Hello - I am planning on installing a regency I2400 insert and both regency dealers and chimney-sweep/installer told me I don't need a blockoff plate at the damper. The manual doesn't mention it either. I haven't decided if I will install it myself or have the sweep do it. Should I use the blockoff plate? My chimney is external and is just shy of 14 feet. There is not enough room to insulate the liner. Thanks for all your help.