Passive House Windows: U-values (U-factors) / R-values
For example, a window that has a thermal transmission measurement of 0.20 for its U-value has an R-value of 5. In the U.S. these values are expressed in Imperial units of BTU/(h °F ft²), whereas throughout most of the rest of the world, they are expressed in metric or SI units of W/(m²K). The conversion from Imperical (BI) units to metric (SI) units can be achieved by multiplying the Imperial units number by 5.678263 (or more simply 5.7).
European manufacturers generally provide a more precise and stringent rating--which makes it more difficult to directly compare energy efficiency ratings to those manufactured on this continent. Passivhaus' PHPP modeling software takes into account a host of these individual performance specifications, which are much more easily obtainable from abroad.
Obtaining this information from manufacturers in the U.S. and Canada can even be more difficult as these manufacturers don't typically publish, provide, or (in the case of Loewen) even know what those ratings are!
From a insulation performance standpoint, of course, higher R-values (lower U-values) are preferred and it is generally accepted that windows need to have an R-values of 6-7 or better (U-values <0.16) to truly be worthy of Passive House construction. Obtaining windows which such stellar insulation performance in the U.S. has been more difficult and expensive. I believe this is one of the handicaps builders here currently have to contend with.
To achieve such high thermal performance, passive house windows are often of triple pane design comprised of special low emissivity (low-E) metal-oxide coatings, again something that is challenging to find from U.S. manufacturers.
Beyond the number of glass panes, the spacings between them are typically filled with a noble gas such as Argon or Krypton--which have superior insulating performance relative to air of 150% and 300%, respecitively.
Furthermore, the actual spacing distances (gaseous volume) between each individual pane can also have an impact on insulating performance as well as the composition of the spacers between the window panes.
It's important to consider the insulating performance of the overall window--which includes its frame or casing and its other components. Many window manufacturers specify center of glass (CoG) values. What is more important is how the window performs as a whole. Unusual as it may sound, the insulating performance of the glass is generally superior to that of the frame, which is why these manufacturers tend to specify performance in terms of center-of-glass (CoG).
Window frames come in a variety of materials including wood, fiberglass, and vinyl and typically include insulating materials such as foam or (in Pazen's ENERsign windows, cork). Each type of material and how it is constructed will have an impact on window energy performance. So, be sure to consider the frame material in your selection process as well.
Vinyl tends to be the least expensive. Fiberglass--often used in high-performance windows manufactured in Canada--provides good insulating performance. Not being as flexible as other materials, though, can make for more difficult manufacturing of windows that have radiuses or curves. Wooden frames are often found on the most expensive windows. To protect the wood from the elements, the frame is generally cladded on the exterior with a protective material, such as aluminum.
The type of window also has an impact on thermal insulating performance and how the glass panes are set within the frame. Popular window configurations are operable double hung, casement (and awning), horizontal slide, (and abroad tilt-and-turn) types as well as the non-operable picture type. Direct setting of the glass panes into the frame will improve thermal insulating performance relative to a conventional setting with the windows frame.