You do not have to mess around with insulation or extra timber.toolstation sell something called "loft flooring legs".they come in packs of 12 and cost around £17.They fix to the top of the joist and raise the floor by 175mm and anyone can do it.They are only suitable for lofts though and are not structural so cannot be used if you intend on using the room as "HABITABLE".

sounds like every one can do DIY `s and when it all go`es wrong they blame the product they are useing or I was told it would be ok , the damage done in time using legs or 3x2 timber over your old ciling joist its all weight on top of your ceilings that the old joist are not designed for ,they are only to hold your ceiling up if any one carries out work like this in 12 or so months if you are lucky you will be replacing your ceilings and all the insulation it may seem like a load of money to do it right but it will cost a lot more in the end place a add on here and ask for a loft conversion expert not a DIY`er

The joists will need to be increased in size to allow the new birds to rest on and to be screwed into. Compacting the insulation will cause the plasterboard to either bow or crack.

Kingspan or celotex is an option but British gas will not supply this. For a 100mm board it costs around £38

Cheapest way would be to buy some loft insulation legs(no good putting down 4x2 timbers if you have already had your insulation layed). Once they are screwed down to the old ceiling joists you can lay the floor boards on top of the legs, cheap and easy to do even for a basic diy'er
Thanks Russell
MTR LOFTS

easiest way is to lay 3 or 4x2 across ceiling joists then board on that. you will find it much better in long run as there are all sorts protrusions on joists.

You will need to increase the height of ceiling timbers, if you put extra insulation then fit the boards, when you push it down it will probably push the ceiling down.
If you didnt have the free insulation, I would have said lay solid insulation boards over the existing joists.

Hi in short no as this will compact the insulation and may cause the plasterboard or ceiling underneath to belly, you would be best to either put kingspan insulation in it's place and board over that but kingspan can be dear or lay 100mm timbers the opposite way to existing joists to raise floor to allow for insulate and re cut insulation between the two
Layers of timbers then board, this would not be a strctural food but good enough for storage

Thanks
Good luck

Putting boards directly on top of insulation can almost halve its performance, there are engionous ways of raising the height of your flooring by using stilts you can research these online and buy them, and if your good at diy it’s a relatively simple project to do yourself