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FWIW I have stuck PIR backed PB to all the bare-brick external walls of my cavity walled Edwardian refurb directly. Mostly using PB adhesive. Instastik foam isn't so good if the wall isn't smooth - dabs give more scope to knock the boards level. Overboarding internal walls with standard PB which were flat, I used instastick more. 3 years since this was done - it's all still there and the house is toasty.