living space ventilation: Passive and active measure designed to ventilate the occupied space should be adequate to ensure that generated moisture is removed at source and does not impact on the building fabric or enclosed voids.

Ventilation above underlays

With some tight fitting, interlocking roofing systems, e.g. fibre cement slating, metal tiling, there may be a need to ventilate the batten void between the roofing system and the underlay. Manufacturer's recommendations should be followed to ensure effective diffusion of water vapour to the outside.

Noise

With lightweight membrane underlays there is a risk of vibration or 'drumming' under high wind loads. During installation, care needs to be taken to ensure that the underlay is sufficiently draped to ensure drainage but sufficiently taut to prevent the underlay touching the underside of the slating or tiling.

Safety

Most lightweight plastics underlays are smooth faced and inherently slippery. During installation, particular care needs to be taken to ensure operatives' safety; manufacturers' site procedures should be followed.

Summary

Lightweight, vapour permeable roofing underlays offer an opportunity to create simple or complex roof configurations without the need for additional ventilation of enclosed roof voids. However, when specifying them, it is important to follow the recommendations of manufacturers and third party certifiers.