External & Cavity Wall Insulation

I am renovating a house and have to remove retrofit insulation from the cavity as there is damp. I am, also, upgrading the wall ties so was thinking I could put PIR in the cavity and since I want to clad with brick slips or render could put some external wall insulation (either EPS or PIR) on in order to achieve the required U value. What I don’t understand is why some PIR boards are not identified as for cavity or external use when their descriptions are the same, other than a difference in price. Would standard PIR boards be okay to use for the applications I have stated if I use membranes between each layer e.g. over brickwork then over external insulation before cladding?

 

PIR foam is ok for use in a cavity (although I'm not clear on how you are going to get it in there?) but you will need to follow the specification for the type of foam board. There are basically two types designed for full fill or partial fill. Full fill is self explanatory - if you use partial fill you must retain a 50mm clear cavity. Full fill boards have an additional external layer - usually plastic - and are houses into each other. (a bit like tongue and groove) Standard boards for partial fill are square edged.

You can use standard board for EWI. PIR will be a bit more expensive but is about twice the thermal value of EPS. So you could either have half thickness compared to EPS for the same thermal value or the same thickness as EPS for twice the thermal value.

 

You won't be able to fit or secure any rigid board in a cavity retrospectively.

 

Maybe a silly question, but have you ruled out all other possible causes of damp before deciding its the cavity insulation?