What type of installation?

Metal was used as flashings for 100's of years prior to FlexWrap and is more proven in the end.

Yes. I would have him do it in either PVC or enamel coated coil.

Wood opening. Does aluminum butt up against jamb on sill and caulk or does aluminum get folded/ and go up jamb an inch or two?

Most window guys will do a butt. Most siding guys will do a fold, but they don't always caulk it, either.

Obviously a fold WITH caulk is best. But a tight butt with the proper caulking (silicone or better) is actually fine. You just don't want a large gap improperly fitted and then compensated with latex caulk. Latex is cheap for a reason.

Hope this helps.

Ok, i think we are on the same page. When he does the install, he places new window on top of old sill.. Its the old sill that he covers with aluminum( 1/2 the sill ) and then "L's" it out. Then puts new window on top. Is that what you guys mean?
Is there an advantage of aluminum over flex wrap?
Thanks for the help window washington and ecostar.

The aluminum will be cheaper and look more seamlessly consistent with the wrapping on the rest of the home/windows.

ok.. thank you all for your responses.

Some of those guys would use coil stock for a table cloth if you let them.

That's a bizarre comment - how does it apply and what does it mean?

I know flex wrap is hip these days, but I've seen it around window nailing fins totally not sticking, or sticking just long enough to get the siding up so no one can see it coming off. Granted, I haven't personally experienced it more than a 1/2 dozen times, but building methods change all the time. (Anyone remember gyp-lap?) As far as flex wrapping a sill - maybe for new construction it might make sense if you can feel confident about it. I would still rather use a sill pan. For replacement purposes, flex wrap wouldn't apply the way it was intended, so I don't see the point. Plus it wasn't intended to show as a finished surface, whereas aluminum is.

I have coil stock as my drink coaster right now.

Works great.

+1 on the comments about the flashing tapes. They are great but only when used properly and not in this application.

A friend of mine wrapped the lower panels of his van in aluminum to cover the rust. It looks pretty and iwould not have caught it if it wasn't for the rivets.