When you deploy a new Web Forms target, a temp directory is created in the Inetpub\wwwroot\application_name directory, where application_name is the name of your application, and several subdirectories are created in the Inetpub\wwwroot\application_name_root directory.
Files are written to and deleted from these directories, therefore the IIS_IUSRS group must have full permissions on temp and application_name_root.