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Over the past months we have posted a few blogs on a new sustainable greenhouse prototype we are developing.  We have received comments from a number of traditional greenhouse users with concerns that this new prototype will not be capable of plant starts because of inadequate light levels.  The prototype does push the envelope of greenhouse design but through the use of careful daylighting strategies not typically implemented it is capable of effective plant starts.  The Research and Development greenhouse that is the basis for our prototype started over 1,000 tomato plants last spring without the use of supplemental light.

To understand how this works a typical all glass or plastic glazed greenhouse has no reflective surfaces and the glazing reduces light transmission into the greenhouse by up to 40%, or more.  This means that the light levels entering a traditional greenhouse are first significantly reduced by passing through the glazing and then any light that does not immediately hit a plant surface is either absorbed by the ground or passes back through the glass and out of the greenhouse.

The sustainable greenhouse prototype uses some of the clearest glazing available and a number of reflective surfaces to increase light levels inside the greenhouse.  First, reflective surfaces on the outside of the greenhouse, i.e. light shelves and reflective roof surface, bounce additional light into the greenhouse.  This reflected light allows more sunlight through each window than would direct light alone increasing the effective aperture size without more or larger windows.  Next, the clear glazing allows more of this light to pass into the greenhouse compared to a similar area of glazing on a typical greenhouse.  Finally, although the greenhouse is made up of a number of opaque walls and roofs each of these is covered with a white reflective surface.  The reflective surfaces inside the greenhouse mean that any light that does not immediately hit a plant or the ground is bounced around inside the greenhouse until it does.  Through a combination of thoughtful daylighting strategies our prototype greenhouse achieves adequate light level for plant growth and plant starts with fewer windows than a traditional greenhouse.

Building a sustainable near zero energy greenhouse is not an insurmountable task.  Despite the basic strategies required, I am not aware of a manufactured greenhouse that combines all of the necessary elements to achieve high performance.  By combining a high R-value envelope with effective thermal mass and intelligent daylight harvesting, near zero energy greenhouses can be easily achieved.

Most greenhouses do not implement any of these strategies.  They are constructed primarily of glass, plastic glazing, or even plastic sheeting supported on metal frames with little or no insulation.  These materials tend to have low R-values, on the order of 0.5 to 2.  Although these materials are capable of letting in a lot of daylight, the daylight is not intelligently harvested.  Traditional greenhouses are over-glazed (contain too many windows) leading to overheating in the summer from too much solar heat gain and are difficult to keep warm in winter because there is little or no high R-value envelope to retain solar heat.  Well-designed greenhouses balance the amount of glazing with the amount of opaque insulated walls to allow the greenhouse to maintain comfortable temperatures year round while still allowing enough light for plant growth.

Greenhouses usually contain some mass that impacts the thermal performance of the building.  However, because of a lack of insulation and poor design the mass can actually contribute to maintaining cold temperatures in the winter and hot temperatures in the summer.  A well designed greenhouse uses thermal mass to reduce temperature swings throughout the course of the day (and year) by absorbing energy when it is available and releasing it when it is needed.

The first thing many proponents of traditional greenhouses notice when looking at the Hutton Architecture Studio/Synergistic Building Technology greenhouse is the greatly reduced area of windows.  The window area is carefully calculated to allow enough light in to heat the greenhouse in the winter and allow for productive plant growth, while reducing the area of low R-value envelope.  The area of the building that is not windows is highly insulated opaque wall assemblies that reduce heat loss.  Finally, to complete the envelope the reduced area of glazing is covered with automatic insulated shutters (built by SBT) that close on winter nights to greatly reduce heat loss.

In addition to this insulation above grade, the insulated envelope continues at least 3’ below grade.  This isolates a large volume of thermal mass that taps into the steady earth temperatures beneath the greenhouse, helping to stabilize the temperature inside.  By carefully controlling the daylight contribution, and hence, heat to the greenhouse, the thermal mass is able to help keep the greenhouse warm on cold winter nights and cool on hot summer days – exactly what thermal mass is intended to do!

By combining these strategies, the Research & Development greenhouse (as discussed in a previous blog) is able to maintain stable warm temperature without overheating throughout the summer by leaving the doors and insulating shutters open.  Evaporative coolers that were installed in the greenhouse were never turned on.  In the winter the greenhouse maintains warm growing temperatures, typically in the upper 50’s to lower 70’s with only solar heat by closing up the envelope and operating the shutters on automatic mode (open during daylight hours and closed at night).   Implementing all three of these simple strategies together has allowed the HAS/SBT team to develop the next generation of reliable near zero energy Green Greenhouses.

By Gardner Clute