Bananas in Boulder
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 hydronic heat supplied by 3 solar hot water panels on the greenhouse roof. These panels are set up as a part of a drain back solar hot water system that turns itself on and off as needed based on thermostats we have set up to control it. The system heats hot water pipes buried deep under the ground. Using this system we can keep the soil temperatures in the greenhouse up in the 70s all winter long- the plants love it. Best of all it takes less than 20 watts to move the water from the solar panels into the ground where we need it (way more efficient than the earth battery which uses closer to 1000 watts to move heat around).
We are currently growing guava, avocado, banana, lime, orange, starfruit, pomegranate, passionfruit, cherimoya, cinnamon and allspice along with more standard Colorado annual greenhouse offerings.
We designed our own greenhouse but do not provide greenhouse design services. We do give educational greenhouse tours for anyone wanting to learn from our greenhouse experiences. We charge $150 for these tours and give copious amounts of useful information for would be greenhouse owners.