Smoke comes out of the Stove Door

[Greg1881]

I have a Quadrafire 3100ACC with a 6" stove pipe going straight up 16 feet exiting the roof. Pipe is clean with no blockage. Some days when I open the stove door I can feel the cold air coming down the pipe, other days I don't. When I start the fire with kindling I get a nice 12" flame going straight up after 5 minutes, if I try to add a piece of wood I start to get smoke coming out around the door seal and air control. If I crack open a window the smoke stops coming out the door. I've had this stove for five years and never had a problem like this. I did add 18" of insulation in the attic this last fall since it only had three inches.

Any ideas what I might be doing wrong? Thankyou

 

[bholler]

You no longer have enough air leaking into the home to supply combustion air to the stove