Impact of new windows

[Diane]

I had new windows installed earlier this year. They are the newest double pane, low e, UV protection coating type windows (and sliding glass doors).

Does anyone know what the effect of these types of windows is on light quality, intesity, etc..? I'm just wondering if the UV filtration might be a bad thing, or that the UV coating (which has a slight tint) could be reducing the usable sunlight.

Also, does the UV coating reduce the possibility of burning plants?

[Jmoney]

your plants might find it harder to get a good tan.

seriously, I have no idea.

[Diane]

Guess that means I should lighten up on the SPF 30!

[mde]

I can not state for certain, but would suspect that this would diminish light intensity. Did you ever take a footcandle reading there on a sunny day but after any direct light came through? You could do a comparison. I know I was challenged on this statement once on this forum, but a local species grower told me that footcandles will be 1/2 of the outdoor strength after passing through a double pane window, so on a summer sunny day footcandles are usually around 10,000 and so you could expect around 5000 indoors within 1-2 feet of the glass. This is exactly what my f-stop reading was this summer around July 1st (see below). If you have screens and/or if even a slight film of dust accumulates, it will reduce the light intensity on the inside of the glass even further.

If you have a manually adjustable camera you can read the f-stop reading with the camera focused over a piece of white paper and get an estimate of the footcandles and compare this to the recommended light intensity for the species you have growing at these windows. If you can get a reading on a peak summer day and compare it to a cloudy winter day to get your maximum and minimum exposures and adjust supplemental light or shading to keep in the proper range.

Here is the general guideline in case you have a manual camera:

f/2.8 = 200 footcandles (fc)
f/4 = 370 fc
f/5.6 = 750 fc
f/8 = 1500 fc
f/11 = 2800 fc
f/16 = 5000 fc

Of course you can buy light meters too, but I've found this estimate works well.

Matt