I dont think insulation is your answer. I would measure the opening, and find a good quality window, to put there. if its an odd size, you can have it custom made. It would eliminate having a big patch on the outside of your house and take care of the cold air that is coming in, at the same time. A much nicer and better solution to the problem.

Do you have access from the interior? If so take a picture.

@cat_ky Hmm thanks. We had a similar vent in the back of the house that I was able to remove the cover and insulate and it's done a great job keeping out the cold. Perhaps insulate it + a glass window? Or maybe I could stuff batt insulation in there, then some rigid foam insulation cut to size, behind the vent cover? Wouldn't that in theory do the same as a window? Just a way to block cold air from entering from the outside? Not trying to make this a big project and I worry the window would be complicated and also look odd out there.

Most times you just seal it up from the interior with rigid foam and caulk and leave the vent as it.

It appears to be a wood, louvered foundation vent. If you don't need to vent your foundation, I suggest you have a mason remove the vent and brick up the opening after you insulate. That will eliminate the maintenance and make it more difficult for critters to enter your home. If you opt to keep the louvered vent, I suggest replacing it with one made of PVC to eliminate the potential for rot.

@millworkman here is the interior. The exterior vent feeds a duct thats about 3ft long into our mecanical room. It was venting our meters. I stuffed insulation in there and rigid foam from the inside, but could air is still entering the vent and escaping into the home/the mecanical room. I'ts very cold.

Nope there's no hot water tank or furnace in there. We have our gas meters (its a 3 family building) in there. I was told that is why the vent was there but that we don't need it.