Wider timbers on narrower steel - how to meet building regs??

[Eggnog]

I'm mid replacement of a first floor single bedroom above a bungalow.  Original work done very badly so major steel schedule and timber support frame being used to stabilise.

Frame sitting on padstones sited mostly on single skin walls so bulk of steel framing is 203x133 UB.  Single gable frame above also 203x133 with remaining support in timber sited on base steel frame. 

Engineer has specified 50-140 studs to sit on top. However, as insulation reqts now at 120 plus breathe space we need to use min 6inch or probably 7inch wide support timbers for remaining structure.  Meaning timbers will be overhanging the steel they are sited on.

Engineer has not specified a welded wider base plate on 133 steels.  A 133 (140 with small overhand at rear) timber beam will be attached to the 133 beam first to fix joist hangers.

 

My questions are:

• Will 140 studwork accommodate current insulation requirements?
• If not and we use 160 (or 175) stud work - How do we site studwork on the narrower base?
• What will building control want to be assured of here??

Any help gratefully received

Happy to share design if it helps

Not my first major project but not come across this problem before when insulation standards were lower

[George]

You'll need to add some pictures because I don't understand the issue 

[markc]

As above, whatever stud size would be on similar sole plate on top of the steel or other support.