A few years ago I came across ultra-insulated high efficiency fridges, like this now discontinued SunFrost design.
Extra thick insulated walls are only part of the reason those are so efficient, but I figured, is it possible to at least get kinda close by adding extra insulation to the outside of a normal fridge? For many fridges, the answer is yes!
This old post from a 2012 eco-modding forum claims that, using just regular old styrofoam, they were able to cut down the energy use of their old energy-hog fridge from 274kWh per year down to 128kWh per year! (And they wrote a little guide on how they did it too).
Those sorta numbers kinda blew my mind. Not only is it possible to save more than half in yearly electricity costs, it also keeps an old fridge out of the landfill instead of being replaced with a modern, more efficient one. And bear in mind, that's just with styrofoam, which according to this handy little chart, has an R-Value of 4 per inch (High density EPS, i.e, Expanded Polystyrene, is what is usually referred to when we say styrofoam). Imagine what sort've numbers they could've got with proper insulating foam with an R-value of 10 or 13 per inch!
I was able to find another post on this topic over on reddit, which brings up the biggest downside of doing this modification: It's ugly.
I think that could be negated somewhat if you're creative and painted the foam (it'd probably look cool if you painted it white or black and sprayed a solarpunk logo with a stencil on it), but I suspect for most, this will be a dealbreaker for a main kitchen fridge (not to mention, you may not have enough clearance for the extra foam in your... Fridge slot?)
Still though, those power saving numbers are difficult to ignore, and I could see people going for this on secondary a garage fridge, like the dude in the second link did. It would have a dramatic effect in energy consumption if the ambient temperature around your fridge is usually pretty hot, as hotter outside temps dramatically increase energy useage.
Now you can't do this to any 'ol fridge or freezer. Some designs dissipate their heat through the metal skin of the fridge itself, and covering it with foam would quickly destroy it. But if your fridge has a visible condenser coil somewhere, either on the back of the fridge, or underneath, you're probably good to go as long as those coils remain uncovered and can get adequate airflow per their design specs.
I think this modification is probably best suited for older, less efficient fridges, where it'll give the most noticeable benefit in energy and cost savings. But if you did apply it to a modern high efficiency fridge, you could probably get near the efficiency of a factory-made ultra-insulated fridge (which currently go for a whopping $2,150!) for a fraction of the price.
EDIT: Found some more numbers in this video. Using 3/4" foam (less R-value), the guy in the vid achieved a 20% reduction in energy use. In comments, another person reported a 45% reduction in energy use after applying 1" foam.
