Answers

There are companies that offer energy evaluations and some even use thermal imaging and a tool / process called a blower door reading / test.  This will help identify the cause and opportunities for correction.  You can do some easy items as well like ensuring all like plate covers have insulated pads behind the covers, check for insulation around your homes perimeter if you have an unfinished basement, check your homes exterior for small gaps or holes left from old utilities and service work performed over the years.  Best of luck!

The best move would be to simply make sure the existing vapor barrier product is installed properly (flush to ceiling below) and add blown insulation over top to to get the best R value and at the least expense.

No need for s econd mositure barrier which is what the paper-face is. Just roll out unfaced insualtion atop the existing. Or, as an option, rent a blower and use loose-fill insulation. 

Rockwood insulation should do the job

The caulking mentioned is fine to seals the gape but you may need somethingto back it other thatn backer rod. in that case you may need a small amount of Rockwool insulation to pack into the void befor applying the caulk. This is a link to the product https://www.rockwool.com/applications/exterior-walls/firestopping/

If you only need a small amount check with a local insulation company to see if they will sell or even give you the smaill quanity you need.

I agree with Jared. You will need a high temp sealant.

Good Morning,

Nice space.  Its hard to tell from the photos just how big the space needing to be filled is however 

Dap 18854 High Heat Mortar Raw Building Material, Black

should work well. It can likey be found at you local hardware store of online

https://www.amazon.com/18854-Mortar-Building-Material-Black/dp/B0006MXS4C/ref=asc_df_B0006MXS4C/?tag=hyprod-20&linkCode=df0&hvadid=167127061369&hvpos=1o5&hvnetw=g&hvrand=16226564078373131268&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032160&hvtargid=pla-307130896304&psc=1

Best of luck and Happy Holidays

Jared

Blow in, but will have to hide it with a new ceiling I believe...

Blow Insulation 

Energy efficent windows are a good investment.