Preventing damp and mould in solid wall home - foam insulation with plasterboard?

[JP_]

OK, so I'm renovating away. Plan is to remove, treat and then insulate below suspended floorboards, put 50mm PIR on the main wall and plasterboarded (all with everbuild pink foam).
But the side wall is partly external - about 40cm has the porch to the side and the floorboards and skirting is rotten here.

So, I was thinking of putting 25mm insulation and boarded over along this part, leaving a stepped wall. Wife happy with the idea. But, if that overkill? If I use plasterboard with lots of foam, to act as insulation, would that be enough to ward off the condensation damp issues?

Happy to use insulation board, just wondering if I can get away without it to save a couple of cm, and a few pounds.

Not got a good photo, but this is the corner I'm talking about. Wallpaper side is the part that sides with the porch.

 

[JP_]

A better pic. External wall fits up to the utility bar.

 

[endecotp]

Consider insulating that wall further into the room.

Where an internal wall joins a solid external wall, some heat is lost from the room via first the internal wall and then the external wall. So when insulating the inside of an external wall, it it worthwhile to also insulate the first part of internal walls that join it. In reality people don’t often do this because the resulting “step” looks odd. But in your case, you’re going to have a step anyway. So you might as well go beyond the point where the wall becomes internal. Maybe one 1200mm board width?

Do it properly with an insulation board, not just spray foam.

Note that you should probably use a few screws at the top of each board as well as the sticky foam; this if for fire safety, so that the boards don’t fall off the walls and block your escape route when the foam melts.

 

[JP_]

Yeah, i read that before. If a fire is so hot that it melts the wall behind the bed, i reckon I'll be past the walking stage of life!