Energy Efficiency in Historic Buildings
Insulating pitched roofs at rafter level- warm roofs
This guidance note provides advice on the principles, risks, materials and methods for insulating pitched roofs at rafter level. When insulation is placed at this position the roof is often referred to as a ‘warm roof’.
The simplest and most common way to insulate a pitched roof is to add insulation above the horizontal ceiling of the top floor. However, if the top floor is open to the rafters and is used as habitable accommodation then insulation will need to be provided at rafter level.
Loft spaces are often converted to provide extra space. In this case insulation has to be inserted around the sloping rafters. Retro-fitting insulation in this situation is a difficult and disruptive operation. It will require either the sloping ceilings or the roof coverings to be removed, and is really not a DIY job.
Insulation can be added above, between, or beneath the sloping rafters. Insulation above rafters is called ‘sarking’ insulation, and is the best technical solution because it can be installed in one continuous layer. Insulating sarking boards can be bought with tongue and groove edges to ensure a good joint. The disadvantage of sarking insulation is that it will raise the roof level, requiring adjustments to gutters, eaves, gables, ridges and abutments with chimneys.
Insulation fitted between the rafters will have the least impact on the internal and external appearance. This is where insulation is typically added when lofts are converted. It is even more important to ensure that there are no gaps left at the edges between the insulation and the rafters. The amount of insulation that can be added between the rafters is limited by their depth.
Alternatively insulation can be added beneath the rafters, in some cases even without removing the existing ceiling. This will reduce the amount of usable space beneath, which may be critical as headroom is often at a premium. If the room was converted recently there is a good chance that at least some insulation was added during the conversion. If so the extra benefit from adding extra layers of insulation is unlikely to be worthwhile.
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