With additional insulation in the roof, there is potentially a higher risk of interstitial condensation. Guidance on avoiding this is given in BRE report 262 Thermal insulation: Avoiding Risks (2002). Also, Approved Document F contains information on the provision of ventilation to reduce the risk of condensation in roof spaces.
As buildings become more highly insulated, the effect of air leakage through the envelope on energy consumption becomes more important. Remember that the roof system is only one of the areas of the building that contributes to the leakage. Junctions and openings such as rooflights, smoke vents and service penetrations are likely to need more attention to counteract this problem.
With thicker insulation, the roof will absorb less heat, which means a mastic asphalt roof will be warmer when exposed to the sun. As a result, the reflective coverings applied to protect the asphalt become more important, and should be reapplied more frequently.
When upgrading, consideration must be given to the weight the existing structure was designed to hold (including snow loading and localised working tolerances). Guidance should be sought from the client relating to the original design. If in doubt, consult a structural engineer.
Part L requires that the roof should be constructed so that there are no significant gaps in the insulation layer (or layers) within the various elements of the fabric, at the joints between elements, or at the edges of elements (such as those around rooflight openings). It is also necessary to design out the additional heat loss that may result from features such as safety harness posts or rafters that project to support a canopy or gutter.
Roof abutment details
When increasing the thickness of insulation on roof refurbishments, remember that upstands must be extended to remain 150 mm above the roof. Ensure that the extended 150 mm upstand height will not compromise the function of the DPC. In certain circumstances, the provision for a new DPC at a higher level may be required to provide a secure detail.
On project refurbishments with extra insulation, all pipe penetrations have an upstand height of at least 150 mm. A weathering flange attached to the pipe should protect the leading edge of the membrane upstand. Existing cast iron or plastic pipes will need to be extended.
Building Regulations www.dtlr.gov.uk
The Metal Cladding and Roofing Manufacturers Association www.mcrma.co.uk
National Federation of Roofing Contractors www.nfrc.co.uk
Mastic Asphalt Council www.mastic-asphalt.com