Without more information, particularly the shading co-efficient of the IGU's (insulating glass units) used in your windows, I can only offer some generalities. First - UV light is not particularly important to plant health, except that too much causes all sorts of problems. Far more important is the amount of visible light transmitted through the IGU's. A single piece of 1/8 inch glass (transmission depends on thickness, but most residential IGU's are fabricated using 1/8 inch thick glass) allows about 89% of visible light to pass. An IGU with only 2 lites (pieces) of glass allows about 79% visible light transmission. An IGU with a low-e coating allows about 75% visible light transmission. Argon or krypton gas fills have virtually no impact on light transmission.

However, there are many "other" factors that can significantly increase shading co-efficients. The use of tinted glass, suspended heat mirror, other other additions to the IGU's can significantly impact light transmission. Ask your installer to supply the spectral transmission statistics of the product. Some IGU's may favor transmission of wavelengths nearer the blue end of the spectrum, which is good for foliage, but not for blooms. Other products might favor the warmer wavelengths (red) which is better for blooms than for foliage.

I'm wondering if Jen (Oojen) is listening in? I bet she did some research before deciding on a glass product for her new addition. Her input is always helpful.

Al

Terrets, I had the same problem when we had new windows installed..There's a gas that cuts down infra-red..I was quite concerned about this..the salesman said, if it effects your plants, I'll buy you new ones..Right, I'd like to see him attempt this..lol
To be honest I don't notice any difference as far as foliage plants..the only room where we still have the old windows, are almost floor to ceiling, but double paned..It makes no difference w/flowering/foliage plants..
I'd question the saleperson..if they can answer..Toni

Some of my windows were recently replaced, I had no say in what was used since the landlords did it, but I've noticed no ill effects. I can't give particular info about all the things Al mentioned, so this is FWIW. Thanks, Al, for that explanation. I was worried about the effect on my plants but all I could do was adopt the ol' wait and see attitude. I love the new windows and my heating bill will too.

Thanks for the vote of confidence, Al, but you've already said it better than I could!
Yep, visible light is the main consideration. If you lose a small percentage (about 4% according to Al, who knows his stuff) for low e coating, it's worth the sacrifice for energy savings, plus your drapes, carpets, furniture, paintings, etc. will have less sun damage (color fading) and your plants will be less likely to burn (too much UV can damage plant tissues just as it can our tissues). Window screens will cost you more of your visible light than UV coating will...I wouldn't anticipate any problems at all. As a matter of fact, strong UV reduces germination rates in many kinds of seeds-- even seeds that require visable light in order to germinate.
We've had our windows in for a year now, and everything's going fine. I did compensate by putting in more, and larger windows. I think I made up the 4%
;)

Good point about the screens - wish I'd have thought to mention it. Remove them on windows you don't open (that have plants near) and during the winter, especially.

Al

Wow, Thank you guys, this ended up being very informative. I really appreciate it!

I am looking to enclose my screened in porch (for my plants of course) and in searching for window info, came across this post. Does anyone know more about the "heat mirror" glass. It is touted as having better noise control and more insulation, with an R value of 9.1, compared to 4.0 for low-e glass. However, it will be a total of $2000 more for the heat mirror glass. Is this worth the energy savings, and will plants respond well to it?

Any thoughts appreciated!

You decide.

Heat Mirror 88 is designed for northern climates, where warmth from the sun is desirable for supplemental heating. Heat Mirror 77 and 66 are designed for climates where overheating is the primary concern. The VLT (visible light transmittance) varies by product, but here is something of a comparison:

A typical IGU (insulated glass unit) without low-e glass allows about 80% VLT, depending on who you listen to.

The same IGU with low-e transmits in the neighborhood of 75%.

An IGU with low-e and heat mirror transmits about 67% for TC88 and about 60% for SC66.

Can't answer whether the 2M outlay is worth it. You need to factor in whether the room is heated & air conditioned all year, how long you intend to live there, expected increases in energy costs and resultant savings, what the increased comfort level is worth to you, and how much you expect to recoup in increased resale value of the home.

Al

Thanks for the detailed answer, Al. I don't think the plants would much enjoy that 60% VLT on the SC66, which is what I believe they want to sell me in NC. Even though I am in a southern climate, my only concern is keeping heat in in the winter. Summer heat doesn't bother me a bit, as long as the windows open. So, I guess I'm really in the market for a northen glass. We plan to live here a long time--15+ years, and I can't see us recouping much window cost on the resale. I guess I'm leaning towards no heat mirror. Do you know the stats for VLT for heat mirror without low e coatings? I don't think the windows I was shown were low e--just heat mirror.
One more question--I see the noise reduction is slightly better with heat mirror. 5 points I think. Does that translate into much of a noticeable difference?

Thanks again for sharing all your expertise.

Low-e coatings don't reflect much of the visible spectrum, so if the product was only heat mirror w/o low-e, the VLT would only be a few % more - nearly insignificant.

Sound deadening occurs because as sound waves pass through the dissimilar materials glass, air, HM, air, glass, each medium vibrates with a different frequency which diminishes transmission. Argon gas is also effective at noise reduction in IGU's and the thicker the IGU, the better the sound reduction.

A single piece of 1/4" plate glass has a STC (sound transmission class) rating of 31. 1" conventional IGU's are rated at about 34 and the same product with heat mirror is rated at 36, so I would rate the difference as fairly insignificant & not too important as a consideration. Though I cannot quote exact numbers without looking it up, I can say that I believe argon gas makes a considerable difference in noise reduction. Without fail, when we install argon filled windows in a noise-rich environment, the home owner invariably comments on how much quieter the home is. Part of the reduction is the superior sealing of the brand of window we sell, but even when we replace wood windows that already have IGU's in them, the difference is significant.

Al

Fantastic info! Thank you thank you thank you. I'm going to go with low e, argon windows, probably with a high solar gain that hopefully won't be too uncomfortable if the windows are open.

Can you tell me what brand of window you sell? If you say it is superior that quite a recommendation.

The primary focus of our business is contract (storefronts) and residential glazing, mirrors, shower doors, stained glass, closet shelving, and vinyl replacement windows. We looked for 5 years before deciding on a quality supplier of replacement windows. We chose Polar Seal brand windows, built by a company in Grand Rapids, MI. Good luck with your project. ;o)

Al

I looked up polar seal's website and there are no dealers in my area--breaks my heart!

Thanks again for everything, Al. You've been super. Now how about moving down south?! ;)

The building we are moving into has 3" windows with two layers of glass, and two layers of film, and I am concerned about our plants.
Here are the specifications of the glass:
Shading Coefficient: .39
UV Transmission: .10%
Visible Light Transmission: 47.5%
Solar Transmittance: 23.5%

Any thoughts on the effect on our plants? Should we plan on supplimenting the light with grow lights? If so, any suggestions for lights that won't look too out of place in our living room?

Thank you,

Gabriel

Well, Shading Coefficient (SC) indicates how much heat gain from light is blocked. It compares how well a particular glazing product reduces solar heat gain to a single pane of 1/8-inch double-strength clear glass. SC values range from 1.0 for clear glass (without low-e) to 0.08 for heavily reflective glass. A standard insulated glass unit (with no coatings or suspended films) has an SC value of about 0.87.

The uv transmission is unimportant (where plants are concerned)

Visible Light Transmittance (VLT) is the amount of visible light that penetrates a window expressed as a percentage, and it is the most important of the considerations you list, as plants use light in the visible range. 1/8 inch clear glass has a value of 90 percent transmission. A standard IGU (again, with no coatings or suspended films) will transmit approx 75-80% of visible light & is reasonably satisfactory for growing most plants.

My office/showroom has blue tinted glass with a low-e coating and has about the same VLT as your windows (47%). I find that plants tend to be quite leggy, even when grown fully in front of east and south windows. Sorry.

Al

A source says that UV rays may be important for the development of some plants for flowering:

"In addition to the light visible to humans (400 nm – 700 nm) plants
“see” or use other radiation too. The 400 nm–700 nm wavelength range is
called “Photosynthetically Active Radiation” or PAR. Much of the light
that plants need is in this range, but for optimal growth result, UV
light (280 - 400 nm) and/or far-red light (700-800 nm) might be
important. For example far-red is critical for the flowering of many
plants. All light is not equal to plants, ie. some areas are more
important than others."

Source: http://www.valoya.com/plants-light

Understandable given all plants evolved outside, not behind glass. It doesn't mean all plants will be affected negatively, but there may be some differences, such as with plants that flower. I know growing NIcotiana that if you leave a plant inside at a window during the summer months it will never flower, but put a geranium in the same spot and you'll see it flowers. Move the geranium outside and it will flower more.