Ask Andersen or Pella, they may be able to explain it.

If not, visit the energy collaborative website.

Michael

Here is a link that might be useful: efficient windows

Heat travels in three ways: conduction, radiation, and convection. All three of these factors affect your comfort level and all three of them affect, and are affected by, the performance of your windows.

Conduction is the spontaneous transfer of thermal energy through matter.

One really good measure of conduction is to take an aluminum bar and hold one end in your bare hand and one end in an open flame. You wont be able to hold on to the bar very long  that is heat conduction. When you insulate your home and you use insulating materials that have a certain R-value, you are using materials with a calculated "resistance" to heat flow.

R-value measures the resistance to conduction of heat thru a material.

Radiation or, radiant heat, is what you feel when the sun warms you as you are lying on the beach (except when you gain warmth from the sand around you, that would be conduction, again).

Radiant heat is what you feel when you stand close to (but not touching) a steam radiator or a radiant heater. Radiant heaters use infrared radiation (IR) to warm you (or other objects), and IR is also the heat you feel directly from the sun.

Convection heat is warm air movement such as a forced air heating system. A convection current is set up as the warm air moves and is replaced by cooler air that is then warmed and the cycle then continues. There is no specific measure of convection current in NFRC ratings, but convection currents can have a significant effect on comfort in your home dependent on window performance.

As mentioned, R-value measures a materialÂs resistance to heat flow whereas U-value (or U-factor) measures the actual heat flow thru a material. It doesnÂt sound like a big difference, but there can be significant differences when dealing with the overall heat loss of an object that has different materials used in its construction and when different avenues of heat loss are available.

This is why window performance should always be measured and rated in U-value (or U-factor) and not R-value.

NFRC has U-value measurements of different window systems.
U-factor rating is going to be between 0.10 and 1.20, the lower the U-value, the greater a window's resistance to heat flow and the better its insulating value. While there are different requirements for different parts of the country (Energy Star, for example), the lower the U-value, the better. Below .33 is a minimum requirement.

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight.

SHGC is the actual measurement of solar radiation, or IR energy, or solar heat (pretty much all the same thing in this example), that passes thru a window either directly transmitted or absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.

Again, there are different requirements for different areas of the country. If you live north then a high SHGC may be an advantage where your primary concern is heating your home. If you lie south then the lower the SHGC number, the better because you want to keep heat outside.

If one is considering Air Leakage, AL is a rating of the "equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft)". The lower the AL, the less air will pass through openings in the window assembly. Simple enough  low number good, high number bad.

I am also including Condensation Resistance (CR), a measurement where the higher numbers are better than the lower numbers. CR measures the ability of a product to resist the formation of condensation on the interior surface of the product.

This is an optional field on the NFRC rating sticker and many manufacturers don't include it.

The CR will not say whether you will see condensation on your windows or not, rather it is a comparison between windows on their "helps prevent condensation" performance. There are several factors involved in condensation, and that is why no one who understands the process will ever say that you will never get condensation on your windows.

Better windows may significantly reduce condensation but, if there is enough moisture in a home, and there are objects that are cool enough to be below the dew point  you will get condensation.

Next is VT or Visible Transmittance. VT measures how much light gets thru the glass. This is also a rating between 0 and 1 and like CR the higher the number the better  usually. The higher the number, the more light the unit passes  but VT also takes into account the percentage of glass versus the percentage of frame in the window as well  in other words, a window with a wide frame/sash and a VT of .5 might actually allow more light thru the glass than will a window with a narrow frame/sash and a VT of .6.

A typical piece of clear annealed glass has a VT of .93 or so. Meaning 93% of the light that hits the glass passes thru. Add a second lite and VT goes down to about 88% or so. These are general figures because different types of glass can vary.

Add a tint and VT goes down more. BUT, now your eye comes into play because we see different colors differently.

Simply put, a measurement of one tint may reveal less actual VT than another tint, but our eyes donÂt see it that way. We may think we see much more light thru the tint that actually allows less light thru it  simple right?

Anyway, bottom line is that in general, and taking into account the previous factors, the higher the VT the more light that gets thru the window.

Crash course  feel free to ask specific questions.

The NFRC is a nonprofit organization that has developed a window performance energy rating system. Prior to the NFRC there were numerous ways to measure and report a window's performance, and some by the NFRC's standards were greatly exaggerated. The NFRC brought uniformity to the industry and provides the definitive standard by which all window companies must now measure a window's energy performance. They're great! For energy performance ratings they currently report a window's U Value and Solar Heat Gain Coefficient (SHGC). The ratings are tricky to read for some because each manufacturer could report dozens of different values because they offer more than one glass option. So if a manufacturer offers for example 22 different possible glass and spacer options, you might have to sift through 22 different U Values and SHGC values.

The NFRC also publishes values for air infiltration, visible light transmission, and condensation resistance. However, those values are not always shown at the current time because they are optional, not mandatory, to report.

To me the two most important variables to look at are U Value and air infiltration (or their DP rating, which is an air/water/structural rating similar to but different than air infiltration). U Value will indicate how good of an insulator the window is, and air infiltration will indicate how drafty the window is.

Since you're comparing Pella and Andersen, two well-respected companies, what you'll essentially find is that they're pretty similar in performance, providing you select similar features in both. An area where they will differ is in the bells and whistles: type of cladding, aesthetics, additional glass options, hardware & other accessories, warranty, etc. Good luck with your research!

Thank you so much for the answers to my post. Because they were so detailed and informative, I feel more confident asking questions and understanding answers from the window people. My research will be much easier. Thanks again.

Oberon did you mean to say that .35 or below is the minimum U-Value requirement? You stated "below .33 is a minimum requirement." Knowing somewhat the way you think (although we've never knowingly met - yet) I'm guessing you meant the former and not the latter. Many double glazed Low E windows with argon gas meet the Energy Star .35 requirement but not necessarily a .32.

Oops! Ya, I meant .35 - I was typing faster than my brain was thinking (I have to reread better!). Thanks!

Of course in my defense, below .33 would be better! ;-)

oberon - Your post was generously informative, well written and extremely helpful! Wow, and thank you!

Well, thank you!