roof insulation

[roygbiv86]

i want to improve insulation of my house. solid brick walls. roof is in 3 parts:the apex which can have usual 250mm insulation. however the other two parts have to be insulated within the slope of the roof - just below the visible tiles (no other protection just tiles - used to be plaster between tiles but most has dropped out over the last 100 years)
what can i use that is windproof waterproof good insulater easy to fit and cheap?? any suggestions would be welcome. thanks again

 

[noseall]

cellotex, kingspan?

not cheap though.

 

[^woody^]

Polystyrene, jablite?

not much good though

(But cheap)

 

[AndersonC]

Would there be a flammability risk with using plain polystyrene?

I am in a similar boat, moreso since the fibreglass lagging is completely worn out/sagged/full of dust from the back-pointing. I really need to make a judged decision quickly on what to do, and cannot afford anything other than DIY!!