insulating floors, in limited crawlsapce depth

[mcmook]

i've been doing a lot of googling and searching on this group re insulating floors - but need some clarification on my own particular setup. i have the opportunity to insulate a ground floor because i'm lifting the floorboards and laying a ply subfloor followed by bamboo T&G planks. the floor joists 6" x 2" are resting on bricks on a solum to which a thick layer of tar has been applied (DPC I presume?). So in all there is only 7" - 8" airpace. The floor is is over two rooms. all in the floor area is approx 70m2 (7.5x5 &

6.5x5). The front is ventilated with two grills, but I cant see any other ventilation, and because the ground slopes up the joists towards the back are under ground level. There is no sign of any dampness at present.

... is it practicle/beneficial to insulate this space, what thickness can i use without comprimising the airflow?

[The Natural Philosopher]

Yes it is practicable,

Use about 2-3" of celotex.

[mcmook]

Thanks... what about a vapour barrier? over the ply perhaps. Polystyrene is cheaper would this be a suitable alternative to celotex?

[John Stumbles]

Sounds like underfloor heating without the heating. Er, have you considered .... ?

[The Natural Philosopher]

Celotex is foil coated and installed with foil sealing tape.

Polystyrene is only half as good - need twice as much - and might be a fire/toxic fume risk. I really only like to use it inside a screed. It really is *very* nasty in a fire. And it burns very well.

Celotex simply chars..it won't sustain combustion. I tried..

[fred]

Very good point.

Maybe rockwool batts would be the way to go for a budget solution,

100mm would be equivalent to 65mm of Celotex and still leave a healthy airgap.

[The Natural Philosopher]

Not good if there are underfloor draughts.