I have an existing framed chimney with a 10 inch double lined insulated chimney with a rain cap. I am going to install a fireplace insert so I have to line my chimney with a 6 inch liner. My question is do I need to get a new rain cap or can I use my existing rain cap and attached the new liner to the existing chimney with some type of metal screws? Thanks for any input. This forum has been a huge help to me.