PU Foam

Expanding polyurethane foam in a can. It goes in as a liquid, swells to many times its volume, and cures into a rigid closed cell that fills a gap completely rather than merely bridging it - which is why it is used where nothing else will reach: around door and window frames, in the gap between frame and blockwork, sealing pipe and cable penetrations through walls and slabs, filling voids behind fittings, and stopping draughts and vermin runs. It sticks to almost everything, including hands and clothing, so wear gloves and mask anything you do not want it on. Fill about half the gap and let it expand into the rest; overfilling is the classic mistake and forces frames out of line as the foam grows.