The idea of insulation isn’t the first thing that comes to mind when we think about a greenhouse. It’s likely that many would wonder, “When and where have I ever seen insulation associated with a greenhouse?” Well, chances are we have, but it just didn’t look like insulation to us when we first saw it. Nevertheless, insulation is an element we need to consider if we’re focused on successful solar operations with our greenhouses.
The main purpose of insulation in a greenhouse is to provide a thermal energy barrier, in the form of “dead air,” to slow down energy movement that is caused primarily by convection and conduction. Let’s look at how we can make use of insulation to deal with both issues. If we can do it successfully, we’ll conserve thermal energy and keep it right where we’d like to have it, inside our greenhouse.
Kill the Convection
No matter what kind of glazing we’re using on our greenhouse, the immediate benefit it offers us is thwarting convection. If our greenhouse is sealed up under a cover, then convection currents inside the heated structure don’t have anywhere to go. And that’s exactly what we’d like to see if our aim is to keep the heat inside. In essence, the fact that we have a cover on the structure creates a giant dead air space. The dead air creates resistance to heat transmission simply because it thwarts convection currents that are trying to take that warm and rising air somewhere else.
The lesson for us here is to seal our structures. Sealants aren’t directly giving us any insulation value, but they play an important role in enhancing the effectiveness of the insulation we do have. Energy transfer is enhanced by air movement, so if we can reduce air movement, we can reduce the cooling effect of natural convection.
I don’t remember much from my science classes at school, but I do remember heat rises, so if you’re going to seal up your garden greenhouse, you best start with the top and work your way down. With less dense air finding its way into the upper levels of your greenhouse, it will gently push on everything it can to find its way out.
Stop the Conduction
Conduction is another big heat loss factor for our greenhouse operations. Single layer glazing is very much like a single layer of glass at home, it’s inferior when it comes to being an insulator. With natural convection on the inside of our gardening structure, and air movement on the outside, a single film covering serves as the heat transfer medium with an enormous surface area. Convection inside our greenhouse warms up the film on the inside and then heat is slowly conducted through the thin layer of film, allowing the cold air blowing on the outside to take the heat away.
One way to thwart conduction when it comes to glazing is to install a double layer of greenhouse film. Whether it’s tacked in place or inflated, having a double layer of glazing is very much like having a double pane window, or adding a storm window on the outside of a single pane window in your home. Between the two layers of greenhouse film is dead air, and that’s what will provide us with insulation that reduces the ability of colder outside weather to conduct heat out of our greenhouse.
Another way to reduce conduction is to use rigid clear plastic panels. Such panels come in double and triple wall designs, so there is either a single or a double dead air space inside the panels. For conduction to take place, the energy inside has to migrate through the structure of the panels, via conduction, until it reaches the outside of the panel where it can be taken away by the breeze. Since most of the panel is composed of dead air, the surface area for conduction of thermal energy is greatly reduced.
If you have a greenhouse like mine, the north wall is solid and painted white to reflect light back to the plants inside. This wall is exposed to strong cold winds in the winter. Even though the wall is made of wood, it still represents a place where heat can be robbed away from the greenhouse via conduction. Placing insulation on the wall is one way to slow down migration of warmth. Use closed-cell rigid foam insulation. It won’t absorb moisture that is naturally present inside a greenhouse during most of the regular growing season.
Soil as a Conduction Path
I know that I just said “heat rises” but in fact, it doesn’t always do that. It only does that in media that allow for convection. That type of media would include primarily liquids and gases. In solids, heat behaves a little differently, as it migrates through the solid, following the path of least resistance. In a solid, heat doesn’t know up from down; all it knows is to find the path that represents the best conductor. In the winter months, the ground outside the greenhouse can be at freezing temperature and often well below freezing. As such, this solid provides a conduction path where the warmer soil inside the greenhouse will transfer thermal energy to the cooler outside soil.
The solution is to install rigid closed-cell foam insulation at grade level and below, for at least a foot or two. The thousands of tiny cells in the foam insulation are air pockets that cumulatively provide dead air space and as a result, they insulate the warm soil inside from the cold soil outside that would like nothing better than to draw off the thermal energy. The closed-cell nature of the insulation prevents it from soaking up water, so the insulating qualities of the foam are maintained regardless of the environment indoors or out.
Insulation at Work
Insulation is perhaps the last thing we think about when it comes to solar greenhouse operations, yet experiments have shown that it’s perhaps the most important aspect of creating and maintaining an environment that is conducive to growing plants in colder weather. In Wyoming where I live, an experiment was conducted with triple pane poly panels in a greenhouse design that resembled a daylight basement. With no additional heat, the interior of the greenhouse never dipped below 45 degrees, despite below zero temperatures throughout the winter. To be sure, that greenhouse was making good use of thermal mass from the ground, but none of that would matter without the presence of insulation.
In another example, again in Wyoming, I have corresponded with a gentleman who built a greenhouse adjacent to his home. He uses triple pane glazing and a concrete northern wall that is part of his home. Despite outside temperatures below zero, he consistently sees temperatures that hit 100 degrees during the day. His main problem is venting the structure because of the combination of thermal mass and insulation that helps retain the heat inside.
When you think of insulation for a greenhouse, just think of something a friend of mine is fond of saying, “It isn’t the heat you use; it’s the heat you lose.”
Clair Schwan is a vegetable gardener who uses both passive and active greenhouse solar heating systems in his homemade greenhouses. His systems are complemented by thermal mass and insulation to increase their effectiveness and that allows him to garden year-round.
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