ASK THE EXPERT

Douglas Kent, technical and research director at the Society for the Protection of Ancient Buildings (SPAB), answers your queries

Q We are insulating the loft of our 200-year-old home. Should we use spray foam?

I would advise you very strongly not to use spray foam insulation in your roof space. Spray foam is installed underneath sloping roofs to save energy or to stop slates or tiles from slipping. In the experience of SPAB, however, the use of spray coatings is often a false economy, sometimes causing serious damage to structural roof timbers. Such treatments can also reduce ventilation and lead to the decay of roof timbers. They prevent proper inspection of the roof and hinder the reuse of sound slates or tiles when a roof is re-covered. Roof timbers may sometimes distort due to the rigidity of spray foam. There have been reports of mortgage providers not lending on houses with spray foam insulation and those that do generally charge higher rates.

Q We are repointing the walls of our rubble cottage and want to know whether the new lime mortar joints should be finished flush with the wall or recessed?

The finish of a joint is best decided by studying surviving pointing, although you need to distinguish between original work – providing a good model to follow – and inappropriate repointing, which would be undesirable. If not covered with render, the limebased joints in many rubble walls were frequently filled and brought forward to the general plane of the face, often with lightly scored lines to give the appearance of stones. If no specialised pointing exists, a flush or almost flush profile will usually be appropriate. This follows historical precedent, providing an evaporating surface and minimises the risk of decay.