Insulating under the rafters and floor

Folks, sorry if this has been covered but the advise given seems to contradict itself in places.

Firstly I want to semi convert my loft so that it is more friendly for storage as we are moving the central heating boiler to the loft. The loft will not be a permanent room as we will not be installing permanent access and plan to use the current fitted loft ladders.

I have a 50's build semi. the roof is tile, what looks like tar impregnated hessian and 2' x 2 3/4' rafters which 16' apart. The ceiling joists are also 2' x 2 3/4' and 16' apart. There is currently rockwool insulation between the joists and the loft floor is boarded with a mix of 12mm chipboard and I think 11mm OSB.

What I was thinking of doing was to to hang Celotex or Kingspan under the rafters, build a knee wall about 18 - 24 inches high running the length of the loft on both sides which will also insulated with Celotex or Kingspan. I would also increase the depth of insulation between the eaves and the rear of the knee walls.

From what I have read here I think that it is OK to just hang the insulation under the rafters as long as I maintain a 50mm gap as this will provide enough circulation to prevent moisture building up. Is this correct or have I missed something? Also what would be a good depth of insulation to install even though I do not necessarily need to meet building regs.

Secondly I have read posts suggesting that it is wrong to insulate the loft floor and under the rafters but there were other posts suggesting that it is common practice. So clarification would be really helpful as I thought that an insulated 'box' would not be a problem.

Is there anything else I need to consider as I know that I may need to heat the space but was going to evaluate that after the boiler was installed. I do not think that there would be a problem with the electrics as it is only the lighting circuit that is run under the current insulation therefore they would have with the same amount of insulation as before but maybe the boards would have some cover to reduce dust e.g an old carpet.

Thanks
Ent

You either go and have a full loft conversion done or you have loft storage area, nothing in between will work and only cause problems later.

Andy

I don't know what HERTSDRAINAGE2010 is on about but that aside if you want to insulate at rafter level and have a non breathable membrane under the tiles then you will either need continuous ridge vents fitted or fit a roof vent near the top between each rafter. Otherwise you will get problems with condensation.

Thanks for the reply FMT. I think that I am having a bit of a brain fart with this. I just want to check my understanding vs your expert advice.

I was planning on hanging KingSpan under the rafters rather than in between them.

Would I still need vents at ridge level as the 70mm gap would allow a free circulation of air from where the rafters meet the joists at floor level, all the way up to the ridge and back down the other side. I was also planning to have about a 12" gap between the ridge and the insulation panels running along the top making a narrow ceiling

Would standing off the panels from under the rafters make any difference?