Hi, I want to remove a small window - leaving an empty space of approx 29 inch x 16 inch in a solid brick exterior wall, which is about 12 inches thick. I have already built the outer wall to a thickness of about 6 inches, leaving an inner empty space of approx 6 inches, ready for building the interior wall with 1.5 inch thick bricks, which would leave a cavity gap of approx 4 inches. In this 4 inch cavity I want put some approx 20mm cork insulation board, leaving an air gap of around 3 inches. Should I first install a plastic vapour barrier on the interior side of the external wall? Also, would the R value of cork board be about the same using 5 boards x 4mm or 1 board x 20mm?
Thanks