Thank you for your question on the model E64E30SS range hood and 332H exterior blower. The 332H exterior blower should have a back draft damper included with it. First check to make sure that it is working properly. It would also be a good idea to install a 10 inch damper in the duct work, close to where the duct work connects to the hood. Please contact the Technical Service Department at 800-637-1453 for further assistance.