Loft insulation thickness

Arthur,

You can lay 100mm between the ceiling rafters and when you lay the flooring the insulation will simply compress down - assuming that it is of the simple fibreglass wool type of material.

Now with regards to converting the loft to a bedroom, remember that 3" ceiling rafters are not designed to carry any heavy loading and you will need to install floor joists to at least 7" x 2" (depending upon their length you may have to go 'bigger').

Of course..as I said, this arrangement is temporary...a stop gap measure.

Arthur

And depending on the length 6" or even 5" timber may be adequate.

And you can always top up the insulation when your joists are bigger, so it won't be wasted

Dear Arthur If it is temporary, I would not bother to use rockwool or fibreglass as Building regs and commonsense require some 300 mm odd depth now I would purchase (as cheaply as poss - seconds?) TP10 or the like from Kingspan and lay that in between the 3" joists where you want to use it as temp flooring (put acrows in down below to support) and lay the rest in large slabs over the joists ready for cutting up when you do your complete conversion later as the TP10 will have to be 150mm thick in rafters and that means 3" in between (to allow 1" ventilation and

I would stick with 100mm under the boards for the moment. This won't be wasted as it can be relaid between the larger joists later to improve fire transmission resistance.

If you want more than that, then divert your attention to insulating the slope of the roof since this will also be necessary once you convert the space. Foiled PIR foam panels being the material of choice for this.