Traditional Cold Roof Construction
Old methods of constructing flat roofs was by the traditional method of insulating the void between the joists directly above the plasterboard. For habitable rooms this method of construction is no longer deemed cost effective or practicable. Warm air transports moisture in the form of water vapour, and any cold surfaces within a building have the potential to reach dew point when that warm air comes into contact with them. Moisture ingress into the roof space can occur in many ways. Roof ventilation construction was often regarded as “the traditional way” of meeting the regulations. However, from an energy conservation perspective, introducing cold external air into roofs can be to the detriment of the construction by:
- Reducing the effectiveness of the insulation
- Promotes warm air leakage from the building into the roof space
- Increasing air infiltration into the heated building (draughts)
- Introducing dirt, dust and insects into the roof construction via the ventilation
- Introducing external moisture laden air into the construction via the ventilation
Normally cold roofs are now only constructed on external outbuildings such as garages due to the excessive heat loss when used in a habitable living area.
Traditional Warm Roof Construction
From October 2010 and the implementation of Approved Document L1B (Conversion of Fuel and Power) an existing flat roof has been required by Law to comply with rigorous U Value targets, based on any refurbishment of more than 50% of your flat roof (LB1; Section 5.81 and Table 3 Column b). A warm roof system is the most efficient way to achieve these results as required and set out in accordance with current Building Regulations. All new-build flat roofs require a warm roof construction to comply with Part L. A warm roof is made from a rigid insulation board composite with integral foil backing (Vapour Barrier) that is placed above the roof joists and onto which the OSB3 decking and seamless fibreglass membrane is laid. The clear space between the plasterboard ceiling and the roof deck provides an airspace where the air remains warm and the foil vapour control layer protects against condensation. The system insulates whilst removing the requirement for ventilating the roof space unlike cold roof details. This is particularly important for a flat roof over a kitchen, bathroom or utility room where the relative humidity is greater than that of other areas (ideal areas for the onset of condensation / mould growth within cold roof designs).
If you compare this with the traditional method of flat roofing where the insulation (quilt) material is placed between the rafters above the ceiling without proper ventilation the warm air condenses and can lead to degradation of the structural timbers and eventual collapse and/or deterioration of the roof deck. The warm roof system is more efficient as a method of insulating the room below compared to the traditional cold roof construction. Fibretechs fit 105mm thick composite insulation as standard when upgrading/refurbishing an existing flat roof to achieve a U Value equal to or better than 0.18 W/m2K. This enhanced upgrade will not only save you money on heating your property but will also provide you with a roof which is thermally efficient. Should the property subsequently be sold following this upgrade then the enhanced thermal qualities of the roof will be noted in the Energy Performance Certificate (EPC). From the 21st May 2010 any property that is marketed for sale or put up for rent in England and Wales will need an EPC. Advantages of Warm Roof construction are therefore:
- Increasing the effectiveness of the insulation
- Prevention of warm air leakage from the building into the roof space
- Reducing air infiltration into the heated building
- Avoiding dirt, dust and insects into the roof construction
- Prevents external moisture laden air into the construction
If you are about to have an existing roof upgraded and a contractor has not advised you of the current regulations and the requirement to insulate the roof externally then you should proceed with caution.