| Inside Temp (°C) | Outside Temp (°C) | Length (m) | Width (m) | Height (m) | Cover Preset | ACH | Heat Loss (W) | Heater (kW) |
|---|---|---|---|---|---|---|---|---|
| 18 | 2 | 6 | 3 | 2.2 | Double poly | 1.2 | 2875 | 3.45 |
| 20 | -5 | 8 | 4 | 2.5 | Twinwall polycarbonate | 0.8 | 5200 | 6.5 |
- Qcond = U × A × ΔT where U is W/m²·K and A is m².
- Qinf ≈ 0.33 × ACH × V × ΔT using air heat capacity at typical conditions.
- Total heat loss: Q = Qwalls + Qroof + Qfloor + Qinf.
- If you enter R-values, the calculator uses U = 1 / R.
- Choose units, then enter inside and outside temperatures.
- Enter length, width, and average height of your structure.
- Select a cover preset, or choose a custom U-value.
- Set ACH to match leaks, vents, and door openings.
- Optionally include floor loss using ground temperature.
- Press calculate to see totals and component breakdown.
- Download CSV or PDF for planning and documentation.
Heat loss inputs that match real garden structures
Use inside and outside temperatures that represent your coldest routine night. A 15°C difference can double heater demand compared with a mild evening. Measure length, width, and average height; volume controls air-loss calculations, while surface area drives conduction. For arched roofs, assume roof area is 5–10% higher than flat footprint. If outside is warmer than inside, ΔT is treated as zero.
Cover and insulation values for quick planning
Typical U-values (W/m²·K) run about 6.8 for single film, 3.9 for double film, 3.0 for twinwall polycarbonate, and near 1.5 for insulated panels. Lower U means less heat escaping per square meter. If you know R-value, enter it and the calculator converts using U = 1/R to keep units consistent.
Air changes per hour as the hidden load
Air leakage often equals or exceeds wall losses in hoops and tunnels. A tight structure may run 0.5–1.0 ACH, while leaky doors and open vents can push 3–6 ACH. The ventilation term uses Q ≈ 0.33 × ACH × volume × ΔT, so lowering drafts by one ACH can save hundreds of watts on small houses.
Reading the breakdown to choose upgrades
The results section separates walls, roof, floor, and air movement in watts. If roof conduction dominates, adding an inner layer or thermal curtain usually provides the biggest drop. If infiltration dominates, focus on base sealing, zipper doors, and repairing tears before adding more insulation. Floor loss matters most on benches or insulated pads; use a realistic ground temperature to avoid oversizing.
Translating watts into heater capacity and energy
Total heat loss is shown in watts, kilowatts, and BTU/hr for comparing heater labels. Daily energy converts to kWh/day for budgeting. Add a safety margin for wind and door openings; 15–30% is common. For fuel heaters, adjust for efficiency to estimate input energy. After sizing, use the CSV or PDF export to document assumptions and revise them as covers age. Track changes after repairs and upgrades.
FAQs
1) What does U-value mean in this calculator?
U-value measures heat flow through a surface per square meter per degree difference. Lower values indicate better insulation and lower heat loss. If you only know R-value, the tool converts using U = 1/R.
2) Which outside temperature should I enter?
Use the typical coldest night you want to survive without crop damage. For heater sizing, choose a conservative winter low. For cost estimates, use an average night temperature for the season you are planning.
3) How do I choose an ACH value?
Start with 0.5–1.0 ACH for a well-sealed structure and 2–4 ACH for common hoop houses with door gaps. If you frequently vent or run fans, use a higher number to reflect real air exchange.
4) When should I include floor heat loss?
Include it when the base is exposed, insulated, raised, or separated from warm soil conditions. If your greenhouse sits on natural ground, floor losses may be smaller, but benches and pads can lose heat faster.
5) Does the roof area assumption affect results?
Yes. The roof is treated as a flat footprint for simplicity. For arched or gothic shapes, real surface area is higher, so heat loss will be slightly higher too. You can compensate by selecting a larger roof U-value or adding margin.
6) How do I turn the results into a heater choice?
Use the suggested heater size in kW as a minimum rating, then match fuel type and efficiency. Add extra capacity if you expect strong winds, frequent door openings, or rapid recovery after venting. Confirm electrical supply limits before purchase.