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There are two approaches, but there are pros/cons to both. It's not always essential to do either. Note that dot & dab is a fast/cheap/bad approach for insulating internally: thermal bridging, poor sound proofing values, potential condensation risks on foil and within low-airflow areas.
Working with the existing mass using direct application fibre board will limit you to ~60mm of fibre board, or a U-value of ~0.5 W/m2K (based on a double run of 102.5mm bricks), but on the plus side you would have few concerns regarding moisture ingress over time. You would be prioritising permeability which can be critical for pushing back moisture levels within wet/dark environments, or for ensuring the long-term health of heritage buildings.
Working against the mass by building a new sub-structure (NOT dot & dab!) means you will have to carefully calculate your dew point and consider ways of bringing down the moisture level within your ventilated cavity, but on the plus side you will have no limit to the amount of PIR you could use - but only if cavity moisture is modeled/managed effectively. You would be prioritising performance/utility and would accept some external moisture ingress risk as you already have dry/sunny elevations in summer.
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JonoMarshall
@ectoplasmosis
I’m looking to internally insulate the solid brick Victorian walls in a couple of our bedrooms; have been advised that using wood-fibre boards stuck on using full-surface breathable adhesive is essential, as opposed to PIR/EPS backed plasterboard attached via dot & dab.
Any tips on type and supplier of wood-fibre boards for this purpose? So much out there… or are they all basically the same and I should choose based on price?