Greenhouse Heating Calculator
An undersized heater leaves plants freezing; an oversized one wastes energy and money. Enter your greenhouse dimensions, glazing type, and temperature requirements to calculate exact BTU needs. Get estimated monthly costs for electric and propane heating so you can budget accurately.
Heating Calculator
Greenhouse Heat Loss Formula
BTU/hr = Total Surface Area × U-Factor × (Inside Temp − Outside Temp)
This formula calculates the rate of heat loss through the greenhouse glazing. Your heater must output at least this many BTU per hour to maintain the desired temperature at the coldest conditions. It's wise to oversize by 20-25% for safety margin and to account for air infiltration around doors and vents.
Glazing Comparison
| Glazing Material | U-Factor | Relative Heat Loss |
|---|---|---|
| Single-layer polyethylene | 1.2 | 100% vs single poly |
| Double-layer polyethylene (inflated) | 0.7 | 58% vs single poly |
| Single-pane glass | 1.13 | 94% vs single poly |
| Double-wall polycarbonate (6mm) | 0.65 | 54% vs single poly |
| Double-wall polycarbonate (8mm) | 0.58 | 48% vs single poly |
| Triple-wall polycarbonate (16mm) | 0.36 | 30% vs single poly |
Real-World Example
A 12×24 foot greenhouse with 7-ft walls, 6/12 roof pitch, double-wall polycarbonate, maintaining 50°F when it's 5°F outside:
- Wall area: 2(12×7) + 2(24×7) = 504 sq ft
- Roof + gable area: ~380 sq ft
- Total surface: ~884 sq ft
- BTU needed: 884 × 0.58 × 45 = ~23,072 BTU/hr
- Heater recommendation: 25,000-30,000 BTU unit heater
- Monthly propane cost: ~$280
Ways to Reduce Greenhouse Heating Costs
- Upgrade glazing. Switching from single poly to double polycarbonate cuts heat loss by 50%+.
- Add thermal mass. Black water barrels absorb heat during the day and release it at night — 2.5 gallons of water per square foot of greenhouse floor is a good target.
- Use row covers inside. A floating row cover adds 4-8°F of protection, effectively reducing the temperature your heater must maintain.
- Insulate the north wall. The north wall receives no direct sunlight — insulating it with foam board dramatically reduces heat loss without reducing light.
- Seal air leaks. Caulk around the base, doors, and fan openings. Air infiltration can account for 20-30% of heat loss.
Plan your growing season with the seed starting calculator or estimate soil needs with the garden soil calculator. Compare energy costs using our EV vs gas calculator.
Frequently Asked Questions
How do I calculate BTU needed to heat a greenhouse?
Use the formula: BTU/hr = Surface Area (sq ft) × U-factor × Temperature Difference (°F). Surface area includes all walls and the roof. U-factor depends on glazing material (single poly = 1.2, double poly = 0.7, polycarbonate = 0.58). Temperature difference is between desired inside temp and coldest expected outside temp. A 12×20 greenhouse with double poly in 0°F weather needing 55°F inside requires roughly 42,000 BTU/hr.
What is the cheapest way to heat a greenhouse?
For small greenhouses (<200 sq ft), electric space heaters with thermostats are simplest. For larger structures, propane or natural gas unit heaters offer lower operating costs. Passive strategies reduce costs significantly: thermal mass (water barrels absorb daytime heat), insulating the north wall, double-layer glazing, row covers inside, and composting beds generate heat. Many growers combine passive methods with a thermostat-controlled backup heater.
What temperature should I keep my greenhouse in winter?
It depends on what you're growing. Cool-season crops (lettuce, kale, spinach) survive at 35-45°F — just frost protection. Warm-season seedlings need 60-70°F. Tropical plants require 55-65°F minimum. Every 5°F increase in target temperature roughly doubles your heating costs. Most cost-effective strategy: grow cold-hardy crops and only heat to prevent freezing (35°F minimum).
How much does it cost to heat a greenhouse in winter?
Costs vary wildly based on size, glazing, climate, and target temperature. A well-insulated 10×12 greenhouse maintaining 45°F in USDA Zone 6 might cost $50-150/month with electric heaters. A poorly insulated 20×48 commercial greenhouse keeping 65°F in Zone 4 could cost $500-1,500/month with propane. Double-wall polycarbonate reduces heating costs by 40-50% compared to single-layer poly.
Do I need a heater for my greenhouse?
In USDA Zones 8-10, an unheated greenhouse can maintain above-freezing temperatures most of winter using thermal mass alone. In Zones 3-7, you'll likely need supplemental heat for consistent frost protection. The real question is: what do you want to grow? If you're only growing cold-hardy greens, a simple frost cloth inside an unheated greenhouse may suffice even in Zone 5. Seedling starting always requires supplemental heat.
What is a U-factor for greenhouse glazing?
U-factor measures how easily heat passes through a material — lower is better (more insulating). Single polyethylene film has a U-factor of 1.2 (poor insulation). Double-layer inflated poly drops to 0.7. Twin-wall polycarbonate ranges from 0.58-0.65. Triple-wall polycarbonate reaches 0.36 — nearly as insulating as a standard house wall. U-factor directly multiplies your heating requirement, so better glazing pays for itself quickly in cold climates.
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