Understanding Differences in Visible Transmittance

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leftontheinside
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Understanding Differences in Visible Transmittance

#1 Post by leftontheinside »

So I get that visible transmittance is a measure of how much light is passing through but I can't really figure out the impact in regards to the numbers numbers. In general I never toy with small differences but what about 10 points?

Will there actually be a noticable difference in the amount of light between a window that is rated 45 and one that is 55?

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Windows on Washington
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Re: Understanding Differences in Visible Transmittance

#2 Post by Windows on Washington »

Yes. Quite a difference. Roughly 20% by comparison.

Think about it in terms of relative difference because no windows is going to let in more than about 0.70 even in a new construction format. Double pane clear glass (no low-e) will only let in about 0.82 on its own. You then have losses of VT to the frame of the window, grids (if any), and Low-e. This why a glass package that has a VT of 0.70 will normally only have a total unit VT of around 0.50.

You are going to be losing VT to the fact that the window in likely an insert and has low-e. If the windows have similar performance, go with one with more VT if you like natural light.

Make sure the numbers you are comparing are apples to apples (i.e. both with grids if you are getting grids).

leftontheinside
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Re: Understanding Differences in Visible Transmittance

#3 Post by leftontheinside »

Makes sense, thanks!

One last thing...my sales guy has a double pane and triple pane option. He isn't pushing one or the other as he says there are tradeoffs.

But I am not sure what to get. One has the the 0.23 ufactor and the other a 0.29 ufactor. But the 0.23 ufactor has a 45 VT and the 0.29 has the 55 VT.

So bascially I sacrifice energy efficency for the level of natural light and vice versa? I'm think the 10 points in VT might be a better choice?

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Re: Understanding Differences in Visible Transmittance

#4 Post by Clevelander966 »

The VT is directly affected by the type & functionality of the different coatings of LowE. LowE is the reflective glaze that redirects both radiant heat & solar heat BACK to their respective sources. In other words, the coating reflects the heat from the sun back towards the outside during the summer and back towards the interior of your home during the winter to reduce your utility usage and subsequently lowering your bills. SB70(Solar Ban 70) works to spectrally select the rays that enter your home, thus blocking out UV and keeping your belongings from fading & discoloring. How much depends on the levels used in the manufacturing process for the glass pack itself. SL7138 keeps the heat from exiting your home through the windows and keeps you warm & cozy. The better it works, either way, the darker the "tint"(looks greenish) and thus the less visible transmittance. Less equates to a lower VT number. Triple should have a lower U-Value & SHGC and may come with some sort of savings that might represent valuable added energy savings if you are replacing all of the windows in your home. If not, then you probably wouldn't need the third pane. The third pane does not alter the VT at any level. The coatings are applied on the insides of the outer two panes (Surfaces 2 & 3 on double pane, 2 & 5 on triple pane)

Hope it helps.

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Window4U (IL)
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Re: Understanding Differences in Visible Transmittance

#5 Post by Window4U (IL) »

These are Softlite Elements windows with a U-Factor of .16 and a VT of .37. You are also looking through screens. Personally, I see hardly any difference with the view through triple pane vs through double pane.
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leftontheinside
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Re: Understanding Differences in Visible Transmittance

#6 Post by leftontheinside »

As a general blanket statement is it safe to see that the actually visible difference to the human eye of VT 45 and VT 55 would probably go unnoticed?

Windows4U, when you say you can't see much difference are talking about a sample of each side by side?

To me the windows in the pic look like they are tinted but it could easily be the light enviroment and the fact you can't compare it to something else in the same conditions.

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Re: Understanding Differences in Visible Transmittance

#7 Post by Windows on Washington »

Everything is a trade off to some extent. You can notice the difference between 0.45 and 0.55 if you eyes are working at 100%. That being said, the triple pane window will likely be more efficient over its lifespan.

Where is the home located? That may necessitate or eliminate the need for triple glazing.

leftontheinside
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Re: Understanding Differences in Visible Transmittance

#8 Post by leftontheinside »

I'm in zone 5, the season low doesn't drop below -10 and the season high doesn't go above 95.

My windows are about a .45 ufactor and my average ANNUAL energy bill is $105.00

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Re: Understanding Differences in Visible Transmittance

#9 Post by Windows on Washington »

Where in Zone 5? Where and what directions do the windows face?

leftontheinside
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Re: Understanding Differences in Visible Transmittance

#10 Post by leftontheinside »

I just moved to Brookfield, WI from Fond Du Lac....Milwaukee suburb. The majority of the windows face west and north and are in bedrooms and living room.

Thanks!

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Re: Understanding Differences in Visible Transmittance

#11 Post by Windows on Washington »

Very heating degree dominated climate.

http://www.climate-zone.com/climate/uni ... milwaukee/

Given the lack of southern exposure, I would say you should opt for triple glazing. Is there a better choice of window for you out there? A 0.23 U-Factor out of a triple glazed window is nothing spectacular.

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HomeSealed
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Re: Understanding Differences in Visible Transmittance

#12 Post by HomeSealed »

I agree with WOW, you could probably do better on that U-value. Check the cost to upgrade to Krypton through the dealer that you are working with... May or may not make sense for you depending upon what he's charging.

leftontheinside
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Re: Understanding Differences in Visible Transmittance

#13 Post by leftontheinside »

"Is there a better choice of window for you out there? A 0.23 U-Factor out of a triple glazed window is nothing spectacular."

Right on, exactly why I went with a double pane @ a 0.28 u-factor with a VT of 55.

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