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Please help me understand Variable Transmittance

Posted by cvette (My Page) on
Sat, Sep 25, 10 at 8:27

Hi everyone. We're looking to purchase Andersen's A-series DH windows. The regular LowE4 has a VT of .52 and meets the .3 U-factor and SHGC for the tax credit.

Unfortunately, the LowE4 doesn't meet in the 200/400 series gliding windows and the picture windows we're looking at for another room. I'd have to go with the SmartSun to get the credit on these windows.

My question is: Is VT the ONLY measure that drives the way the window looks when you look through it? i.e. how tinted it looks? For example, would a lowE4 window with 0.52 VT look the same as a SmartSun glass with 0.52 VT?

In general, how is VT measured and why does it change with window size? I imagine it would be a measure of light per unit area. So why, for example, would a DH have a 0.51 VT but a glider or picture window with the same glass option be 0.48 VT (assuming no grilles in all cases)?
Sorry for all of the questions - Thanks.


Follow-Up Postings:

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RE: Please help me understand Variable Transmittance

VT is a number that represents a percentage of visible light that enters a given window. VT 0.52 = 52% of the light that hits that window (including the frame) comes through.

VT is a function of frame profile (thickness) and the type of glass and Low-e Coating.

The same Low-e coating that is used in the Double Hungs will not allow the slider or picture windows to be qualified because it will exceed the SHGC maximum of 0.30. This is because there is more glass in the slider and picture window as compared to the double hung.

They must use a thicker low-e (SmartSun) to get the SHGC low enough on those window and the VT number will drop as a result.


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RE: Please help me understand Variable Transmittance

i would also add I doubt you could see the difference between a VT of .48 and.52.


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RE: Please help me understand Variable Transmittance

Got it. Thanks windowsonwashington and skydawggy. I thought VT was per unit area but it is for the entire window. So it's not a difference in VT I'm seeing when I compare LowE4 and SmartSun, it is really the thicker layer of coating/tint.

That's too bad. I need to find someone with a good glass sample to take home. Andersen doesn't supply glass samples and neither does the builders supply or HD. How can I ensure I like the SmartSun without taking it home and looking at my white fence in the sun? That's rhetorical unless you know how I can get Andersen glass samples :)


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