Hello All, Thank you for this amazong forum and website! We have recently bought our first home, a BISF Type A1 in the Windsor area. We are so excited and took this an opportunity to do some refurb works.
We discovered with quite horror that the partition wall between master and second bedroom is a thin sheet of plyboard + one sheet of plasterboard, some wardrobe doors closed with lintels...No sound proof, no insulation at all.
So we are going to create a stud wall frame 3x2 inches, Blue plasterboard one side, grey plasterboard the other (funds are dwindling down).
I did some research and I am thinking of putting Kingspan TW55 (the 40mm to give some air gap?).
Is Kingspan TW55 ok to avoid rusting of steel frame in partition walls?
Can we use alternatives?
Any advice would be much much appreciated.
Ele
@Ele Welcome to the forum Ele and congratulations on buying your new home!
You'll be pleased to know that once you become familiar with the way the house was originally constructed (which shouldn't take too long), BISF houses are really quite easy to renovate and improve.
I can understand your shock though at finding out that the walls have no insulation or sound deadening material inside them. Many 1st floor bedroom walls were covered in a thin sheet of hardboard, whilst the lucky ones got plasterboard. You may also find that some ceiling boards are fibreboard too, or if you're lucky, plasterboard.
The good new is that there isn't any supporting structural steelwork or stanchions inside the partition wall between the front and rear bedrooms. Although there will be a boxed in steel flue support cage, usually right in front of you, as you walk into the front bedroom. It is situated just before the airing cupboard door (Depending on you layout).
The steel flue cage itself isn't strictly structural, although it is braced/connected between a large bracket in the loft-space, and a second bracket just below the floorboards in the bedroom, before reaching the fireplace below (Again this depends on your layout, because in some BISF houses, the fireplace is situated on the party wall). You don't really have to worry about potential rusting of this support component because it's not a structural vertical support stanchion. (there are many stanchions located in the external walls of the property, and two more vertical stanchions inside the wall between the front and rear rooms of the ground floor, but these internal vertical stanchions only go from ground floor, up to the ceiling where they join horizontal frame supports hidden under the floorboards of the 1st floor. In other words, unlike the external wall stanchions, they do not stretch from ground floor to eaves level.
Take a look at one of our former members image galleries HERE where you will see an entire house strippped down, and the dividing wall between the two bedrooms, completely removed. That should give you a better idea of your options, and of some of the structure behind the internal and external walls.
If you do choose to remove and replace the dividing wall, you may well have to replace the ceiling boards, depending upon their condition, or degree of damage the work causes. If that is the case, then I would also strongly recommend you consider fitting a larger loft access hatch, possibly with drop-down ladders at the same time. They aren't too expensive and they may serve you well in the future.
You will also find the party wall is concrete block, so not nearly as bad as the partition wall. Saying that though, it is still possible to hear a loudly snoring neighbour, but only just. 
If you have any further questions, don't hesitate to share them.
Best regards