Could someone talk me through insulating my 1930s slate roof?

[StealingSleep]

It's the original roof, all the slate tiles are in good condition and the roof itself is watertight. But, for a long time the lime mortar has been falling off and making a mess of the loft. I think it's pretty much all gone now as you can see all the slates from inside. I'd like to put up some insulation which will also act to stop the last traces of the mortar from falling onto the stuff in the loft. A roofer I talked to mentioned Kingspan but I can't find a simple DIY guide to what I need to do, what layers/membranes/materials to use. The roofer made it sound like you just buy sheets of this stuff, cut it to size and nail it to the rafters, but there seem to be lots of different options. Any tips?

I can't tell you about membranes but rigid insulation (of which kingspan is one make) is very easy to use. You need to measure the depths of your rafters then buy the insulation of that thickness (or a few mm thinner). You then measure the width of the space between two rafters and cut a length of the insulation to fit it - you need to use a handsaw but it is very easy to cut. Hopefully it will be snug enough to hold itself in place, if it's slightly loose, pack it out with some offcuts. Then repeat until all the gaps in between the rafters are filled. Ideally plasterboard over it. It's very simple - the only drawback is it smells slightly and kind of flakes everywhere when you're sawing it.

That's great, sounds very do-able :) Thanks lala :)