The Visible transmittance rating (VT) will not in itself have much to do with energy efficiency, however SHGC (solar heat gain) often goes hand in hand and certainly will.

1) Regarding what is desirable, a moderate to high shgc will be generally preferred in your region. This can be fine tuned of course as you mention by orientation, facing direction, landscaping/trees, etc. Most homes are not designed to take advantage of this however, so again you are looking at the major points. You would not want a super low shgc(and subsequent VT) in your area.

2) Regarding a noticeable difference from .53 to .42, I would say yes, it would be noticeable. More or may not be enough to make or break it for you personally, but there would be a noticeable difference at that number.

3) The better performing products on the market will actually achieve the same or better u-factor without any sacrifice in shgc and vt, so in this case you CAN have your cake and eat it too.

Its all a trade off in the end.

Will the room be a dungeon, no. Will it be darker than clear glass and will triple be darker than double...yes.

Most folks that have aversions to triple don't even really notice it after a week or so.

+1.
To clarify:

-I was referring to the more aggressive double pane low-e products like 366 glass where you may get a bump of a point or two in u-factor but a major drop off in shgc and vt. This is not a worthy trade-off, as its really not a trade off at all. It will be detrimental in most cases.

-On triple pane vs double pane, it is certainly a more legitimate consideration. If you can go from a u factor near .30 down to the lower .20's, that light and shgc may be worth the sacrifice, and in fact in most cases it is, particularly in colder climates.

Thank you both for your responses.

HomeSealed_WI, just last night a sales rep showed me a product with the 366 glass. The demo was pretty impressive, but I noticed the glass looked quite shaded to me; the VLT was 0.5 with a SHGC of 0.21. Based upon the information you and windowsonwashington provided, I cannot imagine that I would be very happy with a VLT of 0.4...but maybe it is just a matter of giving it time.

At any rate, I would prefer to "have my cake and eat it too" with both a good U factor and bright rooms; any suggestions of great windows that might fit the bill?

Low-e 366 is not needed for your climate region.

You should be looking at Low-e 272 in a double pane and a combination of Low-e 272 - 180 or Low-e 180 - 180 in a triple pane application.

Windowsonwashington, thank you. Should these glass options be available with any window brand?

Any reputable brand will have glass packages available to suit every climate region that they serve. As WoW stated, 366 is most often not necessary, and to take that a step further, it in fact can actually be detrimental to overall efficiency in many circumstances that are not warm climate regions (where most energy dollars are expended on heating as opposed to cooling, which is most of the country). It is a feature frequently used as a sales tool by inefficient (read: low quality) products to get a better U factor , as that is about as deep as most consumers (and Energy Star) dig in their research.
What are the products that you are considering at this point? You can get a pretty objective opinion here by simply providing option A, B , and C.

+1 to HomeSealed's feedback.

As a quick sidebar, the glass combos that I reference are from Cardinal. Cardinal is the largest and probably most advanced maker of residential glass in the country.

That said, there are other glass packages that are close in performance and specification.

Get some quotes and we will help you sort through them.

What I'm finding is that the window guys who come to assess our home have no idea what I'm talking about when I bring up glass options (other than double vs. triple pane). A few days ago, it was a rep from Sunrise/Solace and right now (he's currently measuring with my husband) it's someone regarding Okna Starmark...both of which seem to have pretty nice products.

Sunrise/Solace has low-e 366 (double glazed, U-factor 0.28, SHGC 0.21, VT 0.50).
Okna Starmark offers Dual Glazed XR5 (U-factor 0.25, SHGC 0.28, VT 0.50).

They both get a deer in the headlights look when I inquire about other glass options. Does this mean these companies just don't offer the other types of glass you've mentioned? Or that the reps are not aware that other options can be requested?

loriafopiano,

One thing to keep in mind, if you are not already aware of it, is that VLT is calculated using the entire window opening and not just the glass.

In other words, two different windows with the identical glass system in place can have significantly different VLT rating depending on the width of the frame/sash, on whether the window has muntin bars or grills, etc.

Conversely, it's entirely possible to have a higher VLT rating on a window using a darker coating with a larger glass area versus a window with a lighter coating but less glass area.

Just to compare options, here are a few glass-only numbers using Cardinal glass and coatings:

clear - U .455, SHGC .778, VLT 82%
366 - U .242, SHGC .273, VLT 66%
272 - U .250, SHCG .414, VLT 72%
180(3) U .260, SHGC .686, VLT 80%
I89(3) U .291, SHGC .746, VLT 80%
I89(4) U .338, SHGC .704, VLT 80%

All numbers calculated using LBNL windows 7.2.
Numbers based on IG units using 3.0mm glass and 1/2" airspace with 90% argon fill.
Surface 2 coated unless otherwise specified.

(coatings - "one" 80 and "eye" 89)

This post was edited by oberon on Sat, Nov 15, 14 at 8:48

Sunrise makes a nice window, but that Starmark will be the superior option for you. If you were to put the same glass in each of those windows, the Okna would add another .02 or so to the difference. Better glass for your situation, better window.

+1

In that comparison, the glass specified in the Okna window is the better match for your climate given the higher SHGC on the South facing windows.

The comparable glass in the Sunrise window would be the Low-e 272.

I still think that triple pane is the way to go for you either way. Get triple pane quotes and go from there.