The external insulation stops lateral damp and internal condensing on outer walls, The was a member called Nonem on here who was selling some kind of damp prevention injection system, I think he got lynched.
Martin
DPC fluid (water bassed Damp Proof Course) is the most effective product for proper brickwork. It has a siliconate structure which turns to a solid on contact with salts found in the ground. However if theres no proper poor structure it will never work
Remember DPC fluids and creams are poor liners not poor blockers. DPC fluid applied as a surface coating to an external wall will simply lubricate the water going into the wall and make it worse. The poor lining required for an efective DPC is measured in 1000s of a mm (Microns)
Water repelents like Thompsons water seal are a MUCH higher grade of silicone and are designed to reduce the poor size in a structure to a dimension what allows water vapor through but not physical water. They are very tricky to use correctly and are usually solvent bassed and can damage all sorts of things especially nearby parked cars
DPC cream i the new best thing but I struggle with it. The system is injected into mortar beds using a mastic gun so an easy DIY fix but the whole "vapor phase" spread concept seems a bit far fetched to me. Many of the cheaper ones were proven to have insufficient active ingredients to actually work? conditions for it to work are IMHO limiting.
Generally in the industry Safeguard Chemicals is considered a good product and its often rebadged.
https://www.safeguardeurope.com/products/dryzone-system/dryzone-damp-proofing-cream
Where i do like the cream based products is on external walls. The storm guard is much easier and safer to apply than solvent based water repellents.
https://www.safeguardeurope.com/products/stormdry-system
IMPORTANT
Remember if you water proof a wet wall externally using a vapor reducing or vapor proof product the water has only one way to go. Inwards to the internal plaster.
A wall at 8% moisture content + (typical of rainwater penetration) means 8% of the physical weight of the wall is free water. That's a LOT of water.
Try to do external waterproofing/cladding etc at the end of a long hot dry spell
ATB