Chemical damp proofing is a way to prevent walls from drawing water up from the ground through a sponge-like action; a process known as rising damp. To combat this, modern buildings are constructed with a tough Damp Proof Course (DPC) sandwiched between bricks all around the house just above the foundations.
This damp proofing course forms a barrier, preventing damp from rising up the walls. Unfortunately, however, many older buildings did not have a DPC applied during their construction or the materials originally used have broken down. In either case this allows dampness to penetrate through.