Wow! You have been doing your due diligence! Yes, the way the Europeans are calculating U-Values is somewhat different than how we're doing it, but I'm getting in a little over my head here. We have a guy at Mathews Brothers that focuses on this and all our testing. He could speak to this far more intellegently than I, so I can reach out to him later today and find out. As for us, we're producing some windows with very respectable U-Values (0.20 range), but I would caution you on focusing solely on the U-Value. Oftentimes, manufacturers will sacrifice performance of the other metrics (Solar Heat Gain, Visibible Transmittance, Condensation Resistance, Structural Performance, to name a few), in order to achieve the Holy Grail of 0.20 U.
We just produced an informational pamphlet that hasn't even made it back from the printer, which explains all the performance tests. A lot depends on which part of the country you are in, orientation of windows, etc. I can send you a PDF if you'd like. Please email me at
[email protected] so I can reply privately, unless you want your email out there for the world to see.
By the way, you're obviously familiar with Quanex... we're using their(Quanex Mikron) Energy Core profiles with their (Quanex Truseal) Duralite spacer in the product line I've referenced here.