Institutional building design teams in the U.S. often look to Europe for leading-edge fenestration technology, targeting reduced thermal transmittance, expressed in the U.S. as overall window U-Factor (BTU/hr.sqft.°F), or sometimes its reciprocal, the more familiar R-Value. U-Factor includes the effects of both glass and frame.
HIGH-PERFORMANCE BUILDINGS
When approaching near-zero energy building (ZEB) performance, like that achieved on the National Renewable Energy Laboratory's Research Support Facility 
in Golden, Colorado, it can be inadvisable to rely on traditional paradigms regarding system interaction. Significantly reducing solar heat gain, waste heat from artificial lighting and fans, internal loads and mechanical system inefficiencies can lead designers to consider complementary reductions in conductive heat flow through fenestration.
In most commercial buildings, cooling loads dominate. Hence, in typical commercial construction, triple glazing is often deemed unnecessary and U-Factor is considered a secondary design parameter. Daytime conductive heat loss through windows can even be beneficial during "swing seasons" in some climates.
However, for ZEBs in some climates and occupancies, changes in U-Factor can have a greater percentage impact on overall energy consumption. Since the overall building load profile is much lower to begin with, even small changes in heating load due to U-Factor reduction can be deemed significant.
FRAMING DESIGN
In European markets, window frame U-Factors often are reported separately, placing a higher market emphasis on design features that reduce heat transfer through framing areas. With the heavier sightlines generally evidenced by European windows and curtainwall, effects of frame area performance are intensified.
Framing design idiosyncrasies aside, thermal performance expectations in Europe can be more stringent for other reasons. Northern European energy costs are much higher than in the U.S., recognition of associated environmental burdens is more widespread, and longer payback time on incremental investment in energy efficiency may be accepted.
Other general characteristics of the European market include project-in and tilt-turn operation, exterior operable shading, and a supply chain structure employing local fabricators of operable windows.
Innovative composite framing design creates an AAMA AW100 Class architectural window series with best-in-class R6 fixed and R5 operable performance. No-compromise product selection is made possible by a unique combination of 45% aluminum extrusions and 55% engineered polymers (by volume).
Glass options include spectrally selective low-e, double- or triple-insulating glass; Viracon RoomSide™ low-e where condensation is not a concern; and integral between-glass blinds with 5/8" aluminum slats. Other features include:
- 3-11/16" frame depth
- 44mm polyamide thermal barrier and foam cavity fillers
- Engineered polymers and aluminum extrusions used where their inherent material properties are best suited
- Narrow sightlines and flush-face, convection-baffled, operable vents
- Fixed; in- or out-swing casement; top-hinged, hopper or awning vents
- 15 psf water resistance; 0.10 cfm/sqft air infiltration
- 1" exterior glass offset to complement curtainwall and storefront systems
- Heavy butt hinges or concealed, stainless steel, four-bar friction hinges to carry triple glazing with ease
- Dual-color frame finishes
