Bit of a problem with Inset.

 From:  Michael Gibson
5508.12 In reply to 5508.11 
Hi Martin, I see - but that seems like it could be odd to use Inset for such a purpose since the overall purpose and UI for Inset is focused on making something with a border around it.

Maybe the kind of thing that you're talking about would be better as a separate command or some kind of other option in Offset or Shell instead...

You can get the basics of it right now by using Edit > Separate on the face, then doing a shell, then deleting the bottom face of the shell and then joining it back in.

- Michael