Calculator Inputs
Example Data Table
| Canopy Type | Layers | Area | Outside | Soil | Wind | Humidity | Est. Inside | Reduction |
|---|---|---|---|---|---|---|---|---|
| Plastic Low Tunnel | Double Layer | 10 m² | 2°C | 8°C | 2 m/s | 70% | ≈5.0°C | ≈55% |
| Row Cover (Fabric) | Single Layer | 6 m² | -1°C | 6°C | 3 m/s | 50% | ≈1.5°C | ≈35% |
| Mini Greenhouse | Bubble Wrap | 12 m² | 0°C | 10°C | 1 m/s | 80% | ≈7.8°C | ≈75% |
Formula Used
The calculator estimates heat loss through a canopy using a simplified heat-transfer model:
- Q = U × A × ΔT where Q is heat loss rate (W).
- U is overall heat-transfer coefficient (W/m²K).
- A is canopy area (m²), and ΔT is inside–outside temperature difference (K).
For the canopy, overall resistance is modeled as a series:
- U_canopy = 1 / (1/h_out + R_eff + 1/h_in)
- R_eff combines material, layer, and air-gap resistance, adjusted for leaks and venting.
Estimated inside temperature moves from outside toward soil temperature as R_eff increases. This reflects better trapping of ground heat under tighter, better-insulated covers.
How to Use This Calculator
- Enter outside air and soil temperatures for the evening period.
- Select canopy type and insulation option that matches your setup.
- Set wind, humidity, air gap, seal level, and vent level.
- Press Calculate Heat Retention to see results above the form.
- Download CSV or PDF to save the run with your notes.
Canopy R-value and why air gaps matter
Heat retention improves when total resistance increases. This tool builds an effective resistance from canopy material, added layers, and the still-air gap. Tight edges stop warm air escaping through gaps overnight. A practical air gap of 3–8 cm often improves performance because trapped air slows conduction. Very large gaps can circulate and deliver less benefit, so air-gap gains flatten once mixing becomes likely.
Wind exposure and convective heat loss
Wind is a major driver of night heat loss. The calculator increases the outside convection term as wind speed rises, which raises the uncovered U-value and increases canopy demand. Canopy type changes sheltering: tunnels and rigid frames reduce inside air movement, lowering internal convection and helping the cover hold soil heat through the coldest hours.
Humidity, sky clarity, and overnight cooling
Clear, dry nights often cool faster because surfaces radiate heat to the sky. The humidity input slightly adjusts the effective radiative term to reflect this trend. Treat it as a directional modifier, not a local forecast. For conservative planning, choose “Leaky” sealing and higher venting when your cover has gaps or you expect strong breeze.
Using U-value and kWh to compare options
U-value tells you how easily heat passes through the system: lower is better. The energy retained estimate converts the reduction in heat loss into kWh over your selected duration, helping you compare upgrades such as double layers, thermal liners, bubble wrap, or better sealing. If you set a target temperature, the heater outputs show the power and energy needed to hold that target.
Field calibration and practical improvements
Measure outside air at plant height and place a probe thermometer under the canopy to validate results. If predictions are high, reduce the seal level or increase venting until estimates match your readings. Improve retention by weighting edges, patching tears, reducing night vent openings, and adding thermal mass near sensitive crops. In windy sites, a windbreak or inner fabric layer can stabilize temperatures.
FAQs
1) What temperatures should I measure for the best input?
Measure outside air near plant height, not at roof level. Measure soil temperature 2–5 cm below the surface near the root zone. Use evening values that represent the cold window you are planning for.
2) Does soil moisture change heat retention?
Yes. Moist soil often stores and releases heat more steadily than dry soil. If your soil is very dry, the effective heat source may be weaker, so treat the inside-temperature estimate as an upper bound.
3) Why does a “Leaky” seal reduce retention so much?
Leaks allow warm air to escape and cold air to enter, which can dominate conduction through the cover. Gaps at edges and seams behave like an unplanned vent, so sealing improvements usually produce large gains.
4) Should I vent at night to reduce condensation?
Small vents can reduce condensation but also reduce heat retention. Use “Slight Vent” when humidity is high and frost risk is moderate. In hard frost conditions, minimize venting and verify with a thermometer.
5) How do I choose a reasonable air-gap value?
If your cover sits close to plants, use 2–4 cm. For hoop tunnels, 4–8 cm is common. If the cover flaps or wind enters, choose a smaller value because air mixing reduces the benefit.
6) Can this replace a frost alarm or weather service?
No. It estimates performance for your setup using your inputs. Use forecasts for timing, and keep a thermometer under the canopy for safety. Combine both for reliable protection decisions.