The cold frame heat retention calculator estimates the minimum nighttime temperature inside your cold frame based on outside low temperature and glazing type. Use this to determine if cold-hardy crops will survive winter without supplemental heating.
Cold Frame Parameters
Temperature Estimate
Enter outside temperature to estimate.
How to Extend Your Growing Season with a Cold Frame
A cold frame is essentially a mini-greenhouse without a heat source — it relies on solar gain during the day and retained heat at night. With the right glazing and some thermal mass, a cold frame can extend your growing season by 4-6 weeks on each end and allow year-round harvests of cold-hardy crops in USDA zones 6-9.
Siting Your Cold Frame
Place cold frames against a south-facing wall or fence for maximum solar gain and wind protection. The wall acts as a thermal battery, absorbing heat during the day and slowly releasing it at night. A brick or concrete wall is better than wood. Orient the glazed panel to face true south, tilted at an angle equal to your latitude plus 15 degrees for maximum winter sun exposure.
Adding Thermal Mass
Fill dark-painted gallon milk jugs with water and line them inside the cold frame perimeter. Each gallon stores about 8 BTU per degree of temperature change. A row of 4 jugs along the back wall can add 3-5°F of nighttime protection. Black-painted concrete blocks or bricks work similarly — they're better because they don't freeze-crack.
Managing Ventilation
The most common cold frame mistake is forgetting to vent on sunny days. A closed cold frame on a clear 30°F day can hit 80-90°F inside by noon — enough to wilt or kill plants. Install an automatic vent opener (Univent or similar) that opens at 50°F and closes when it cools down. These cost $20-40 and require no electricity.
Frequently Asked Questions
Is this cold frame calculator free?
Yes, completely free with no signup required. Estimates are based on typical R-values for common glazing materials and standard cold frame construction assumptions.
How much warmer is a cold frame than outside?
A typical glass cold frame is 5-10°F warmer than outside temperatures at night. Twin-wall polycarbonate (R-2) provides 10-15°F of protection. Heating cables or thermal mass (water jugs) can add another 5-10°F. On sunny winter days, cold frames can overheat — ventilation is essential.
What can I grow in a cold frame in winter?
Cold-hardy crops that overwinter well in cold frames include kale, mâche (corn salad), spinach, claytonia, Asian greens (tatsoi, mizuna), winter lettuce, and scallions. These tolerate being frozen and thawing repeatedly. Root crops like carrots and parsnips can be left in the ground with a cold frame over them.
What is the best glazing for a cold frame?
Twin-wall polycarbonate (6mm or 10mm) is the best balance of insulation, weight, and cost. It transmits 80-85% of light, has R-2 to R-2.5 insulation value, and weighs much less than glass. Tempered glass has higher light transmission but lower insulation. Single-layer poly film is cheap but has the worst insulation value.
How do I ventilate a cold frame on warm days?
Prop the lid open with a stick or use an automatic vent opener (temperature-triggered, no electricity needed) set to open at 50-55°F. Overheating kills plants faster than cold. On sunny days in March, a closed cold frame can exceed 90°F even when outside temperatures are below freezing.