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Heating Your Greenhouse - BTU Calculator

Explaination: This calculator is designed to give an estimate of the BTU (British Thermal Unit) heating needs of a greenhouse. Simply enter the area to be heated and select the climate adjustments. This is to be used for a general guideline only.
Greenhouse Area - The total square feet of exposed surface area of your greenhouse not including the floor (this is not length * width). To find out the area your greenhouse, click here.
Coldest Outside Temperature - You will want to enter the lowest temperature expected for your area. Not sure? You can click here for the USDA plant hardiness zones which provide an average annual temperature rating for areas within the United States.
Maximum Inside Temperature - This is the highest temperature you want to maintain in your greenhouse.

Heat Loss Value - Check the table at the bottom of the page to find the heat loss value for the covering used on your greenhouse. Some values may vary with manufacturer. If you know the R-value of your covering, you can convert it to a heat loss value using this formula: Heat loss value = 1 / R-value.

Resulting Minimum BTU - This is the minimum BTU output your heater should have. Most heaters use a BTU input rating. BTU output = (heater efficiency) * (BTU input).
This result is  Area x (Max - Min) x Heat Loss
Covering Insulation (R) Values for Polycarbonate, Polyethylene, Glass, and Fiberglass
Covering Material R Value Covering Material R Value
3 mm Glass (single layer) 0.95 4 mm Twin-wall Polycarbonate 1.43
Two layers of Glass (insulated) 2.00 6 mm Twin-wall Polycarb
8 mm Twin-wall Polycarb 1.61 4 mm Polyethylene
10mm Twin-wall Polycarb 1.89 6 mm Poly 0.87
16 mm Triple-wall Polycarb 2.50 6 mm Poly double layer (inflated) 1.43
Fiberglass / Polycarb (single layer) 0.83 11 mm woven Poly 0.95
R value: a unit used to measure the effectiveness of thermal insulation.
A higher insulation value indicates greater heating and cooling efficiency.

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