Heating Your Greenhouse for Four-Season Growing!
What you can grow in your greenhouse is only limited by how warm your heat is. First, select an energy source. Electric heaters are generally less expensive to purchase and work well if you have electricity wired to your site (no unsafe extension cords, please!) Gas heaters use either natural gas or propane. They are inexpensive to operate and require no electricity. Next, decide how warm you will need to keep your greenhouse. If your goal is to winter-over plants, you need only keep your greenhouse above freezing. If you plan to get an early start with seedlings, you will need a more consistent and constant heat supply.
Heaters are rated by the amount of BTU's they produce. In order to determine the correct size of heater you'll need, you'll first need to gather some information on the size of your greenhouse, the minimum outside temperature you expect for your area, and
the minimum temperature you want to maintain in your greenhouse.
| Greenhouse BTU Calculator |
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Calculate
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Formula
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Example
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Find the total SF (square feet) of exposed surface area of your greenhouse.
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288
9 x (4 x 8) = 288 (square feet)
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Nine (4 x 8) panels would be
9 x 32 = 288 SF (square feet)
(using our Solexx™ panels)
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Determine the maximum inside temperature you want to keep the greenhouse. (Depending upon the type of plants.)
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60°F
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You’re growing seedlings, so you want the inside temperature to stay at 60 degrees Fahrenheit
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Determine the minimum outside temperature you expect for your area.
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20°F
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You live in Oregon and you expect winter temperatures to be down to 20 degrees F .
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Determine the heat loss factor for the greenhouse covering material.
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0.8
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Corrugated plastic has a heat loss value of .8
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Multiply the SF (square feet) by the difference in temperatures, then by the heat loss factor.
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288 x 40 x .8 = 9216
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The total sq. footage of the covering surface area is 288. The temperature difference is 40 degrees F. The heat loss
factor is .8
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This is the minimum heater BTU rating to keep your greenhouse at the desired temperature during the coldest time of year.
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9216 is the minimum BTU rating to keep your greenhouse at the desired temperature during the coldest time of the year. Electric heater provides 5,118 BTUs to heat your greenhouse. Non-vented gas heaters provide 20,000 BTUs for those larger greenhouses.
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Insulating Ability & Heat Loss
R Factor is the measurement of ‘insulating ability’ of the material. The higher the R Factor number the greater the insulation value.
U Value is the measurement of ‘heat loss’ through the material. The lower the U Value the less heat is escaping
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Material
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R-Factor
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U-Value
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Solexx (5mm)
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2.30
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0.43
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Solexx (3.5mm)
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2.10
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0.48
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8 mm Tripple Wall Polycarbonate
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2.00
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0.50
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Double Pane Storm Windows
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2.00
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0.50
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10 mm Double Wall Polycarbonate
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1.89
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0.53
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8 mm Double Wall Polycarbonate
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1.60
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0.63
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6 mm Double Wall Polycarbonate
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1.54
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0.65
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4 mm Double Wall Polycarbonate
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1.43
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0.70
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Single Pane Glass, 3 mm
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0.95
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1.05
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Poly Film
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0.83
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1.20
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