Hard to comment on specifics without knowing more detail, but it sounds good so far. One thing I can say for certain, is that ANY builder using a sill pan of any nature bis light years ahead of most.

+1

Props to the builder for using some good building practices and not skimping.

You do not seal the bottom in these case so that the water can get out if it gets in.

Spray foam the interior gaps though.

So it's ok to use black plastic instead of an adhesive such as flexwrap by DuPont and leaving the bottom seal open? Or do you think it would be better if we went with a pre fab plastic sill pan?

Pre-fabs or systems that are designed to work together would be better but I would rather have a guy that knows what he is doing with tar paper than a guy who is clueless with a system.

Sill pans are a great idea. Plastic sill pans under windows are "okay". I have seen them crack under doors (too heavy). A metal pan is a better idea but more expensive. Making your own sill pan from flex wrap or some other self adhesive flashing is just another way of doing it and as others on the board have said is a good practice.

+1. Sounds like a winner to me.

There is nothing wrong with his methods. You just heard a few different ones but if this is the way he does his sill pan, letnhim do it his way. He obviously knows what he is doing.

The fact that he wants to use a sill pan is a plus in my opinion. As stated there is nothing wrong with a plastic sill pan. Also like previously mentioned I prefer metal for doors as well as opposed to plastic.