Sliding Glass Doors for Greenhouse Glazing?

(Question)

Hi

I want to know if UV glass is suitable for a greenhouse? Is normal greenhouse glass preferred to be UV coated?

I am planning a garden to grow vegetables. I want to extend the season as long as possible and as cheaply as possible. I would want the 20ft x 50ft garden to be enclosed in glass during cooler months, and open or screened in during warmer months.

Sliding glass patio doors are about 3ftx7ft tempered glass in a frame, are UV on one or both sides and R-5.

Are they suitable for greenhouse glazing? I am worried about the UV coating. Do plants need UV?

A greenhouse made from sliding glass doors could have the doors removed during the summer. The biggest reason for using sliding glass doors is the price. Used sliding glass doors cost less than $20 each. Many are free to take away. Also they are R-5. a double layer is R-10. This means heating the greenhouse all year round becomes possible with solar panels. The greenhouse could be open or screened, or single layer R-5 or double layer R-1 by just inserting or removing different sliding glass doors.

The only negative is the weight. They are heavy and would require a substantial frame and foundation. However, 2 layers of glass doors would result in an R-10 greenhouse with easily removable doors. Sliding glass doors can also be used to make thermal or electric or thermal/electric solar panels. These solar panels could provide extra heat during winter and any electricity during the whole growing season or year round.

Any comments?

Peter

(Answer)

Hello Peter,

Thank you for writing with thoughts, and questions, about your greenhouse project and  garden plans.

Regarding using the sliding doors: in very general terms, R-rating of glass refers to the amount of interior heat loss the glass will allow. The higher the rating number, the lower the heat loss. So an R-5 glass provides greater prevention of heat loss than, say, an R-2 glass.

But, the trade-off here is that fewer sunlight rays will penetrate the higher-rated glass, essential for plant growth. Photosynthesis combines carbon dioxide, from the air, with water from the soil, in the presence of light energy to form sugar and oxygen.

Regarding UV rays: photosynthesis utilizes visible light from the blue to red part of the light spectrum. Ultraviolet and blue light are needed to keep plant stems sturdy and short, while red and far-red wavelengths, in turn, keep plants from being too short, and control the flowering and germination of most plants. Untraviolet light is at the low end of the spectrum, and, in very large quantities can be harmful, but, a certain amount of UV light waves are beneficial for balanced plant growth. Historically, most greenhouses have been built from single glass panes, to allow as much light as possible to reach the plants.

Yet if you have access to inexpensive glass in the form of pre-fabricated doors, you may want to take advantage of them, but the light waves that will reach the plants will be reduced. Your other question regarding solar panels for heating will require some expert engineering calculations, particularly as your proposed area is 20′ x 50′ — and you didn’t mention where your greenhouse might be located.

The Toronto Master Gardeners mandate is ”to inform, educate and inspire the residents of Toronto to create healthy and vibrant gardens, landscapes and communities through the use of safe, effective, proven and sustainable horticultural practices”. So while TMG neither sells, nor endorses, manufacturers’ or retail products, included here is a link to a Cornell University’s “Greenhouse Horticulture”, which, in the left margin, provides handy links to several very useful resource publications for homeowners and gardeners. This includes, among numerous other important considerations for greenhouse construction, detailed information of framing and glazing. This will hopefully get you underway with answers to your specific questions re the pro’s and con’s of using sliding door panels.

Cornell University

Once you have your greenhouse planned and constructed, and have selected your vegetables, TMG can definitely give you specific guidance on cultural practices including soil, sunlight, air temperature and water. There is much information of the TMG site on growing vegetables, including:

Organic Vegetable Gardening

In the meantime, we suggest that, in addition to online research into greenhouse construction options and solutions, you may wish to visit the Toronto Botanical Garden Weston Library collection. There are a number of books on greenhouses and the growing of fruits/ vegetables all year round. Also, if you haven’t already done so, you may wish to visit a variety of greenhouse locations in person and perhaps speak to the horticultural staff at Allan Gardens, Spadina House or  Toronto Botanical Gardens, (all in Toronto) or the Royal Botanical Gardens, Hamilton. They would likely be able to share some detailed information on the considerations for greenhouse management.

All the very best on your new year 2016 project !