Your unplugged deep freezer already proves the principle: in a Parry Sound winter, outside is a freezer. The fridge is the inverted problem — outside is too cold, the kitchen is too warm, and the cabinet sits in the wall between them, metering cold air in only as fast as house heat leaks through.
Freezer: solved — keep unplugging it. This sheet covers the fridge only.
A super-insulated cabinet against a north exterior wall, with two openings through the wall — intake at the cabinet floor, exhaust at the top. Cold air is denser: it spills in low, warms against the food and the house-side wall, rises, and exits high. A thermosiphon, no fan required. Each opening wears a bi-metal automatic vent — a coiled spring of two metals that opens the louvers as the cabinet warms past ~4.5 °C and closes them as it comes back to temperature. The metal itself is the thermostat: no motor, no wires, nothing to wake you up.
All winter, outdoor air is colder than +4 °C. Cooling is free and unlimited — the design problem is never "enough cold," it's too much.
Heat from the woodstove-warmed kitchen leaks through the cabinet's house side at a few watts. That constant trickle is what keeps the lettuce above zero.
Vent opening sets how fast cold comes in; house bleed is fixed. Where they meet is your setpoint. The bi-metal coil finds that point by itself — warmer box, wider louvers; at temperature, shut. No controller, no alerts, no you.
2×4 frame against a north exterior wall, sheathed in ½″ plywood. Interior around 24″W × 20″D × 48″H gives ~13 cu ft — roughly the fresh-food side of a full fridge.
North wall · no direct sun · near the kitchen, not in it if you can2″ XPS or polyiso minimum (R-10), seams foil-taped. Insulate the house side just as heavily — that face sets your warm bleed, and you want it small and predictable.
2″ XPS ≈ R-10 · foil tape every seam · foam the floor tooFrame two rectangular openings sized to your vents (standard foundation vents fit between studs): intake at cabinet floor level, exhaust at the very top. Vertical separation drives the thermosiphon — take all the height you can get, 36″+. Slope the sills slightly down-and-out for drainage.
Between-stud framing · seal to vapour barrier with acoustical sealantOutside: screened hoods facing down (mice, snow, wasps). Inside: a bi-metal automatic foundation vent on the cabinet face of each opening, so the coil senses cabinet air. Closed at ~4 °C, opening progressively as the box warms — this is the entire control system, and it has no moving parts you power.
Air Vent RAGR / Temp-Vent class · test open/close temps in a cooler firstFoam-core door on the kitchen side with full-perimeter gasket (fridge gasket or good weatherstripping) and a cam latch that pulls it tight. Air leaks here swamp everything else.
Test: close it on a strip of paper — it shouldn't pull out easilyTwo 4 L water jugs on the bottom shelf as a thermal flywheel, and a cheap thermometer for the first couple of weeks while you confirm where the vents settle. Alarms off — the vents are the control now, not you. After shakedown, glance at it when you grab the milk.
Any fridge thermometer works · no alerts, no 3 a.m. BluetoothBoth versions open the vents when the box runs warm and close them at temperature, with no daily attention. They differ in precision and in whether any electrons are involved. Click a card to price that version in the parts list below.
The −30° air is bone dry — no problem there. The hazard is warm, humid kitchen air leaking into the cold cabinet and condensing inside your wall assembly. That's how you grow mold in a wall cavity. Air-seal and vapour-barrier the cabinet on the house side, seal the duct penetrations to the existing poly, and treat the cabinet walls like the exterior walls they effectively are.
The neighbours will suggest it. It doesn't work: below ~10 °C ambient, a compressor fridge's thermostat stops calling, the fresh-food side freezes and thaws at random, and the compressor oil thickens. Cold ambient breaks a normal fridge — it doesn't help it. (Your chest freezer survives unplugged for the opposite reason: it's just a well-gasketed insulated box, which is exactly what this build is.)
| Item | Spec | ≈ Cost |
|---|---|---|
| Rigid foam, 2″ XPS | 2 sheets, 4×8 | $110 |
| Plywood + 2×4s | ½″ sheathing, framing | $120 |
| Screened exterior hoods × 2 | face down, rodent screen | $50 |
| Bi-metal automatic vents × 2 | closed ~4 °C, self-opening | $60 |
| Door gasket + latch | full perimeter, cam latch | $40 |
| Fridge thermometer | shakedown checks, no alarms | $15 |
| Sealant, tape, fasteners | acoustical seal, foil tape | $40 |
| + Inkbird ITC-308 | dual-stage controller | $50 |
| + 4″ inline fan | ~10 W | $60 |
| + 25 W heat cable · NC damper | Option B freeze guard | $85 |
| Option A total | zero watts, automated | ≈ $445 |
A weekend and about the cost of two months of generator gas — after which the winter refrigeration bill is zero, permanently.
January means run about −9 °C with nightly lows near −12 °C — the box holds fridge temperature effortlessly whenever daily highs stay below ~+4 °C. Solid service December through March, usually late November and early April too. The other months you're back on the powered fridge — but those are the months the generator was cheap and the woodstove was cold.
Solid outline = reliable passive service · dashed = shoulder weeks, watch the thermometer.