I took a call yesterday from a gentleman who said his chimney cap would get very dirty and clog up and was looking for a solution. He has an insulated class a chimney and a wood stove. The issue that causes this is the insulated chimney does such a good job of keeping the flue gases hot that it will hardly accumulate in the chimney. Once it hits the cold metal of the chimney cap it builds up rapidly. There is no real solution, however this chimney cap cleaning may help. A lot of caps come with a 5/8" spark arrestor mesh to comply with California regulations and tend to clog the holes in the mesh between chimney sweeps. There is a 3/4" mesh available that eliminates this problem in most situations where the smaller mesh is not required by code.