Chloe

Hi everyone, I’m new to this forum and are looking for advice on an internal wall insulation (IWI) project for a 1930s semi-detached chalet. It has original solid brick walls (no cavity), but a complex mix of exterior claddings on our freezing North-facing elevation. The Setup (North-Facing): Ground Floor (Lounge): The exterior has an exposed original brick plinth at the bottom, and cement pebbledash on the upper half. Inside, there is a large original-style bay window with integrated shutter blinds. First Floor Right (Master Bedroom): The exterior here is entirely covered in cement pebbledash. Also has a large bay window. First Floor Left (Ensuite): This has a unique diagonal split exterior—the top half is vertical tile hanging, and the bottom half is cement pebbledash. Inside, we currently have severe black mould in the shower area on this external wall. My Dilemma: My builder has recommended Kingspan K118 (PIR) drylining for its high U-value and thin profile. However, I’m highly concerned about trapping moisture behind a non-breathable foil-backed board, especially where the exterior is wrapped in waterproof cement pebbledash. I'd love your thoughts on these three specific issues: 1. Breathability vs. PIR: Given the cement pebbledash, would a vapour-permeable system (Wood Fibre + Lime Plaster) be definitively safer for the ground floor Lounge and the Master Bedroom to prevent interstitial condensation? 2. The Bay Window "Shutter Trap": I have integrated shutter blinds in the bays. If I use a breathable wood fibre system (typically 60mm+), the shutters will hit the new thicker walls and won't fold back. Has anyone safely used a thinner PIR board here by applying a breathable hydrophobic cream (like Stormdry/ProPerla) to the exterior pebbledash? Or is Aerogel the only safe thin option? 3. The Mixed Ensuite Wall (Jackoboard): My builder suggested stripping the mouldy ensuite wall back to brick and using Jackoboard (XPS) to create a waterproof, insulated "warm box" for the shower. Since the exterior of this specific wall is half breathable (tile hanging) and half non-breathable (pebbledash), is sealing the inside with vapour-impermeable XPS safe for the brickwork across both of those exterior finishes? Any advice would be hugely appreciated! Thanks so much!!