Solid insulation boards, typically from the big manufacturers such as Kingspan and Celotex (but other brands are available too) are widely used in the modern construction industry. These products are not ‘breathable’ and should be viewed as being as unsuitable for use in any historic building. They can be easily cut to size using a bread knife and I have seen them squeezed into awkward spaces and between roof rafters and floor joists. Old buildings need to ventilate to allow for moisture exchange and these products restrict this and can lead to condensation problems. Foams and polystyrene products are to be avoided in historic buildings.