Divide a root cellar into three zones by height and distance from the door: the coldest, at floor level on the back wall, at 1-4 °C for potatoes, carrots, beets, turnips, parsnips and apples; a middle zone at 4-8 °C for cabbage and winter squash; and the warmest, near the ceiling and door, at 8-12 °C for onions, garlic, cured meats and cheese. Control humidity with ventilation and water, remembering that warming air lowers its relative humidity.
Part of Root Cellars & Cool Storage
Temperature and humidity zones are the cooler and warmer, damper and drier spots inside a cellar, used to match each stored food to the conditions it keeps best in.
Cold Air Sinks, Warm Air Rises, And Relative Humidity Moves With Temperature
A cellar is not uniform. Cold air is denser and settles at the floor and the back wall; warm air collects near the ceiling and the door. That makes the zones a natural consequence of the room, not an arrangement you impose. The cellar as a whole holds 2-10 °C and 85-95% humidity, but individual shelves sit at different temperatures within and slightly outside that band.
The other half is humidity. Relative humidity is the water in the air as a fraction of what the air could hold at that temperature, and the amount the air can hold rises quickly with temperature. So the same air has a different relative humidity at different temperatures, which changes how ventilation behaves.
Two cautions on the zone figures. The 8-12 °C warmest zone goes slightly above the 10 °C top of the whole-cellar range, so expect the top shelf near the door to be the least reliable. And potatoes sit in the coldest zone: potatoes held near freezing turn sweet as starch converts to sugar (recalled, not checked here), so keep them at the warmer end of the 1-4 °C zone, not against the cold back wall. Apples give off ethylene, a ripening gas that can spoil neighbours such as potatoes and carrots (also recalled), so store apples in a separate part of the cellar with their own air.
What You Need
- A thermometer, ideally recording minimum and maximum, hung on a cord at floor, mid and ceiling height
- Pans of water to add humidity and damp sand and crates (see Shelving & Organization)
- Vents you can open and close, low for inlet and high for outlet
- A habit of checking the cellar each morning in the first cold weeks
Method: Set Up And Tune
Step 1: Map the cellar. Take readings at floor, mid-wall and near the ceiling over a few days, and note where the cold and warm spots are.
Step 2: Place the foods. Put potatoes, carrots, beets, turnips, parsnips and apples in the coldest zone; cabbage and winter squash in the middle (squash prefers slightly drier conditions); onions, garlic, cured meats and cheese in the warmest.
Step 3: Raise humidity if needed. Place pans of water on the floor, or pack root vegetables in damp sand in crates.
Step 4: Lower humidity if needed. Improve ventilation. Open the door on cold, dry nights.
Step 5: Close up in warm, humid weather. Do not ventilate with warm, humid outside air (see the sum below).
Step 6: Record. Keep a log of the minimum, maximum and any problems, and adjust next year.
Worked Example
Relative humidity from the saturation vapour pressure, using the Magnus approximation (recalled from a standard reference, constants 17.62 and 243.12 °C): e_s(T) = 6.112 × exp(17.62 × T / (243.12 + T)) hPa. Values: e_s(2 °C) = 7.06 hPa, e_s(4 °C) = 8.13 hPa, e_s(8 °C) = 10.71 hPa, e_s(10 °C) = 12.26 hPa.
Winter air brought in. Outside air at 2 °C and 80% relative humidity (assumed) carries a vapour pressure of 0.80 × 7.06 = 5.65 hPa. Warmed to 8 °C with no added moisture, its relative humidity falls to 5.65 / 10.71 = 52.7%, about 53%. Cold, dry nights therefore lower the cellar’s humidity, as the method says.
Summer air brought in. Outside air at 10 °C and 90% relative humidity (assumed) carries 0.90 × 12.26 = 11.03 hPa. Meeting a cellar wall at 4 °C where e_s = 8.13 hPa, the relative humidity there would be 11.03 / 8.13 = 1.357, or 136%, which is impossible: the excess condenses as water on the wall. The dew point of that air is about 8.4 °C, so any surface colder than that gets wet. Warm, humid air in a cool cellar leaves the walls and shelves running with water, which feeds mold.
Warming alone dries the air. In a sealed cellar at 5 °C holding 90% humidity, warming the air to 10 °C without adding water leaves 0.90 × 8.72 / 12.26 = 64%, so a cellar that warms in the day dries out its air unless damp sand, pans or the produce itself replace the moisture.
Common Mistakes
- Ventilating on warm, humid days — walls sweat, produce molds.
- Potatoes against the coldest wall — they turn sweet.
- Apples beside potatoes and carrots — ethylene spoils them.
- Trusting one thermometer at one height — the zones differ.
Safety
Bad air in a closed cellar
Stored produce uses oxygen and gives off carbon dioxide, and a poorly ventilated cellar can hold air that cannot support life. Ventilate before entering, never enter alone, and do not test the air with a flame, which goes out only after the air is already too thin and ignites methane. If someone collapses inside, rescue from above with a line rather than going in.
What’s Next
- Learn next: Shelving & Organization — putting the foods in place
- Combine with: Interior Moisture Management — condensation in buried rooms