Thank you.

Good windows usually have air infiltration ratings between .01 and .06. anything over .1 is just too high in my view.

Air leakage has nothing to do with Ufactor and thats why so many pro's encourage homeowners to delve into air infiltration and structural data when evaluating a window. Ufactor does measure thermal efficiency but the benefit of energy efficient glass can be mitigated by air infiltration. A window with not only a low ufactor but a low air infiltration rating will support your expectations when purchasing new windows or doors.

wood windows are notorious for air infiltration since wood is prone to drying out and warping.

Thank you. Like I said a dumb question. Did not realize that the U value related to the glass and not the entire package.

Uvalue relates to the glass and the frame. Your question is a good one, its not stupid.

The windows overall ufactor is important due to it being able to prevent energy transfer. This can easily be shown with a heat lamp demonstration. Air infiltration is a liitle different and can wreak havoc on a window with the lowest ufactor in the world. A an airtight window should always be part of the equation.

Good advice above for the most part, although the one important point requiring clarification is that the U value is NOT just for the glass. The NFRC sticker on every window measures total unit u value, meaning the fully assembled product. There is a measure of u value for glass only called COG (center of glass), however that rating is of little relevance to consumers, and is commonly used by unscrupulous window companies to falsely make their product look better than it really is.

As the others mentioned however, air infiltration is a pretty important consideration for obvious reasons.