UV Transmission

sheilabbFebruary 1, 2010

I am replacing the Hurd Mirror 88 glass units due to fogging between the panes. The windows guy first suggested Oldcastle Glass - PPG Sungate 500 on Clear Low-E #2, with Argon fill. Visible light Trans =74%, UV Trans=42%, U=.31 and SHGC=.62, Solar Trans=52%. When I complained about the amount of UV (our Hurd is only .5% UV transmission), he suggested the following:

Pilkington Energy Advantage Low-E #2 with Argon fill, visible light transmission, 69%, UV Trans. 28%, U=.28, SHGC=.59, Solar trans=39%. Is the UV low enough to protect furnishings? Are the other changes okay? What I want is to gain as much heat in winter and as much visible light as I can without overheating in summer. We have 2 foot overhangs (passive solar with concrete/tile floors with lots of south facing windows) and trees around the house to help in the summer.

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Windows on Washington Ltd

He is quoting glass numbers only and not the actual numbers as they would be observed in the case of an installed window.

Most low-e coatings will block +75% of UV rays right off the bat. Take that number and extrapolate it into a window and the lack of transmission through a frame and that number will go to about 90%.

What number are you referring to when you say Solar Trans?

They have UV specific coatings that will drop the UV transmission rates to below 0.5%. If you are worried about fading, you can opt for a coating like that.

All things that are going to block UV and increase thermal performance are going to hurt SHGC and passive solar numbers.

    Bookmark   February 1, 2010 at 10:47PM
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He is just replacing the glass unit in the windows so that is why the numbers are just for the glass unit. As for Solar Trans%, I believe it means total solar transmittance.

    Bookmark   February 2, 2010 at 3:36AM
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Windows on Washington Ltd

I figured as much based on the description, however, there is no measurement for that on the NFRC.

The SHGC represents the measurement for the sun's ability to heat the home. Not sure where your contractor got this other number or how it is even applicable.

    Bookmark   February 2, 2010 at 7:23AM
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I found the following definition of terminology at http://glasselect.oldcastleglass.com/UserControls/Terminology.aspx?MonoLami=IG&Category=Targets

Ultraviolet (UV), Visible and Solar transmittances are based on laboratory spectrophotometric measurements weighted by an appropriate weighting function using LBNL Window software in accordance with NFRC 300 methodology. The wavelength ranges of the suns energy used to calculate properties:
Ultraviolet = 0.30 to 0.38 microns
Visible Light = 0.38 to 0.78 microns
Near Infrared = 0.30 to 2.5 microns

    Bookmark   February 2, 2010 at 11:23AM
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Windows on Washington Ltd

I think you are starting to get hung up on the minutia a bit.

If you worried about passive solar heat gain, the number you need to concern yourself with is SHGC. You will want as high an SHGC as you can get while balancing the thermal numbers.

As far as the UV transmission goes, most major glass manufacturers will have an SPF (SPF is Guardian's version) coating option that will block 99.5% of all UV radiation.

Standard Low-e will block +75% on its own and when evaluating the window on the total effect, it will usually run North of 90% on the UV transmission data.

When considering passive solar and balancing U-factor with SHGC, there are several things to consider. Take a look at the link below.

Here is a link that might be useful: Lawrence Berkeley Labs study

    Bookmark   February 2, 2010 at 8:05PM
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The information given above about "+75%...and whole window effect 90%" is not accurate, when evaluating UV light. If you are concerned about UV light transmission, you are concerned about the amount of harmful fading sunlight that comes through your glass. Therefore, it likely doesn't matter to you whether it is through an inch of the glass or the whole window.

The bottom line is that if you want to block UV light like your old glass, you must add plastic to the glass. Unless there is a plastic substrate added to the glass, you will be unable to block out more than approximately 85% of the UV light. When plastics are added, this number reaches about 99-99.5%, since plastic absorbs or will not allow much of the UV light to penetrate.

Your old glass had a piece of plastic centered between two panes of glass (like a triple glazed assembly). Another variety, that would offer similar UV light protective elements, is to use a laminated glass product -this sandwiches the plastic between two pieces of glass (as opposed to being "suspended" between two panes of glass).

This information should get your search started. Keep in mind that the additional information provided by the other poster is accurate in that you will need to decide which benefit is most important to you. Generally, it is difficult, if not impossible, to find a product that excells in all parameters. You will likely weigh the benefits of each, and decide which is the most "balanced" glass for your needs. As an example, the Heat Mirror product you are replacing excelled at UV protection, but it was poorly limited in all the other benefits found in typical Low-e glass -these would include reflective properties, color, clarity, energy performance, solar heat gain, etc. There is no "perfect" product, but there are perfectly balanced products available for your region and your requirements.

    Bookmark   February 3, 2010 at 1:53PM
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Another bit of information that may help, but keep in mind that I am not a glass expert. When attempting to achieve a high SHGC for passive solar purposes, the glass substrate is different from commonly used glass. Therefore, most glass manufacturers offer these glass types, but they are somewhat limited (because of a lower demand). Personally, I've seen better balanced numbers than the ones you noted from your "windows guy". Pilkington makes this specific high SHGC glass, as does PPG and Cardinal. Generally, a consumer gives up a little in u-value and heat retension when specifying maximum solar heat gains. Add the plastic into your equation, and you'll just have more balancing to consider.

    Bookmark   February 3, 2010 at 2:08PM
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Windows on Washington Ltd

Sorry to disagree but there is actually nothing inaccurate whatsoever in my previous post.

Feel free to re-read my comments in my first post and you will see what I mean.

Thermal efficiency numbers, Visible light, SHGC, and all other measurable performance criteria are given in relation to the window as a unit and not the glass alone. Hence the reason that I related the UV transmission from the glass number to a whole unit number. The reason that I stress whole unit numbers with our clients in the replacement world is because it is much easier to twist numbers when it applies to the glass alone. People quote Center of Glass R-Values, Visible light, and other data that can be entirely unrepresentative of whole unit performance.

On average, a window of average size (double hung) will have a whole unit (i.e. including the losses to frame and sash) UV transmission rate of about 90% with standard Low-e, meaning that 90% of the UV rays that are exposed to that entire window will not enter the house.

The numbers, specifically from Guardian, are 76.2% blocking on standard two coat (7138) Low-e double pane, 90.3% on triple pane low-e, and 99.9% on SPF with Low-e.

I will also re-suggest my SPF specific coating suggestion.

    Bookmark   February 3, 2010 at 3:23PM
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I see your point, but it seems silly to average out the UV protection by the square footage of the entire window. This will inevitably confuse a consumer. As an example, a window salesman could offer a UV-conscientious consumer Window A or B. If Window B has a 6" smaller glass area (on all four sides), then the UV protection would be enhanced by the percentage of the overall square footage of the window, as UV doesn't penetrate the sash or frame on any window (in regards to fading qualities) - i.e. A window with plywood in place of the glass would have a 100% UV rating. So, the consumer buys Window B and is dissatisfied with their purchase, because they later determined that Window A had a larger glass area, more visible light and a greater glass UV-protection which made it much more beneficial. So, unless a consumer compares Visible Light along with "Whole" window UV-protection, they don't know that they may be receiving a 2'x4' window with 1.5 square feet of visible glass area (instead of 5.25 ft2).

This UV data obviously differs from other window tech talk about low-e, SHGC, etc., as these items will change based on the type and construction of the sash and frame. Thus, these calculations should be advertised as "whole" window numbers, and not glass only.

I have never had a window sales person ever represent their windows with a "Whole" unit UV rating, and if they ever had represented their wares in this manner, none of them ever surpassed about 85% protection. That is, unless their glass contained plastics within the assembly.

    Bookmark   February 3, 2010 at 5:02PM
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Windows on Washington Ltd

I get your point too. I was just clarifying my original statement that was pretty straightforward.

UV transmission is not the first consideration for about 95% of the customers out there so it is a second tier concern and especially when you consider that standard Low-e blocks about 80% right off the bat.

    Bookmark   February 3, 2010 at 8:07PM
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