Yes, Windows 10 install without an internet connection. Rather, none of the installation processes in itself will need an internet connection.

However, it is strongly advised to keet the internet connected hard-wired to the router while performing a clean boot for the following reasons:

1. The upgrade installer will try to install Windows updates while installing Windows 10, which includes the latest featured update. You will miss that if the system isn't connected to the internet. You could surely install them later as per your convenience.

2. The upgrade installer will download the latest drivers from the internet during the installation. Else, you would have to update the drivers yourself.

3. It would help in recognizing the internet time, and you could connect to the wireless network during the final stages of the setup itself.