Replacement of old lath & plaster wall will plasterboard

If it’s a stone cottage then you shouldn’t be trying to use modern materials you will just cover up the problem Your house needs to breath not be suffocated and an air gap is will be useless as your walls are telling you that they need attention like removing some of the plaster to see the extent of the damage, are the outside walls pointed up with cement and sand if they are it needs removing and replaced with a lime based mix, and make sure a dodgy builder hasn’t been putting cement based products on you house internally

Hi If the damp has dried, and walls repairable where damaged, you could consider applying an insulation paper internally “Wallrock Kv600” I use this in a number of situations and find very effective Google for info Good luck

I mainly agree with Steven’s answer above. 1. Try to avoid using modern materials 2. Although airgap can be a good idea but can you make sure it will be properly ventilated? If not, strict NO to any modern polystyrene or similar materials, as you will escalate the dampness in the cavity. 3. Is the cottage listed? Then you need to talk to the heritage officer if the change in the fabric is not a like-for-like repair. 4. Breathability - salt contaminated plaster and substrate to be taken back (all cementitious material should be taken back) and you can use a lime based material for plastering. There are some thermal plasters which are designed for old buildings, insulating them with their cork content, at the same time they are breathable (lime and clay based). Have a look at this product: https://www.ecologicalbuildingsystems.com/product/diathonite-evolution We have been using that for years on old buildings. Good luck! Adam ECO-Renovation UK

The easiest and cheapest solution is to build a timber stud wall at least 50mm away from the existing wall to prevent any damp getting through and allowing air flow. Then put in insulation then simple plasterboard.