Greenhouse Design Feedback

With winter approaching, I am currently in the process of designing and creating a greenhouse for my apartment balcony and I wanted to get some feedback and opinions on what we have for the design.

Some quick notes about my location and climate:

  • Southeast Wisconsin (temps as low as -30 with windchill in extreme cases)
  • The balcony is West facing and has an cover over the top of it
  • The balcony doesn’t get too much sun during the winter and I expect that the greenhouse can be positioned in a way so that it doesn’t get hit with direct sunlight

My fiance and I have created the following design for the greenhouse with measurements:

The front and top of the greenhouse will be able to be opened and we are planning on using either a solid piece of plywood or 2x4s for the bottom and can be sealed off. The idea right now is to use twin-walled polycarbonate panels for the siding.

I think my biggest worry is heating and air movement. The space off to the right side will be for my trees that do not fit on my bench so there will be limited spacing for something such as a space heater. I do have temperature controlled heating pads but I don’t know if that will be enough. I’ve heard of people using ceramic and chicken coop heaters in their greenhouses so I’m going to be looking into those. I also plan on having a fan or two within the greenhouse to keep air circulating.

In the case that the greenhouse does get hit with direct sunlight on the side or top, I’ve been told by a friend to use board insulation panels to block it out from making it too hot. Obviously on warm days I’ll be able to open it up and let air in and out.

Any feedback or opinions would be greatly appreciated as this is somewhat nerve-wracking being the first winter to try this.

Overall a good looking design. You have about 5 m^2 of surface area not including the back. 6 mm thick twin-wall has an R-value of 1.54. Assuming -30F and 40F inside the greenhouse gives a temperature difference of 39 degrees C. That means heat loss of 126 W with no wind chill and probably 175-200 W with wind chill. As a result, a small electric heater on a thermostat should work very well in combination with a fan to circulate the air.

For reference I heat a fairly well insulated 22 ft. long, 12 ft. wide, 8 ft. tall temporary greenhouse to 34-35F using a 1500 W heater and Inkbird thermostat. I add supplemental propane heat when the temperature is going to drop below about 5F, but have to make sure I turn it off before the temperature reaches about 15F if I don’t want to heat the greenhouse too much. My walls are 6 mil (0.15 mm) plastic, bubble wrap with the bubbles facing the plastic, and about 0.7 mill plastic on the inside of the frame giving about a 1 inch air gap.


Wow, this is amazing. Thank you for taking the time to give that information. I feel better now about the heating and air situation. Just need to make sure that the temperature fluctuation isn’t too drastic for the trees but it sounds like that won’t be too much of an issue.