It might be better to refer to this as a copper chase flashing than a cap. Most people think of the cap as the vent termination. The mfr. specifies how a chase can be flashed and if it must be ventilated or not. Yes, you must protect against dissemilar metals. This vent appears to be crooked near the top. There is no visible support in either pic. There must be a listed firestop and fireblocking per the codes and listing. If you find plywood under the chase flashing, it must be cut out to meet clearances. If you have combustible membranes on top of the chase, you're looking to burn the house down.
Stated clearances are air spaces. The unit is covered with two types of insulation which must be removed. What is that framing cut out? The elbow strap does not appear installed. If the vent penetrates and insulated space, there must be an attic insulation shield.

So, how did you get into the chase to get these pics?

Bob Harper

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disgusted with some people on this forum. Out of here!