You certainly need one. Not just for air but also for insects etc. Very few commercial hoods come with them internally but they usually offer one as an external transition. I used a wall vent that had one built in for my modern aire hood.

You do need one for the reasons mentioned. Does your wall cap have one in it? I have a roof mount from MA & it included a damper, but I put an inline one in anyway.

Thanks for the info. There is only a screen on the outside wall vent, no damper. Will discuss more with my hvac guy.

really shouldn't be a big deal to add one at the starting colar on the hood. I have one integrated into my roof mounted blower. However, I allot of cold air still coming in. Probably from the duct work being cold in the attic in the winter and convection. Thus, I added a Fantech 10" damper in the run from the ceiling to the starting collar. Or they can replace the outside wall cap with one that has a damper. Never saw a wall cap without a damper especially for a rangehood.