Insulating/ceiling Steel Frame conversion

[daunker]

I am converting a steel frame barn, it is single storey and plan is to have vaulted ceiling - it is single pitch roof. Ideally I would put insulation below the steels, but once I build up insulation on floor height will be restricted on one side of property. So my hope is therefore - to sit insulation between the I rafters, and on the lower "exposed" face of the rafter to fix 50mm (or more) strip of insulation to break its contact. 

 

The steel beams have existing angle irons above - to which ply and zinc roof will be attached. Ceilings are to be timber due to likely cracking in wind load with plaster (Structural Engineer).  The span between rafters centre to centre (and stanchions) is 4.5m. Span between Angles is 1.4m. The rafters have 130mm space in the I, so it would kinda be perfect for insulation.  Just am not sure how it would be possible to support fix insulation and ceiling there with such wide spans. In my head I was visually celotex but perhaps there is something different/lighter/smarter that could occupy this gap, and maybe fixing (after a ventilation gap) to the underside of the ply for Zinc would work?

 

Have thought about insulation above rafters (timber joists in between the I), however this keeps the steel within the thermal envelope at this point, but it leaves this zone as it bridges to the outer wall, which is outside and creates significant area for heat loss.

 

Have attached photo- steels painted with zinc phosphate - timber rafters below support dropped ceiling supported on block - but this is being removed.

Edited June 6, 2022 by daunker