Hi, we are renovating out Victorian Maisonette set over three floors, it has very thick brick walls of around 400mm and has had some moderate damp issues in the walls (seemingly due to a cement parge coat in places and leaking window ledges)

The plan is to use Kingspan k118 on the solid brick walls with timber battens to achieve a U-value of 0.3.
I am aiming to get the building as airtight as possible so one concern is that the insulated plasterboard won't create a particularly good seal against the very draughty brick work.

I was thinking should I do a parge coat in lime on the brickwork before installing the IWI?
Is that a bade idea as it would reduce ventilation to the air gap behind the insulation?

Also Was planning to continue the insulation down between the joists maintaining the air gap by foaming an insulated board between the joists. Again is this a bad idea due to reducing the ventilation? (We already have put 200mm of rockwool between the joists.

Thanks in advance

It is a solid brick wall of ~40cm in depth (may be that thick due to the height as it is 4 very tall storeys)

I don't like hiding timber battens on the cold side, but Kingspan's document
https://www.google.com/search?q=installing+kingspan+k118&oq=installing+kingspan+k118&aqs=chrome..69i57.8552j0j7&sourceid=chrome&ie=UTF-8 says:

''Adhesive Bonding
Drywall Adhesive Dab Bonding
 Drywall adhesive dab bonding must always be used in
conjunction with a bonding agent applied to the foil facer,
as per the following sitework instructions.
 This method is suitable for use on brick, block or concrete
cavity or rendered solid walls, which are dry, stable and
free from moisture penetration.
 ***This method is not suitable for use directly on non rendered solid masonry walls where there is a risk of
moisture penetration***, or on timber or steel frame construction''.(***my emphasis, Nick P ***)

I have used PIR and warm battens direct to the wall. Using 'raw insulant' gives you a better chance of getting a tight VCL. How ydo you ensure continuity of VCL on insulated plasterboard?

I would definitely use a lime parge coat.

But above all, don't use IWI over pre-existing damp issues unless you are jolly sure you have cured them.

(Note that a preference for rendered walls - material not defined - could mean (if the render is cement-based) that you may be cutting off the 'breathing path' to the exterior, having blocked that to the interior with the VCL.

Thanks for the advice, EWI is definitely not going to happen as council would never allow it.
Parge coating seems to be a good option for air tightness just my hesitancy is as to whether I should even be trying to get the gap between the insulation airtight.
Is the air flow due to leaky brick work expected to help dry out any moisture that makes it's way in there?

I would not expect the VCL to be anywhere near perfect with insulated plasterboard but we can try our best with sealant and foam.

I looked at using separate insulation boards (K112) but it was working out more expensive on materials alone let alone extra building costs.

I would also note we are not able to apply IWI everywhere anyway due to limited space in hallways and stairs so sadly so really just aiming for the best we can do (hopefully this won't cause too many issues?)

The government has recently published guidance on this. It's a good guide covering all the relevant details.

https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice