The U Value for triple pane is about .003 more than double glass.
Most important benefits are the usual. Quality construction, 7/8" spaced double strenght (1/8") glass withe low E and full argon gas fill. Kryton optional.

Warm edge spacer technology which separates the two pieces of glass is also much better than any metal spacers. It protects you better from mold and mildew.

And finally the installation. This can make or break the windows lifetime warranty. It says it right in the exclusion section.

So make sure their installed by the best.

All the best,

the PorchGuy

Here is a link that might be useful: My Album

Good morning,

Much depends on where you live. In most parts of the country a double pane with LowE coating will be more than adequate for your needs. But, in some parts of the country the triple will offer some advantages in energy performance that would make the triples more appealing despite the (normally) additional cost.

It really depends a lot on your requirements.

First, a dual pane window with a LowE 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 LowE coating.

As a very general consideration, a "typical" dual pane with LowE and argon should have a U factor at about .28 - .38 and a "typical" triple pane with two lites LowE coated and argon fill will end up somewhere between .18 to .28 - again, those are general numbers and you would need to look at the performance numbers of the windows that you are considering.

A triple with krypton fill will normally have slightly better U-factor numbers than will an argon-filled triple, but it will have a correspondingly higher (or more than correspondingly) cost included.

Are better U-factors worth the additional cost?

Well consider:

R-value as given is used to define the thermal resistance of a material while U-factor measures thermal transference thru a material.

The formula for computing U-value is: Btu / (hr x degrees F x sqft)

The formula for computing R-value is: (hr x degrees F x sqft) / Btu

The formula for computing heat transfer is: (BTU/hr) = (area / R) * ÃT

ÃT = the difference between what is the temperature on one side of the window and what is the temperature on the other side of the window (and that "A" thing is supposed to be the Greek letter Delta. Since the site won't accept the letter Delta, we have that "A" thing).

Consider a comparison of two windows one having a U .35 and the other U .19 (I picked these number for illustration and because they are not "untypical" U-factors for dual and triple pane windows). For computation, it is necessary to convert the U factor numbers to R value - U .19 = R 2.86 and U .35 = R 5.26.

And if we compare the energy performance numbers on a single 10 square foot window we get:

If ÃT = 10°, then (10 sqft / R 2.86) = 3.5 * 10° = 35 BTU/hr
If ÃT = 10°, then (10 sqft / R 5.26) = 1.9 * 10° = 19 BTU/hr

Notice the relationship between BTU/hr and U factor in this first example?

Not really that much difference; but, if we consider a more robust temperature differential:

If ÃT = 40°, then (10 sqft / R 2.86) * 40° = 140 BTU/hr
If ÃT = 40°, then (10 sqft / R 5.26) * 40° = 76 BTU/hr

Again, notice the relationship between BTU/hr and U factor? Using a 10 sqft window makes the rest of the calculations really easy because of R = 1/U and U = 1/R...and I can do these in my head (certain level of laziness involved here).

And finally, if we happen to have a mind-numbing ÃT of 100° (which does happen where I live), then:

(10 sqft / R 2.86) * 100° = 350 BTU/hr
(10 sqft / R 5.26) * 100° = 190 BTU/hr

And remember that is from a single, small window. Imagine a whole house full of glass...

And as Porchguy said, and I agree 100%, installation is vital - the windows MUST be installed correctly to get the maximum performance benefit.

Thanks. I'm so glad I was a math major. So my worst case temperature differential will probably around 80° and average around 60° during the winter months. And I have lots and lots of windows.

It's looking like the price diff will be worth it even with a .23 window.

and you always wondered where you would use all that math in the real world... ;-)

The technical information posted by Oberon is excellent. One error that should be corrected, in order to avoid future confusion, is noted below.

"...convert the U factor numbers to R value - U .19 = R 2.86 and U .35 = R 5.26."

Correction: "U .35 = R 2.86 and U .19 = R 5.26"

In response to additional information requested in the initial post, the U-value numbers stated for Pella vary greatly. It appears that the spectrum of U-values .28-.87 shown for the Architect Series product may include a triple-pane low-E option through to a clear single-pane option within the same window assembly. My experience with both Pella and Eagle (as well as many other wood window manufacturers) has shown that their U-values are very comparable when similar glass construction is used.

The former's U-values might show a slightly better performance rating based on the use of roll-formed aluminum clad sashes instead of extruded aluminum sashes, due to the heavier gauged metal being more conductive. Otherwise, it may be only fine/minor details that separate the two manufacturers shown. I believe that both even use the same glass manufacturer.

In regards to the triple-pane option, a consumer should evaluate the type of construction utilized in the triple-pane insulating glass unit. Some companies will offer removable interior storm panels and refer to them as "triple-pane". There is a benefit to this design, as will be explained later. Another approach to triple-pane IG is to minimize the spacing between the glass panes. This method diminishes the optimum U-value that can be achieved through the use of an adequate air/gas space (i.e. 7/16"-5/8"). Both of these triple-pane options have been designed to allow window manufacturers to fit the wider IG units into "standard" 1.5"-1.75" thick sashes.

A disadvantage to utilizing triple-pane glass in fenestration products is the added weight of the glass. It is our opinion that hardware changes may also be required when utilizing heavier glass options in windows and doors. In casement types, this would include heavier or stronger hinges. In double hung types, this would include properly matched weights and pulleys. Hinged doors might include commercially rated ball-bearing hinges, and/or a greater number of hinges than a dual-pane unit.

Another disadvantage to a triple-pane IG product is the multiple seals. When selected by a consumer, the triple-pane option doubles their potential for seal failures. The consumer should ensure triple-pane warranties match the standard dual-pane offerings. It is our opinion that dealing with seal failures is a nuisance, regardless of warranties. This is likely the only advantage to a removable storm panel type of "triple-pane" option, since it is simply a dual-pane IG unit with an additional pane of glass placed to the interior side and utilizing some type of rubber gasket as a dust/bug "shield". However, unless a hard-coat low-E product is placed on the removable storm panel, the energy advantages are minimized.

Other aspects or information to consider in the purchase of windows or doors have not been requested, but should be noted. Air infiltration ratings are another big consideration that should be made in conjuction with U-values. Additionally, depending on the location of construction, solar heat gain characteristics found in different glass products can prove beneficial when incorporated properly into a building structure's design. Lastly, product durability should be a priority. With the mention of "cold freeze/thaw", there will be a lot of differing movements (expansions/contractions) between the dissimilar materials used within the complete window or door assembly, so the methods of product assembly/construction will be very important.

mcsbldr,

OOPS! Thanks, good catch!

It's so frustrating when dealing with sales people. I asked both Eagle and Pella to provide me with quotes on triple pane (Eagle is just an an aluminum extruded pane to slip in the casement). They both asked me why.

I asked Pella to provide an additional quote for the third pane on the big windows to have a coating of the Low E. Again he asked why. And then was told "well you'll darken your windows."

I'm thinking that I'll be better off tossing a coin and going with that.

Why is this so hard?

Well if anyone cares, here's what I went with. I really just had to toss a coin, because there were things that I liked and disliked about each.

Pella Designer - double pane with the added third pane. On the big windows in the living room, I am having the Low E coating added to the third pane.

The windows that do open are actually fairly small, so I ignored the weight issue.

With the addition of one piece of glass to make the triple pane it will not decrease the light variance by enough for you to notice.

I was going to say that with a $3000 price difference I would maybe start looking at things like customer service, and warranty over a small function for the windows themselves. If a windows cost that much then you want to make sure your sales rep is going to be there for you and that the company itself is going to cover anything that might go wrong. and I know Pella is great for making sure the customer is satisfied.