Protective Wire Balloons, Chimney Cowl fixed over flues to deter and prevent leaves, air born rubbish and particularly nesting birds from blocking downpipes or flue. These simple mesh guards sit at the top and just inside the chimney top. They prevent the ingress of smoke into the interior of the home caused by a chimney blockage. These simple but effective mesh barriers are available to both trade and DIY customers at C & W Berry.