20. Installation – ventilating the void when it is not backfilled. You also need to think about your proposed treatment of the void between the liner and the chimney. XXIn most cases there is an unarguable case for filling the void with insulating material, (see chimney section) but if the void is to be left open it must be ventilated to allow a circulation of air to take place – otherwise condensation can become a problem. XXThe recommended technique for ventilation is to drill a couple of 15mm holes opposite each other into the chimney head just below the cope. These are sloped upwards to discourage rain penetration. Two short lengths of copper tube are then tapped into place (A. & B.) to ensure that the vents remain open and serviceable.