Boxing in steel above bifold doors

[Pauls extension]

Hi i have been slowly building a new extension and am at the stage of plasterboarding. I have a large I beam with a 300mm plate welded on the bottom spanning the opening for the doors. The doors are fitted to the underside of this plate. I need to plasterboard the inside of the steel from the doors back and up the face of the steel to the ceiling. It is the underside i am struggling with as it is a cold bridge from the outside. If i bond plasterboard to the underside, will this not go damp from condensation on the steel?
Can't find anything online that shows how this is done 'as standard' hence asking the question here. Any advice appreciated. Thanks

 

[^woody^]

will this not go damp from condensation on the steel?

Not if you stop moist air hitting it in the first place.

But this might need mechanical fixing anyway for fire protection.

 

[Pauls extension]

Hi woody, the outside is just above the soffit but is still cold. I have insulated as much of the steel as possible, it is just the plate on the bottom, i can feel the condensation on the inside in the morning if its been a cold night.
Would a layer of 25mm insulation then the plasterboard be sufficient or is that still likely to get damp?
Does this need to be screwed for fire regs or can it be bonded?
Thanks

 

[^woody^]

Don't you have any plans to detail this?

The beam should be insulated on the soffit and internal face and this deals with the thermal bridge and condensation prevention. This may be the same as or additional to the insulation requirement for the external wall.

For any fire protection, mechanical fixing is required and normally it needs noggins in the web, and a batten on the soffit - but this often impacts on the door frame, so depths and thickness of materials need careful consideration.

 

[frutbunn]

If its single storey you won't need fire protection.

 

[Pauls extension]

No, plans the architect supplied are very basic.
If boards need to be mechanically fixed on the underside, what would be the best fixings to use seeing as the steel is a 6mm plate welded to a 10mm steel?