Insulating a Steel beam

[crispy_wafer]

Morning, I've got a steel ridge beam and two angled supports for a gable.  I've got a fair pile of PIR offcuts, can I just cut them to shape and fill the steel, then insulate over before boxing in?  Or would I need to leave an air gap.

 

[IanR]

If the steel is fully outside the thermal envelope of the conversion, then it may be better to leave it open and ventilated. If it is within, or partially within the thermal envelope then I'd go with option 1 and ensure there is a robust vapour block layer stopping warm air from getting to the steel and condensating.

 

You'll get better opinions if we can see the whole roof/wall build up and where the steel sits within that overall build up. If the top side, as drawn, is open to the outside, and the insulation is drawn roughly to scale, it looks under-insulated and a cold-bridge risk (with associated condensation/mould risk).

[SteamyTea]

My gut feeling is that option 1 is the better one, but as @IanR says, it will need a VCL if bridging the warm, moist interior with the cold exterior.

More detail.

[crispy_wafer]

Ok, so the ridgebeam, supporting a cut roof over a high pitched ceiling.  Pic below is probably better than my wording.  I've got insulation between the rafters, cut to the top flange of the steel, but not over.  

 

[freshy]

I have a similar situation here, the steel sits bridging the envelope. 

 

How should I insulate the steel beams?