The foam spray stuff is great for prolonging the life of the roof. If the mortar is falling away is the roof nearing the end of its life? If you do not want to re roof then it is a good solution and is now a lot tidier than it used to be. However, it is harder / messier when it does come to recovering the roof.
You can fit insultation between the rafters, and because there is no sarking felt then the area above the insulation is ventilated anyway, you need to keep 50 mm clear.
To achieve near bg regs standard you will need (assuming 100 mm deep rafters), 50 mm fitted between rafters (leaving 50 mm space above) and then a sheet of about 70 mm below. The loft should then be warm and toastie rather than cold and dirty.
Altrnatively look up Tri Iso super 10 which is a thin multi layered foil which is easier to install. It is stapled to the rafters and you could od this yourself quite easily.
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