It works both in summer and winter: Actis Triso Super 10. In winter, it retains heat within buildings. In summer, it reflects radiation preventing overheating of attic rooms. Certified as the equivalent of 210mm of mineral wool but is only 30mm!
Other benefits include: seals roof against wind and damp while retaining ventilation; it saves valuable living space; beams and rafters may be left exposed. It is more tricky to install as you have to double batten the roof, however you don’t have to deal with nasty fibreglass mineral wool, or cut celotex insulation between the rafters, which takes an age. Triso is a much faster option.
Above is a new build roof we did in Hartpury, Gloucestershire, that used tin foil type insulation.