I don't know? I never have anything on my cap really. I'm even running a screen on it this year, nothing on it.I just spent the afternoon cleaning the nastiest tar junk I've ever seen from my duravent chimney cap. First used the torch to convert as much as possible to the dry kind of creosote and then all kinds of brushes, scrapers, and chemicals until I gave up and used a wire wheel on the die grinder. It's clean now but I do not want to do this again.
While I had the cap off I ran the brush down four feet and the pipe is quite easily cleaned. No tar.
Burning all 14% mc wood and never stalling the cat but my mistake was trying to use stat settings too low to keep flue temps up. Don't think that an active cat means you are good. You need an active cat and flue temps high enough to prevent condensation of the junk into tar.
The cat does not eat everything. The stuff I was removing today was absolutely not ice.