A cold frame holds heat poorly compared to a greenhouse, but it holds more heat than open ground — and in northern Canada that difference is often enough to matter. Understanding what governs overnight temperature inside a frame helps in making decisions about construction materials, depth, and placement that go beyond general recommendations.
How a cold frame retains heat
During daylight hours, solar radiation passes through the glazing and heats the soil, frame walls, and air inside. After sunset, the stored heat radiates back upward. The glazing slows the rate at which that radiated heat escapes to the outside air. This is broadly similar to the greenhouse effect, though the scale and materials are different.
The two factors most people notice first — glazing material and insulation on the frame walls — matter, but soil thermal mass is often the larger variable in practice.
Glazing: glass versus polycarbonate
Single-pane glass allows more solar transmission than most plastics, which means more heat enters on a clear day. It also loses heat faster at night because glass conducts heat more readily than twin-wall polycarbonate. In mild climates, the extra daytime gain from glass roughly balances the extra nighttime loss. In cold climates with clear nights, polycarbonate tends to produce warmer overnight minimums.
Twin-wall polycarbonate (typically 6 mm or 8 mm) provides roughly twice the insulation value of single-pane glass. On clear nights with outside temperatures below -10°C, this difference can be several degrees inside the frame — enough to protect established transplants that glass would not.
Double-pane glass is an option for permanent installations. The insulation improvement is comparable to twin-wall polycarbonate, though glass is heavier and more difficult to vent manually. Old window sashes with original single-pane glass are common for improvised cold frames; they work, but overnight performance in hard frosts is limited.
Soil thermal mass
Bare or dry soil stores relatively little heat. Moist, dense soil — particularly clay-heavy soil — absorbs substantially more solar energy during the day and releases it slowly overnight. This is the thermal mass effect, and it is often the largest determinant of overnight minimum temperatures inside a cold frame.
Practical implications for northern gardeners:
- Water the soil inside the frame the day before an anticipated cold night. Moist soil holds more heat than dry soil.
- A dark-colored mulch (dark compost, for example) absorbs more radiation than light-colored materials and transfers it to the soil below.
- Containers and raised beds within a frame lose heat faster than in-ground soil because the sides of the container are exposed to cold air. A frame built directly over in-ground beds has a thermal advantage over one housing containers.
Frame depth and air volume
A deeper frame encloses a larger volume of air, which acts as a buffer against rapid temperature drops. However, more air volume also means more total heat must be stored during the day to maintain a given temperature overnight. For crops that grow close to the soil surface — spinach, mâche, winter lettuce — a shallower frame works. For taller plants or transplants, depth becomes necessary regardless of thermal considerations.
The back wall of a cold frame is typically taller than the front, creating a slope that sheds rain and improves the solar angle. A back height of 30–35 cm and a front height of 15–20 cm is a common proportion for low-growing crops in northern regions. This gives adequate air circulation without excessive volume to heat.
Frame wall materials
Traditional cold frames used brick or stone for the side and back walls, materials with significant thermal mass. Modern frames more commonly use rot-resistant lumber (cedar, for example) or composite materials. Lumber has modest insulation value but negligible thermal mass.
Adding a layer of rigid foam insulation to the interior of lumber walls is straightforward and measurable in effect. A 25 mm layer of extruded polystyrene on each interior wall surface noticeably improves overnight temperatures in northern winters. This is particularly relevant for frames left in place through late fall and early spring when ambient temperatures are lowest.
Placement and orientation
South-facing slope, if available, increases solar gain in the northern hemisphere. A gentle slope (5–10 degrees) can increase effective solar exposure meaningfully compared to flat ground. Protection from prevailing north and northwest winds reduces convective heat loss through the glazing and frame joints. This is worth considering when siting a frame, though it is often less influential than glazing quality and soil moisture.
The University of New Hampshire Cooperative Extension publishes practical guidance on cold frames and hotbeds for northern climates, including temperature expectations under different glazing materials. See extension.unh.edu.
What a cold frame cannot do
A passive cold frame — no added heat source — provides protection typically in the range of 5 to 10°C above outside ambient temperature. On a night with an outside low of -15°C, the inside of a well-built frame might reach -8 to -5°C. Most commonly grown greens do not survive those temperatures without additional row cover inside the frame. A cold frame works well for the frost fringe (outside lows of -5 to 0°C), but northern Canada's deep-winter temperatures exceed what passive cold frames can address without supplemental heat.
