Do energy efficient windows (low-e) have an effect on plants?

Asked July 6, 2014, 1:09 PM EDT

Do energy efficient window (low-e) have an effect on plants? If so, how much. We are planning a sun room and will keep our tropicals in it for the winter. I have read several articles that say that there is enough visible light for plants. Any thoughts? Thanks, Mary

Charles City County Maryland

7 Responses

Hi Mary,

You brought up some good questions.

The light that plants use for photosynthesis is in the visible spectrum which has wavelengths between 380 and 720 nanometers. Chlorophyll, the main plant pigment in photosynthesis, best absorbs violet, blue and red light as shown in the figure.

Low emissivity (low-e) windows have coatings that reduce the amount of UV light that passes into your home. The advantage of low-e windows is that you can reduce the amount of light (and heat) coming through the windows without reducing visible light and making your home darker inside. There are other window coatings that reflect visible light as well, but you do not have to worry about less light for your plants with low-e coatings.

Visible Transmittance (VT) ratings are used to measure the amount of visible light that passes through an object on a scale from 0-1. Windows with low VT ratings allow less visible light through which could, in turn, reduce plant growth. Compare the VT ratings of the different windows that you are considering.

Assuming the sun room is warm enough you should be able to keep your tropical plants in there over winter. However, because winter sunlight is not as intense as in the summer, you will not have as much growth as during the summer time.

See the link for more detailed information on window ratings.

Thank you for your quick response. I have been in an area with poor internet reception and could not reply. I see that the VT is rated from 0 to 1. Could you tell me at what point the VT would be too low to sustain the plants through the winter or is it just a matter of growth and not giving them enough light be sustained? I am assuming that at a rating of 0.5 it would be halfway between 380 and 720 which would give you half the visible light. Thanks, Mary

Hi Mary,

I spoke with someone at the National Fenestration Rating Council, the organization that does the labeling. The rating is the proportion of light that is transmitted through the entire window area (including the frame) to light transmitted without the window there. Zero would be no light and 1 would be as much as with no window there. A window with more, smaller panes would have a lower rating than one with larger panes since light only comes through the glass.

More important than the window rating is the orientation of the room and the number of windows. Plants vary in their sun requirements, so there is no hard and fast rule for how much light is needed. If you want to maintain the plants' growth then natural light is typically sufficient. If you want your plants to produce a lot of growth or fruit during the winter you will likely need to use grow lights to supplement sunlight.

Thank you for researching this for me. My windows will be floor to ceiling with only 7-8" between each one and the room will face east so as to get only the morning sun. This should eliminate the sun in the heat of the day. Thanks again, Mary

Hi Mary,

Orienting it to the east will reduce sunlight in the summer, but you will also not get as much light for your plants in the winter. Keep in mind that our winter sun rises in the south east. This online tool calculates the sun's position based on location, date and time. If you have not already, it would be wise to consult with an architect knowledgeable in sun rooms or passive solar systems to optimize your design.,-76.9827,14/2015.01.15/07:52

Thanks for all the info. Mary

I'm glad I could be of help. Thanks for contacting Extension!