woodchip

"a lot of the slab stuff sold at builders is the best kind if you can get it"

You're talking about This kind of insulation. Celotex is certainly an excellent product - easy to cut and fit, and has a very high insulation value. It's not cheap, however, and for that reason it isn't often used as a standard loft insulation. Millions of square metres of it go into loft conversion walls and roof pitches though.

wee eddie

Granules are good, but to get enough insulating value you would need to have more depth between the ceiling joists than is normal in an ordinary house. That means that you end up inserting vertical boards at the eaves and covering the joists with granules - it is a fiddly job, and the loft ends up being a no-go area.

My advice would be to bite the bullet and lift the boards in sections, inserting 150mm thick loft insulation as you go. Assuming your joists to be 100mm deep that would mean compressing the insulation as you re-lay the boards, but that's not usually a problem. It would take time, but you'll only do it once, and the result would be worth the effort in terms of the insulation you'll gain.

Woodchip: It was Summerlands Acrylic Roof that burned. Highly flammable

In the late 60's, I produced several statuettes out of moulded Acrylics, only a very few sold, but getting the Acrylic Sheet to the correct temperature, for moulding, was critical. I had several burst into flames, in the oven, when I tried to heat it too quickly.