I am a homeowner, not a pro.

It is USELESS getting only the U value WITHOUT knowing the Energy Star zones of our homes. THe following document STRAIGHT from the government site shows you, by zones, what the minimum current and proposed Energy Star requirements are:

http://www.energystar.gov/ia/partners/prod_development/archives/downloads/windows_doors/2ndCriteriaRevisionProcessAnnouncement.pdf

The document also shows the proposed changes to the Energy Star regions so it is best to locate your home in this new zoning map to see the "new" proposed requirements. This can act as a baseline to firm up your decision.

Another fairly independent website that does not sell any products is:

http://www.efficientwindows.org/gasfills.cfm

This site is a good starting point to understanding window technologies and terms commonly used by the window business folks.

To give you a baseline perspective, I will share our experience with our U=0.34 windows. We have lived with these windows and French doors in the current Energy Star South-Central zone (new proposed Energy Star Zone ES2) for five years. Our local extremes are 105-F in the summer with direct high angle sun through our windows and a rare 29-F in the winter. We often receive strong wind up to 40-MPH gusts with driving rain when it is around 40-F. We do get more sun year round even when there is low temperature. The sun heats the interior of our house in the winter. So far, no condensation, no draft and no curtains needed to block our views. Our A/C easily maintains a comfortable 75-F inside while it is burning hot outside. Our radiant heat system easily warms the house to 70-F when it is frosty outside. As it is, we are quite happy with these windows and doors in our local climate conditions.

In New England, it will be more humid, not as hot, but a lot colder and a lot nastier wind and rain. You should find out from the government doc above the appropriate ES zone for your house and then find an appropriate starting U factor.

As for Argon gas and Krypton gas sealing and performance over time, the following is a paper from Sweden where they did research on how long the gas will remain in the windows and the leak rate over time.

http://www.byv.kth.se/avd/byte/reykjavik/pdf/art_155.pdf

The following is Kansas State's engineering dept post and NOTE THE PART ABOUT Argon leak rate. It can leak but very slow!
http://www.engext.ksu.edu/henergy/envelope/windoors.asp

Hopefully, these INDEPENDENT sources of more reliable information can help you solidify your own position regarding whether to go fro triple pane with Krypton fill.

Thank you so much for your responses. My gut instinct is to follow through with the triple pane. I was already aware of the DOE proposed new criteia, that's what really started my search for triple pane. If I bought the Energy Star U .35 now, it would not be Energy star rated in 2012. If I go with the the .20 now, I will know that my windows are still energy star rated up to 2015 and beyond. I guess when those contractors who thought I'm crazy now are having to install U .20 windows to meet energy efficiency in 6 years will look back and say," Those people were really on the ball." The cost diff is not that much higher for the money i will save in the long run as we plan to stay in this house for the long haul. My friends are installing windows in there home and have decided to go with dbl pane argon. They also think we are silly for spending the extra money. So i guess time will tell for comparison.

In regards to your contractors questioning your window decisions, keep in mind that a 2x4 wall with R-15 fiberglass insulation will yield a R-value of approximately R-11 total value, while a typical "modern" window yields a R-3. Thus, considering the use of triple-pane glass is not a poor choice, based on the fact that such a window could achieve a R-5. This is still far from R-11, but helps you achieve a better balance within your overall wall assembly.

In our opinion, the biggest downside to triple-pane glass is that the exposure to seal failure has doubled, so one should ensure they receive the best warranty available; we've seen up to 20 years and non-prorated for 3-pane. Other negatives include the expense, the increased distortion and decreased visible daylight.

In regards to the comment above about Energy Star guidelines, we review them with the same skepticism as other energy conservation issues. There is a short and long term assessment that must be considered. I fear that the DOE is only assessing the short term benefits. It's similar to reviewing the best fuel economy in an automobile. A light-weight plastic car will achieve excellent fuel economy, but you'll likely be purchasing a few of them for every sturdy steel car you buy. After a few years and the repurchasing of disposable plastic products, where are the cost savings? There certainly aren't any savings to our environment.

Buying Energy Star rated window that meets anything but current standards is not a requirement. It really makes no difference whether the windows are rated to the 2015 standard or not. The difference in saving in Hartford Ct. between a double pane with an overall U-factor of .29 with a SHGC of .40 vs a triple pane with a U-.18 with a SHGC of .40 is about $100. per year based on a 2000 sq. ft. house.

What's more important is purchasing a good quality window from a reputable manufacturer and getting a quality installation.

I agree with Skydawggy that getting a good quality unit combined with a professional quality install is at the top of the list.

Obviously there is no question that you want the most efficient window in the more efficient wall, and your contractor is correct when he told you that better window energy numbers do result in better performance in the more efficient wall.

However, that certainly does not mean that you want a less efficient window only because you have a 2x4 rather than 2x6 wall to start with. While a good U-.32 window, properly installed, will improve your overall comfort and the energy performance of your home versus the windows you have in place, a more efficient window will improve overall wall performance even more so - the question does the additional improvement justify the additional expense?

A good dual pane IG with the right LowE coating may very well meet all of your needs at (potentially substantially) less cost than higher performance triple. If you live in northern Maine or northern Minnesota, I would agree that the triple may well be worth the additional cost. But in Connecticut I would definitely consider the dual pane for the best overall value.

Consider that the U-.2 triples will meet 2015 energy star requirements - but does the rest of your home meet 2015 energy star requirements? Basically, meeting a future energy star level is a non-issue. You simply want the best performance for the dollars that you are planning to spend.

So, to add a few numbers:

Imagine a single wall in a home that is 100sqft (for simplicity). In this example we will assume that this wall has an overall R-value of 10 (typical 2x4 construction)including the effect of studs, outlets, etc (but not windows), and just say that this wall averages R-10 across its total surface.

If we add a 5x 2Â (10sqft) window to this wall and this window has U factor of .32.

Wall = R10 or U.1
Window = U.32
Window = 10% of wall
Wall = 90sqft, window = 10 sqft, overall 100 sqft
Wall = 90 sqft x U.1 = 9
Window = 10 sqft x U.32 = 3.2
9 + 3.2 = 12.2/100sqft = U.122 or overall wall R value of 8.2.

Now if we change to a window with a U.20 Â

Wall = R10 or U.1
Window = U.20
Window = 10% of wall
Wall = 90 sqft, window = 10 sqft, overall 100 sqft
Wall = 90 sqft x U.1 = 9
Window = 10 sqft x U.20 = 2.0
9 + 2.0 = 11/100sqft = U.11 or overall R value of 9.1.

So, changing from U.32 to U.20, in this example, results in an improvement of 63% in window performance, but it also results in an improvement of 11% in the entire wall structure  the higher the percentage of window sqft in the wall; the more the overall R-value improvement if using the lower U-factor window  in this example assuming 10% window coverage in the wall.

If your wall is 15% of the total wall area then the improvement to entire wall energy performance will be increased proportionally.

By way of comparison to a 2x6 wall (per your contractor)- which I am going to say is R-20 because the math is really easy (being lazy here - but it is close enough for this illustration):

So if we change to an R-20 wall we have:

Wall = R20 or U.05
Window = U.32
Window = 10% of wall
Wall = 90sqft, window = 10 sqft, overall 100 sqft
Wall = 90 sqft x U.05 = 4.5
Window = 10 sqft x U.32 = 3.2
4.5 + 3.2 = 7.7/100sqft = U.077 or overall wall R value of 13.

Now if we change to a window with a U-.20:

Wall = R20 or U.05
Window = U.20
Window = 10% of wall
Wall = 90 sqft, window = 10 sqft, overall 100 sqft
Wall = 90 sqft x U.05 = 4.5
Window = 10 sqft x U.20 = 2.0
4.5 + 2.0 = 6.5/100sqft = U.065 or overall R value of 15.4.

Or a potential 18.5% improvement to overall wall performance with the higher efficiency window in the higher efficiency wall (assuming I did all the math right..it is before breakfast on a monday morning...).

Not sure where you are located but there are triple pane units with a U of .15 with a fantastic warranty. Manufactured just outside of Philadelphia. Go to www.nfrc.org look under okna windows 900 series.

i sell what i believe are the best windows you can buy (sorry, but it's not appropriate for me to mention the name here) and my understanding is that argon, not krypton, is the way to go.

what i'd recommend is triple-pane with an argon fill. we also offer a krypton (which is a higher density gas than argon) for those who really want it but the added performance and the very high price differential (primarily due to current krypton gas demand from china) does not make krypton the best value.

in fact the government is working on getting manufacturers (and sellers) away from krypton fill glass packs because the marketing is abused and krypton is not a good value for consumers. often krypton will be advertised but there may only be a 5% fill so they can say "krypton" or there is an argon/krypton mix.

gas leakage should be a consideration. the standard gas fill is only 75%, so opt for a higher gas fill combined with the least gas leakage so your window will continue to perform adequately in 20 years.

now i should mention a few other points; superior energy efficiency is not sufficient.

make sure your replacement window decreases your glass/viewing area as little as possible. loss of window glass is the number one complaint after new replacement windows have been installed.

compare air infiltration rates as much as R-values (and be sure to find out if the R-value is quoted for the entire window or only the glass pack (measured at the center))

the installation quality is as (or maybe more) important as the window.

if it is a superior window, it will have a lifetime non-prorated warranty - this will also increase your home equity value. (by the way, check out costvsvalue.com for percentage of cost recoup on windows)

your window should have insulation in the frame since the frame is 15% of the window. if it is hollow then that is probably not the window you want in your home. if there is insulation, high-density polyurethane (used in freezer doors) is twice as good as the less expensive styrofoam. often manufacturers will slide a styrofoam bar into the frame so they can say the frame is insulated but what good is that with the air spaces around it?

the gas fill is what everyone talks about, but the type and number of coatings are more important.

in general, pay attention to the efficiency of the glass pack, but do not focus exclusively on it; there are other parts of a window that are almost as important.

you should not have to pay more than $1400 for a superior 3x5 double-hung window; many of the big name brands are $1400 and over and do not contain all the features a lower priced window might have - i.e. cost vs performance in not directly related.

Good thread and great links!

Looking over the NFRC site, it is hard to find a "name brand" window with a U under .3!

Try Okna, Gorell, Sunrise and Softlite. These come to mind but there are many others. The other numbers consumers need to look at are air infiltration and design pressure.