Replacing ceiling under existing flat roof - insulation opportunity - confused!

There are insulation products based on space blanket technology that would easily fit in your gap and have the equivalent insulating effect of 20cm of rock wool. E.g.

The problem with these products is that they are good for insulating radiation, but behind plasterboard you don't have any (much) radiation. For that you need the stuff the OP was planning on.

To the OP: Could you shrink the gap between joists with a 25mm strip of celotex cut sideways? It would make a /big/ difference to the cost!

In article , Jonathan writes

Perhaps a daft idea but couldn't you make up the odd small space with "fill foam"?

Dear Lewis Money spend on Insulation is never wasted - it is simply the second best investment after draft sealing. If I were you I would use this opportunity to do a good job ~Suggest 100 mm (full depth) of joists IF and only IF the firring above allow full cross ventilation If not then maximum that will allow air accross If you can ge 4" in then thats ok if not I would lower the ceiling 2" plus depth of PB by putting on a continuous layer of 2" kingspan TP10 (names all vary but it is all same polyurethane based stuff) staggering joints and using either large washers and screws or much quicker the stainless steel nails/ plastic washers designed for the EXTERIOR holding of of this product on walls before rendering. The washers are large 50mm dia pink plastic and the nails stainless which can be quickly driven into the joists I would then plasterboard it (having marked the joists AND fixing lines on the walls first) with a combination of Gypsum Plasterboard ahesive spread with a serrated tile trowel and a further set of pink ss fixings on different centres pink the lot with finish and bobs the job Use foilbacked PB Consider using drywall topcoat as v check Chris

Thanks for the tip.

I'll have a word with the plasterer - I'll be fitting the celotex myself to save a few bob, so we'll see who's best doing which bit. It sounds like the 'between and under' solution, but a bit more complicated!

:)

I'm not convinced by the effectiveness of this solution in anything but a radiated heat context.

Not at all daft. So 'not daft' in fact that I didn't think of it until someone passed me a glimpse of the bleedin' obvious.

Thanks! :)