First caulk every crack - around the doors, windows, along the roof line and where the floor meets the walls. The most important place to insulate is the roof, second the floor (if you have a crawl space...) Third is the walls. Rolled insulation is the easiest.
Hi. Use foam board rather than batten. It's easier to work with and can be covered or left. Sold at big box stores. Good luck.
You could use sheets of insulation and glue them up
They look like large sheets of paneling but there insulation put them along all the walls