I have a new construction home and it appears there was an average job at best insulating our bonus room above the garage. Both sides are flanked by attic space and when I crawl into the attic I see the attic side is sealed with OSB but I believe these walls were 2x4 so only R-13. I’m wondering if I would benefit from doing a proper air seal with a 1” rigid foam and also foam/block the joist cavities under. Any input would be greatly appreciated.

been there.....had the exact same thing but weren't planning on staying in the house so I added register buster fans and that took care of the cold.

https://www.google.com/search?q=register+booster+fan&sxsrf=ALeKk01pX7GUOfVj75bRWZUGomRGx9RF-g:1607768047780&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjXvPnamsjtAhWQuVkKHY4mBtAQ_AUoA3oECA8QBQ&biw=1920&bih=956