So a bit of background. One wall in our upstairs rooms doesn’t get any sun from outside. This means it is always very cold on that wall (the rooms warm). Combine this with they both have a chimney means that there are cold spots and we get a LOT of condensation. Its my intension to get this battened out and insulation put in (about 2”) then plasterboarded over. This will remove the fireplace that is currently there. However I have had a thought that this could be a good opportunity to get some fitted wardrobes along that wall. Is there anything that could go wrong with this idea where I dont get it boarded over and just sandwich insulation between the existing wall and back of a built in wardrobe?

Well, imho you need a vapour barrier on the “warm” side of the insulation, i.e. on the room side, otherwise you will still get condensation on the wall and end up with wet insulation. Also, many forms of thermal insulation tend to be friable, so you may find it shedding fibres or granules if you don’t properly enclose it.
The next owner of the property may not be too impressed if they remove the wardrobes either, although that might not be a concern for you……😐

I think I’d insulate and then build wardrobes, is 2 inches of insulation enough?

Lots of black mold behind the wardrobe and stinky clothing would be the outcome.
Sort the issues first. 25mm foam onto 12mm plasterboard will only take 40mm off the room after a skim

We had insulation board (around 50mm and foil backed) glued and screwed directly to the wall and directly plastered over as part of some grant aided insulation work on the house we rent.

I think I’d still fit the plasterboard. It won’t add much time or cost. No need to skim it. Makes it a proper wall then you can be a bit more flexible with your wardrobe design. We’ve insulated one wall in our bedroom. It was freezing and a real cold sink. 50mm of insulation has made a huge difference. Well worth doing. I taped the joints and wallpapered it. It is now warm to the touch and no signs of damp.

I had an issue with a North facing sheltered wall with an old chimney too. Put so easy proper ventilation toland bottom of chimney, sorted out the damp, but I wouldn’t dream of putting any furniture at that wall.

If you use foam, definitely plaster board over. The foam is relatively easy to ignite and produces seriously toxic smoke. Anything you can do to slow down fire getting to it is time you can use to get out.

Are you talking about covering up the fireplace? You’d want to have it properly blocked up, otherwise it’s just letting cold air (and animals?) Behind your wall. IDK if you can insulate right up to chimney might need a gap to combustible stuff.

My bedroom as a kid was in a corner, so had two outside walls, which they’d made an attempt to insulate with a type of foil and thin polystyrene which we found under the woodchip wall paper. The insulation must have been 1mm thick and was totally inadequate.
That room was bloody freezing at the best of times and wasn’t helped by the single pane sash window that had a 1/2″ gap at one side of the bottom of it so the wind blew in in winter.

Were i to insulate it today I’d strap the wall with 1×2, fill in the spaces with foil backed foam board and double layer gyproc over the top.

Then get a bloody big radiator 😆

Use insulated plasterboard with 50 to 75 mm phenolic foam fixed to it. Can be dot and dabbed or fix battens to the wall and screw to that. Vapour barrier is built in so either skim with plaster or tape gaps and paper once fitted.
Kingspan is the regular brand and their site shows installation methods even if you use a different brand.

Did it to the outside walls of our bathroom in the last house that were N and E facing in the offshoot at the back of a terrace. Went from coldest to warmest room in the house.

Note than rockwool and white foam block have about half the insulation value per thickness of phenolic foam.

Sorry that stuff slowol above is on about above is what was used for us. The plasterboard attached to the foam.

Ah right.
I was thinking there would be little difference between boarded in rock wool and backed plasterboard. It seems there is. Bugger.

The chimney in one room stops in the loft now but I’ve not capped it to allow ventilation. I’m worried if I don’t put a vent in the board it will trap stale air that could cause damp?

I put two vents on outside wall, one top, one bottom, and one inside too.

I’m worried if I don’t put a vent in the board it will trap stale air that could cause damp?

If the chimney is boarded in it should be vented so sounds like something is not right, as does the condensation, if you are insulating the wall it’s not a big job to strip the wall back and do it properly, 50mm gap, then framed with CLS, min 50mm Celotex board but you can go up to 100 inside the framing without losing more space in the room, membrane then board…after bricking up the chimney, if it’s vented at the top just leave it or vent it to the outside wall…the only hard bit really.

If you vent to the inside you will draw loft air into the room if the chimney tops there.