Taking Panes With Plants

We want to include a large indoor sunspace for growing plants in the new house we are building but are unsure what type of windows to use. Low-E glass will save on heating costs, but we are concerned that it may also inhibit plant growth. Should we use Low-E or regular windows? E.W., Toronto, ON

Rather than inhibiting plant growth, Low-E glass designed for residential use will probably benefit your plants. Here’s how: Glass is a remarkable product in that each sheet is about 90% transparent to solar radiation in the form of visible light, but is opaque to thermal radiation. Light from the sun passes easily through the windows of a house and warms the interior, but this heat cannot then pass back through the glass. A window will, of course, absorb and re-emit heat, which is why the glass on an oven door feels hot to the touch and why you feel chilly sitting next to a window on a cold winter night.

Low-E (low emissivity) coatings applied to a window slow the rate at which heat is transferred through the glass, at the expense of only a slight reduction in its visual clarity. An inert gas such as argon can also be added to the cavity between the sheets of glass in a double-pane window to further reduce heat transfer with no darkening of the glass, and other coatings can be applied to block UV (ultraviolet) light or parts of the visible spectrum.

Plants require sunlight-particularly the red portion of the visible spectrum-for growth. Some early Low-E coatings had a bluish tinge which indicated that some of the red light was not getting through, and a greenhouse constructed of this glass (or the highly reflective glass used in some commercial buildings) will result in slow-growing or dormant plants. Most current Low-E windows used in residential construction, however, are only slightly darker than a typical uncoated double-pane window: they let in 75 to 78 percent of visible light versus 81 percent for double-pane and 90 percent for single pane uncoated glass.

A summer greenhouse can get away with single-pane glass and it’s higher light transmission, but if the sunspace is part of your living area or if you want to grow plants year-round, the glass must also insulate against heat loss. This is where Low-E glass really shines. When the outdoor thermometer reads minus 18 degrees C, the interior surface temperature of a typical double-pane argon-filled Low-E window will be a relatively balmy 13 degrees. An uncoated double-pane window will have a surface temperature of 7 degrees while a single uncoated pane of glass will be a frigid minus 8 degrees. Lower heating bills are an obvious benefit of using Low-E glass in a sunspace; you will also be able to place plants closer to the windows without fear or their becoming chilled.

The slight difference in light transmission between Low-E and uncoated double-pane glass may be felt only by tropical plants that require full sun. However, triple-pane or so-called Low-E2 windows that have extra coating layers for even better insulative properties probably should be avoided-these reduce the transmission of visible light by up to 10 percent compared with clear double glazing. On the other hand, Low-E windows with an additional coating to screen out UV radiation may work well in a sunspace; researchers studying the thinning of the Earth’s ozone layer have found that excessive UV light can damage or cause genetic mutation in plants.

Before choosing the glass for your sunspace, ask about the type of coating used as well as its emissivity rating, which is a measure of its insulative ability. Then look through it. If the glass looks clear to you, it will look clear to your plants.
Stephen Carpenter is president of Enermodal Engineering, an energy-conservation consulting service in Waterloo, Ontario. He is also vice-chair of the Canadian Standards Association (CSA) committee, which rates the performance of windows.
Reprinted from Harrowsmith Magazine – December 1993. Illustration by Stephen Quinlan