It’s a bit of a trick to build an efficient tropical greenhouse in Boulder however it is completely possible. The main issue is to use what nature has given us and use it wisely. Here in Boulder we have copious amounts of solar radiation.
The first thing we did was orient the greenhouse to maximize direct solar gain in the winter and minimize it (to the extent possible while still getting enough light to grow the plants) in the summer and instill it with massive amounts of stone for passive heat balancing using thermal mass. Then we put in an earth battery so that we could access additional thermal mass underneath the greenhouse for greater daily solar and seasonal heat balancing. Finally, we gave it humidification and great insulation, used reflective surfaces to enhance the natural lighting, and added passive and solar powered venting.
Our greenhouse is built of Raycore SIPS panels and Lexan Thermoclear Plus Softlite polycarbonate sheets over yellow cedar laminated beams. The Raycore panels have a reflective coating that not only helps with insulation, it also keeps moisture out of the walls and reflects light back into the growing space. The highly specialized polycarbonate glazing diffuses the light entering the greenhouse so that the intense Colorado sun does not burn plants and so that larger perennial plants do not cast shadows over the smaller annuals. The design additionally admits 50% more of the available light in winter than in summer, passively balancing temperatures and growth processes throughout the year.
The building is heated and cooled passively with a huge amount of surface thermal mass in addition to an active humidification system, a fan circulation system and temperature sensitive Gigavents which open and close without electricity. When the house gets too hot, our Aeromist system (this is a fog system- regular misters add way too much humidity) turns on and humidifies and cools the air. AC Snap Fans pull interior greenhouse air through 4" drainage pipes buried in up to 5' of soil. The soil acts as a heat sink, cooling the air on hot summer days and warming it on winter nights. (We were originally using DC Snap Fans that turned out to be a mistake. They did not handle the resistance in the underground piping nor the humidity well and their AC fans are also super efficient.) If this is not sufficient or if the humidity gets too high, additional AC Snap Fans suck air through the Gigavents and out of the building near the ridges.
On extremely cold winter nights that don't come after a sunny winter day, the greenhouse is additionally warmed with a rocket stove heated, recycled, stainless steel hot tub (now converted to a bathtub). The stove heats the bathtub water (while providing CO2 for the plants), which acts as a heat sink and heats the greenhouse overnight. The next day the water is either reheated or used on the plants. We are in the process of hooking up a compost driven, hydronic heating system as we are unsatisfied with the rocket stove back up heat and the earth battery and thermal mass are insufficient on their own. The rocket stove takes too much constant feeding. Hydronic heating from compost can supply heat 24/ 7, rain or shine for upwards of 6 months if built properly and can supply ample heat for our needs.
Tropical plants that need the most warmth are grown near the center of the house where the stove will be located and will be kept above 50 degrees at all times. Plants that need a little less warmth, like figs, are being grown at the ends of the house where temperatures will drop a bit lower. Taller perennial trees will create an overstory canopy with annual crops and smaller perennials grown beneath.
We are currently growing guava, avocado, mango, banana, lemon, lime, orange, grapefruit, starfruit, dragon fruit, pineapple, pomegranate, passionfruit, cherimoya, cinnamon and allspice along with more standard Colorado annual greenhouse offerings.
We designed our own greenhouse. To get your own greenhouse growing in Boulder come visit our greenhouse on one of our Saturday tours to learn about it, then contact Ceres at www.ceresgs.com or go to a greenhouse design class at the Central Rocky Mountain Permaculture Institute.