I have a 1930s bungalow with thick clay roof tiles. The roof felt looks in fair condition and I cant see any water leaking in. The problem I have is that there seems to be a layer of clay dust on the loft insulation. What can I do about it? Would I have to remove the felt and apply a sealant to the tiles? If so, what sealant should I use, and could I put isulation boards between the rafters and instead of refelting? I would really appreciate some advice on this, cheers.