Here’s the scoop on heating a backyard greenhouse – particularly a home or hobby greenhouse. This does involve a bit of math but as soon as you understand how to do this, you’ll be able to decide if you want to try to heat your greenhouse or use it as a season extender at both ends of the gardening season.
You have to know the square footage of all exposed surfaces of your structure. So if the side wall is 12 feet long by 6 feet tall, you have 72 squre feet of exposed surface. Do this with all exposed surfaces including the roof.
Why do this? Unless you know the surface area that is losing heat, you won’t be able to calculate how much heat you have to provide.
A hoop style greenhouse for extending the growing season without heating
Heat Loss of Covering Material.
All greenhouse coverings have a heat loss rating. This rating is applied to the forumla and it determines how fast the heat goes out of the greenhouse.
The two most common coverings in greenhouses are double layer of polyethylene (plastic) and a rigid acrylic board.
Double poly has a heat loss number of 5.0
Acrylic has a heat loss number of 4.65
Your Interior Temperature
How hot do you want to keep your plants. You decide on this number.
Minimum Outdoor Temperature
What’s the average outdoor minimum temperature you have to heat to.
If you’re looking at a northern climate and have to heat right through a -40F night, that’s the number you have to plan for.
The heat loss calculation looks like this.
Heat needed = heat transfer co-oefficient of covering times the surface area of the greenhouse X the difference between the interior temperature you want to maintain and the minimum outdoor temperature.
Example of Calculation
So let’s say our home greenhouse of 12’x6’x6′ has a total survace area of 450 (for all the sides, end walls and roof) And we’re covered with a rigid acrylic. This is a made-up example and will be entirely different than yours.
And let’s also decide that the minimum temperature outside is going to be -20C (-4F) and we need to keep the temperature at 20C (68F)
This means the difference in temperature is 40 degrees. (20-to 0 =20 and 0 to -20 is 20 so 20+20=40
So we’re looking at our heating need being
To Convert Watts to BTU/hour we multiply by 3.413
We now know we need a furnace capable of putting out 285,668 BTU/hour if we want to keep our plants at 20C (68F)
Any difference in the measurements of your greenhouse or in the temperatures used for outdoor or indoor calculations will drastically change these numbers.
My Advice for heating a backyard greenhouse
Over-engineer your heating needs.
Get the right unit to meet those needs.
You’re going to find that that one night where it goes down to -40 (same in C and F) is going to wreck your plants if you’re not able to handle the heating load that’s engineered for -20C (-4F)