Greenhouse Design 101
Phil in Jones, Oklahoma (near OKC) writes: "I recently had a greenhouse built; eight feet wide by 15 feet long by approximately eight feet high (with traditional vaulted roof). The floors are wood with ¼ in. spaces to allow water to escape. The entire greenhouse is raised on blocks about a foot. Most of the roof and sides is/are covered with 8 mil plastic; but for the winter, I'll also insulate the wall panels and roof on the north side with 12 mil translucent greenhouse insulation. I also have panels of Styrofoam insulation around the base.
"My question pertains to a heating source inside the greenhouse. There are all kinds of self-proclaimed greenhouse experts on the Internet that tout various kinds of electric heaters, but when I look at the reviews on Amazon, I see scary one-star ratings, mostly because of potential fire hazards.
"Gas or propane heating is impractical and too costly, so for now, I feel that I must use an electric heater. There are all kinds such as coil, ceramic, and infrared. I think I will go with an infrared heater that Consumer Reports gives high ratings for safety and efficiency. It produces 5000 BTUs, which approximates my need based on a fact sheet from my local university extension service. I will use a correctly rated 50-foot extension cord to power the heater and place a smoke detector with a remote alert via smartphone in the greenhouse.
"Winters in Oklahoma can dip below the 30s on infrequent occasions and never below 20. The heater has a thermostat and safety shut off if it gets too hot. Might you have any other suggestions for safe heat?"
A. Although highly detailed in some regards, Phil's treatise left me with many questions, so we had a follow-up phone conversation. But first, I will reveal that the direct answer to his question is an oil-filled electric radiator; that's what I used in my greenhouse when I grew hot pepper plants over the winter for our major displays at the Philadelphia Flower Show. There's no flame, no heated coils, just gentle radiant heat controlled by easy-to-use settings and a built-in thermostat, so Phil can now stop buying fire extinguishers in bulk.
But I was (and still am) growing in a cold winter area (especially when I was raising a lot of hot pepper plants for one of our 1994 displays, when we were hit by weather so fierce we had to keep shoveling a path through waist-high snow to get to the greenhouse door!) and Oklahoma generally has relatively mild winters; so I told Phil I was more concerned about his greenhouse overheating during bright sunny winter days and asked if his structure had an automatic vent.
"Not exactly", he replied, explaining that he had double doors that he planned to prop open when it got too hot inside, plus the side windows also swung out to ventilate the place. "So you're going to open all this up on hot days and then remember to close everything up tight when it gets cooler that night? That'll get old by day three! Do the windows open at the top at least?" No, he answered, they swing out at the bottom.
"You do realize that heat rises, right? You need to be able to ventilate excess heat from as high up as possible, which is easily done with an automatic vent, where a gas filled cartridge lifts up a roof panel or an arch above one of the doors when it gets too hot and lowers it automatically when it gets cooler." He explained that he and the builder thought about installing one and now he will. I asked if the greenhouse had the traditional bench or table down the center. "Nope; we built large shelves along the inside of the walls..."
"...Where the temperature changes are going to be the most extreme!" I interrupted. "Cursed be YouTube videos!" I yelled! "At least you can put the oil-filled radiator in the center of the room and think about swapping the shelves for a center bench next year."
Let's end with a few other tips we discussed. Greenhouse supply companies carry large, heavy rubber heating mats that cover the central bench to keep the root zone of the plants warm. Numerous studies have shown that bottom heat protects plants from cold much better than things that warm the air temperature. They're made for greenhouse use, so they are, of course, safe to use around water (but they must be plugged into a ground fault interrupter receptacle). They have a probe that goes into the dirt of one of the plants to keep things at the perfect temperature. In Oklahoma, that should be more than enough to protect plants over winter, so it's likely there's no need for a heater. You can also use the mats for seed starting in the Spring.
I also explained to Phil that when we constructed my greenhouse, we ran conduit underground to carry electricity safely, and buried a length of PVC pipe that carried the warm, moist air from our electric clothes dryer inside. The last thing we did every night was to toss a big load of freshly washed clothes into the dryer; even on really cold nights the automatic vent would open, and steam would come pouring out.