loft insulation between rafters or on rafters

[dave1001]

Hi, wonder if someone can enlighten me, ive boarded out my loft and am looking at insulating the pitched underside of the roof with celotex sheets, the rafters are approx 75mm deep.
I know you have to leave at least 50mm air space between the tiles/sarking and the insulation and if i was to achieve this i wiould need to build up the depth of the rafters.
Can i instead of going between the rafters with strips of insulation just use the full sheets of insulation as you would plaster board and glue and screw them directly to the face of the rafters, using some thin ply strips as a type of washer for the screws.
I havent seen any one else suggest this so im thinking there must be a good reason ?. i know there will be a slight loss of head height but im stooping up there anyway.

 

[noseall]

For what reason are you insulating?

Is it a token gesture or are you intending living up there?

 

[dave1001]

Just storage really.i took up the crushed dirty rubble strewn insulation that was the aftermath of having the roof re done.and ditched it.
i beefed up the existing joists by gluing and screwing some similar sized joists on top of them and chip boarded over them , but i didn`t replace the insulation.
It just seems logical that if you place the insulation boards like you would plaster boards, that you have a 75mm air gap between the insulation and the roof covering, and it saves on cutting up the sheets to fit between.
What am i missing ?.