CONDENSATION AND VAPOUR CONTROL LAYERS

Water vapour is always present in varying amounts in the atmosphere, and when it comes into contact with a cold surface the vapour may condense into liquid.

Condensation is liable to occur on the internal surfaces of the walls and roof of a building if the temperature and humidity of the air inside the building is appreciably higher than the outside atmosphere, particularly if the walls and roof have low thermal insulation values.

In practice such condensation is frequently more severe under the roof than on the walls, partly because roofs often cool rapidly at night by radiation to a clear sky and partly because, in the past, roofs have usually had lower thermal insulation values than walls.

When designing a roof the problem of condensation must always be borne in mind.  Any provision required to control condensation should be determined as recommended in BS 6229 but with the calculation method modified to conform with BS 5250:2002 – Code of Practice for control of condensation in buildings.

In the case of a roof incorporating mastic asphalt, a suitable thermal insulation may be included within the system.  This layer must have sufficient insulation value for its underside to remain above the temperature at which condensation can start, even on the coldest nights.  The provision of insulation alone, however, may not be sufficient to prevent condensation.  If the insulation is permeable to water vapour, the vapour will pass upwards through it and condense on the underside to the waterproof membrane.

To prevent this happening a vapour control layer in the form of a vapour check or full vapour barrier should be provided on the underside of the insulating layer.  A vapour check may consist of a single layer of roofing felt, whilst a vapour barrier can be formed of two layers of roofing felt or a proprietary metal lined vapour barrier.  It is essential that all side and end laps are fully sealed.

Consideration should be given to a 13mm thick single coat of asphalt roofing on an underlay of glass fibre tissue which would serve as a vapour barrier. (Fig 1.2)

THERMAL INSULATION

The statutory requirements for the thermal insulation of roofs are set out in approved document L- Conservation of Fuel and Power to the Building Regulations 1991 (as amended).  This document requires roofs to have the following maximum U values.

These requirements are usually achieved by the inclusion of insulation boards beneath the waterproofing in a warm room system or above the waterproofing in a protected membrane system.

There are several types of insulation suitable for use in mastic asphalt roofing specifications.  The following types are the most frequently used.  All must be laid strictly in accordance with the manufacturer’s instruction.

POLYISOCYANURATE FOAMS

Suitable formulations of polyisocyanurate boards are available for use under mastic asphalt.  As mastic asphalt retains a high temperature for long periods after application the board used must be designed to maintain dimensional stability during the application and cooling of the asphalt.  It provides a substrate that is an ideal base for mastic asphalt.

CORK

Cork is resistant to moisture and decay and provides a substrate of good laminar strength making it an ideal base to receive mastic asphalt.

Cork/polyurethane foam composites boards provide good insulation within acceptable thickness limits.

CELLULAR GLASS

This type of board is non combustible, impermeable to water and can normally be laid without a vapour control layer.  The products high compressive strength makes it an ideal base for asphalt, particularly in situations where pedestrian traffic is anticipated.

When asphalt is applied over cellular glass insulation, two layers of non-bituminous building paper are required between the slabs and the sheathing felt to prevent adhesion.

PERLITE/POLYURETHANE

Perlite offers a high  level of dimensional stability with good fire resistance.  Perlite/Polyurethane composite boards provide a stable base for mastic asphalt together with good insulating properties.

EXTRUDED POLYSTYRENE

Extruded polystyrene is used in the Protected Membrane Roof system. Full details regarding the use of this type of board together with thermal insulation values are to be found under the Protected Membrane Roof  system.

TABLE OF THERMAL INSULATION VALUES

DECK: 150mm IN SITU CAST SLAB AND SCREED

DECK: 100mm PRECAST LIGHTWEIGHT CONCRETE UNITS

DECK: 19mm TONGUED AND GROOVED BOARDS

DECK: 50mm WOODWOOL SLAB

DECK: STEEL OR ALUMINIUM DECKING