I agree with Mike in that it really depends on your requirements. There are a few differences that I will discuss - if you have a minute (or several).

A dual pane window with a LowE2 coating and argon gas will outperform - energy considerations - a triple pane window made with clear glass.

The real advantage of a triple pane is that since there is that extra glass layer the manufacturer has the ability to coat two different surfaces with the LowE2 coating.

As a general rule, triple panes come in two varieties - one version has a relatively narrow space between the lites - kind of like taking a wide dual pane and dropping another lite between the first two.

Then there is the triple pane with a wider spacing between the lites - kind of like taking two "standard" dual panes and removing a lite from one and slapping the remaining parts together.

Both work, but there are some differences worth considering.

The narrow airspace version works best when two surfaces are LowE2 coated and krypton gas is used between the lites. Krypton gas performs at its energy-saving best in a narrow space of about 1/4" or so...which happens to be the typical space between the lites in a narrow triple pane.

In this configuration it is quite possible to get an R-10 on the window. But, this is a center-of-glass measurement and is not necessarily indicative of window performance as a whole. Still, this configuration is very energy efficient and works really well...the downside is that this version can be expensive.

The wider triple pane version would generally have an airspace of about 7/16" between each lite. Again, the advantage is in the LowE2 coating on two separate lites. In this case, argon gas would be the most cost effective fill - rather than krypton. Argon is cheap and plentiful and at the 7/16" spacing is almost as good as krypton in performance numbers - not quite as good - but the slight insulating advantage of krypton in this configuration doesn't offset the additional cost of krypton.

The biggest disadvantage of the wider triple pane is the physical size of the IGU - or more precisely the width of the glass package.

While manufacturers who use this version build their sash to accommodate the IGU width, not all companies can do so, so not all companies offer a triple pane package because of both the perception and the actuality of the unit width.

The wider version of the triple pane (argon fill) may be a bit lower than the narrower version (krypton fill) in center-of-glass R-value at about 8.5 or so. But, as a generalization, the wider version may outperform the narrower version if both use argon or air infill between the lites.

And as a comparison, a dual pane with LowE2 and argon will get to about an R-6 center-of-glass.

One interesting downside to the increased energy efficiency of the triple pane is the likelihood of increased EXTERIOR window condensation on cool mornings.

Often, people who replace energy deficient single pane (or dual pane) with more efficient LowE coated windows become concerned that their windows are having "problems" with exterior condensation on cool mornings.

This is actually quite normal and indicates that the windows are performing exactly as they are supposed to do. Well, by increasing the energy-blocking performance of the window it is quite possible to increase the likelihood of exterior condensation. Not a problem – in the sense of a window flaw – but some people do find it objectionable.

Either way, a little sunlight "fixes" that problem pretty quickly. Although in certain areas of the country it is actually possible to have frost or ice formation on the exterior of super-efficient windows in the winter time – which beats having it on the interior I would venture to suggest!

Ultimately, the best window in the world won’t do you a bit of good if the installation is flawed…so once you decide on a particular window; make sure that the installers know their business!

I'm having a hard time getting these guys to give me even a price on triples.

Solution:
Energy Star,Northern Zone windows will be required to have a U rating of .22 by Jan 1, 2010.

Wait till 2010.

I'm giving you a link to a page that addresses some of it but also will give you other resources, like NFRC, that will help.

I'm looking to replace about 6 old single-pane windows, they are roughly 2 feet x 3 feet. The remaining windows were all replaced w/ double-pane by the previous owner. There are 4 old leaded-glass windows with storm windows, which we will not mess with. This is an old house w/ wood siding, lath/plaster walls, and no added insulation. I currently have no plans to insulate the walls, as I can't see how the expense will pay for itself in 10 or even 15 years. We are in Portland OR, so not the Sunbelt but not the Snowbelt either.

I just had a pitch from Penguin Windows, whose triple-pane low-E krypton-filled window with insulated frame certainly seems impressive. But the are not cheap - bid comes to appx $1,200 per window, not including any federal tax credit.

So, here is my thinking. Say the exterior walls of the house are appx 3,500 sq ft and the roof another 1,500 sq ft, total 5,000 sq ft exterior surface. Each window is let's say 6 sq ft. So the six windows in question are about 0.7% of the exterior surface.

I think a typical old lath-and-plaster wall w/ wood siding and no insulation is about R=5. Suppose a good double pane argon-filled low-E window may be R=6, and the best triple pane krypton-filled low-E window is R=9.

It was a typical winter day for us - exterior temp about 37F and windy. I used the Penguin guy's infrared temperature sensor to check the temperature on the inside of various places. Exterior wall was 66F. Double pane window was 65-66F. Single-pane window was 55F (these are the ones we'll be replacing). Single pane leaded-glass window with storm window was 58-59F.

So, I concluded that my existing double pane windows are insulating roughly as well as the exterior wall itself.

And I also concluded that replacing 0.7% of the exterior surface with six R=9 windows rather than six R=6 windows will make essentially no difference to the heat loss from the house.

So - final conclusion - there is no reason for me to spend $1,200 per triple pane window from Penguin or anyone else. I should look for good double pane windows for around $500-600 per.

Does this all make sense? What am I missing?