Energy Efficiency in Historic Buildings
Insulating flat roofs
This guidance note provides advice on the principles, risks, materials and methods for improving the thermal performance of flat roofs by the addition or upgrading of insulation. Adding insulation to flat roofs can lead to a significant reduction in heat loss but thought and care is needed to make sure this is effective and does not cause problems.
Many older buildings have areas of flat roof, typically over porches, dormers, extensions and towers. If insulation work is being carried out to the main areas of pitched roof it is important that flat roofed areas are not forgotten, even if small, otherwise the insulation of the building as a whole will be compromised with risks of cold bridging and subsequent condensation.
Upgrading a flat roof to a desirable standard can be complicated and is not without risks. Generally solutions will need to be individually designed for each situation and professional advice will often be needed.
It is important that the character and significance of a building is not compromised by the removal of flat roof coverings or ceilings to install insulation or by changing roof levels, altering gutters and rainwater outlets. Such changes may require consent if the building is listed. Any change to the roofing material of a building in a conservation area may also require consent. In each case all proposed changes should be discussed in advance with the local planning authority’s conservation officer.
Although the material most commonly used to cover flat roofs was lead, many buildings now have flat roofs covered in other materials such as copper, zinc and bitumen. The insulation of a flat roof will usually require the stripping of either the roof covering or the ceiling beneath to gain access. Insulation can be added between the joists that support the roof, above them or below them. Whichever method is chosen will depend on the practical, aesthetic and conservation considerations of whether the ceiling or covering can then be removed and whether the roof can be raised or the ceiling level lowered. In certain situations insulation can be added directly on top of a flat roof without removing the current covering. A new covering can then be added on top of the insulation
It is recommended in most circumstances that lead roofs and zinc roofs are ventilated on their underside to prevent a build up of moisture leading to underside corrosion. The ventilation layer should be below the decking supporting the metal and above any insulation layer.
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