Small Dorm Fridge Ferm Chamber?

Most of the fermentation chambers I've seen use the larger dorm fridges. Are the small ones feasible to use at all or will it burn the fridge out?
I was thinking of building a foambox off of a small dorm fridge so I could keep my fermenting brews in the garage where the temps get to the upper 90s/100's in the dead heat of summer. This would be to cool 2-3 5 gallon carboys/buckets

Theoretically possible if you increase the insulation value of your chamber walls. Practically speaking without knowing how much refrigeration your unit produces its hard to know how much R value you need. My hunch is you'll need more than the R5 per inch from the extruded polystyrene most people use here.

Most dorm fridges produce ~100 BTU/h cooling...

With a fridge temp of 60 and garage temp of 100 your delta is 40.
Take that times the surface area of the box ( assume 3'x2'x2.5' box for 3 fermentors)
Times the u value of your wall ( 1/r value)

Blah blah you need 296 btuh of cooling with r5 assuming perfect sealing up. In other words, not enough cooling.

Solving backwards you need about R-15 wall construction for theoretical min and practically R-19+

Thanks, that actually makes perfect sense. I have a passing familiarity with R-Values after researching various ways to seal up and re-insulate my attic. Went from an R15 (the house was built in the 80's) to R35-ish with a ton of blown in insulation. HUGE difference in how comfortable the house is.

I could double up the r5 board couldn't I? R10. Of course that effectively doubles the cost of the materials though.

Those little dorm fridges are just so darn cheap though. You can find them for like $40 or less or a nice one, the larger "mini" fridges are $60-120 used.

R values are cumulative so that works. You're probably looking at 4" + of insulation though to be safe.

Although, I've been planning a temp control system and for my lager chamber cost wise its looking cheaper to build a composite wall with XPS inside and a stud wall with fiberglass batts and a vapor barrier on the warm side. Fiberglass on its own won't perform well over time due to moisture infiltration.