Steel lintel/beam above patio doors and front door

[akist]

There is a steel beam/lintel above my patio doors and above the front door. This mass of steel will transfer heat above the doors to the outside, so I will likely get condensation over the doors. Typically the exposed part of that steel lintel would be covered by 12.5mm plasterboard, which I'd glue to it with Sticks Like. But maybe the heat will also be lost through the plasterboard. To improve insulation I have had ideas of covering the underside of the steel lintel with double plasterboard, or maybe bolt some ply onto it first and then the plasterboard. How do other people do it?

 

[bsr]

If you have room you can use foam to glue on 25mm insulation and then 9.5mm plasterboard.

 

[akist]

I have 37.5mm insulated plasterboard I could use, but I am worried about the fixing, it is just foam (rigid but still) and paper at the back. I want something sturdier.

 

[crank39]

 

[akist]

I have a whole lot of these, Marmox boards they are called. I originally got them to tile over shower areas upstairs.