Choose the unventilated cold roof structure that applies to your roofing projects structure.
Breathable, pitched roof systems - sealed well sealed and normal ceiling?
The recent introduction of low resistance, vapour permeable underlays and breather membranes has offered specifiers and designers the opportunity to look at pitched roof construction in a new way.
The function of a low resistance, vapour permeable underlays or breather membrane is to allow internally produced water vapour to permeate through its special layered composition to the external environment, while inhibiting the ingress of wind driven rain or snow.
By utilising the benefits of vapour permeability specifiers and designers are able to reduce the traditional ventilation requirements of cold roof structures.
In this way a significant step can be made to meeting the Government’s objective in Part L of the Building Regulations, which require a substantial reduction in heat loss through the constructional envelope of a building.
The installation of a well constructed vapour control layer is pivotal to the success of a cold roof solution that incorporates the use of a vapour permeable underlay to achieve a reduction of traditional ventilation requirements.
Sealing the ceiling of any building will reduce both moisture transfer and heat loss, thus minimising the risk of condensation in the roof. However, in recent revisions to BS 5250: 2002 (Code of Practice for Control of Condensation in Buildings) it is acknowledged that in practice it is extremely difficult to construct a vapour control layer that is totally impermeable to water vapour (i.e. airtight) and therefore has created a new classification “well sealed ceiling.”
Well Sealed Ceiling
A well sealed ceiling is more achievable than a sealed (i.e. airtight) ceiling, but requires high standards of workmanship by the trades involved in installing plasterboard or other ceilings, plumbing and electrical services. It is important to consider at design stage how construction details can be achieved that are robust over the lifetime of the building.
To meet the requirements of BS 5250: 2002 (Code of Practice for Control of Condensation in Buildings) a well sealed ceiling requires the following;
- The design avoids constructional gaps, especially at the wall/ceiling junction with dry lining construction, and holes in the ceiling.
- No access door or hatch should be located in rooms where large amounts of moisture are produced, including kitchens or bathrooms.
- Sealed loft hatch, sealed pipe and cable penetrations and sealed recessed light fittings. All seals to provide an effective barrier against moisture entering the roof space.
- Penetrations for services and rooflights, to be permanently sealed.
- The ceiling is sealed to the external walls to limit any leakage through cracks.
- The head of any cavity in any wall or partition should be sealed to prevent transfer of warm moist air into the loft.
BS5534:2003 (Code of Practice for Slating and Tiling) also recommends that a condensation risk analysis should be carried out on non ventilated cold roof systems.