How low is too low in terms of VT for windows? 0.5? 0.35? Room with lots of views and windows which we don't want to be too dark. Please put aside questions of U-values and solar heat gain for this question, as those issues I understand better. Time to buy windows for our new construction and this is the main unknown for us. Bonus question... full window U-value I get why it makes sense. Why do they quote a full window VT value? I have no illusions that I will be able to see through the frame... If you are going to quote full window, why not say that humans are 99.5% blind since they can only see through their eyes?? :).

The real answer isn't going to be all that helpful to you, but the truth is that it all depends on you. How bright do you like your house? I a lot of people like their houses fairly dark, though personally I like mine bright. You should look though windows to the outside that you know what the VT is, so you can decide if those are to dark or light.

Personally, I wouldn't want to go under .5 for a window with a "main" view. In my next house, I am planning on using HSHG windows on the south, which is my main view, which also have higher VT (.57), but on the other sides of my house, I will use LSHG windows that have a lower (0.29) VT.

I would agree that how low is too low really depends on you.

To reply to your other question, as you noted VT is the total light thru the entire window and not just thru the glass.

Consider that you can have two identically sized windows, with identical glass packages, but differences in sash/frame size or configuration will affect the amount of light that is available thru either window.

If only the glass was considerd in VT, then there would be no way for you as a homeowner to know which window admits the most light - it is quite possible when comparing two different windows to have the window with the higher glass VT to have a lower overall VT when compared to the other window.

Thanks for the responses. We are likely going with the Serious Windows 725s and I was wondering about upgrading to the 925. You get a 0.03 pick up in U-value, and more than a 0.1 drop in VT. The whole point of building a modern house with lots of glass in an isolated location was to have nice views, have a bright house, while still having privacy. We know that financially the upgrade to the 925 does not make sense (8K+ for a calculated savings of well less than $200), but are considering it anyway for non-financial reasons. The drop in VT though is probably a deal breaker (the $ may have been as well).

As for the VT for whole windows, the whole window thing makes sense for comparing two same size windows by different vendors. It helps less when you have a whole bunch of 8'x6' windows in a room (so the frame will really not matter that much, relatively speaking)... and want to end up with a room that feels closer to being outside than in a auto show room with tinted windows :). In other words we want to err on the side off too bright and using shades then too dim.

Thanks for the responses.

If it doesn't make financial sense and you want to err on the side of too bright, then just use the 725's. If you are concerned about energy consumption, take part of the $8000 you "saved" on the 725's and put that into additional wall/ceiling insulation, or a higher efficiency heat source. Because whole frame R's are usually based on smaller (3'*4') units, on larger windows like 8' by 6' the actual whole frame R value will be a little closer to the COG measurement, because there is more window area per frame perimeter.

Have you ever visited Falling Water, the concrete house in Pennsylvania that Frank Lloyd Wright built for the Kaufman family from Pittsburgh? When he showed the Kaufmans the plans the missus reportedly protested that they could not see the beauty of the creek coming down the mountainside because the house was going to built right over it. There were no windows which provided a view of the creek. Wright's response was that if they could see the creek all the time pretty soon the beauty of it would become just part of the daily background and they would not even really see it any more. By designing the house so that they had to make an effort, like go out onto a deck, or go down some stairs that ended just above the water, to see the creek and its beauty, it would always be fresh. He designed it so one could hear, but not see, the falling water anywhere in the house.

I offer this as food for thought about how much window area you really want in your house. Don't let your view of the beauty of nature become so ordinary that you fail to really see it and appreciate it. The bright part I wholeheartedly agree with!

The reason that most companies state that measurements are "full window" is because the "full window" is what is tested. Window testing is based on one nominal size per type of window (xo, single hung, fix,etc).

Here is a link to a new piece of literature that Serious has published. The document name is SeiousWindows for Passive House. It lists all of the measurements (SHGC, U-Value, VT) in seperate catagories. They give you the full frame measurement as well as center of glass, sill, and jamb. It is actually very interesting and shows that the glass performance is much more efficient when you stop including the frame. Go figure. This should also help you decide between the 725 and 925 series.

http://index.seriouswindows.com/document-library/func-startdown/90/