Steel beam insulation

[Ivor Windybottom]

No just use it for domestic stuff (drying clothes gym etc)

You definitely need ventilation then. Otherwise, even though your lintel isn't very cold it's probably the coolest surface in the room so will be acting as a dehumidifier by causing all the airborne moisture to condense onto it.

You can't just keep adding water to a sealed room, it all ends up somewhere unless you remove it somehow.

 

[freddiemercurystwin]

Spend the money on some thermal socks and a fleece then. What you're proposing seems to have no value, or point.

 

[oliverwilliams18185]

Woody is right. That is a lintel, not a beam, and the holes are just the plaster key. Skip the membrane. You want the insulation tight against the steel with no air gap. The gap is where condensation forms. PIR board with foil backing, cut snug and foamed at the edges, does the job. If the garage stays unheated, do not overthink it. The risk is worst where warm house air meets cold steel. So seal the room side well and leave the garage side alone.