Guide · Terminology
Aluminum window glossary
European window terminology, defined plainly for US projects — from thermal break and sightline to RC2, SHGC and the difference between Uw and U-factor.
Why this page exists
Two markets, two vocabularies.
European and American window trades describe the same components with different words and measure the same properties on different scales. A specification that says Uw 0.78 and one that says U-factor 0.14 can describe an identical window.
These 39 definitions cover the terms that come up most often when a European system is used on a US project — written to be quotable, not exhaustive.
8 terms
Thermal performance
- U-factor
- The rate of heat loss through a complete window, used in US energy codes and stated in BTU/(h·ft²·°F). Lower is better. It is the US counterpart to the European Uw value: divide Uw by 5.678 to convert. A Uw of 0.78 W/(m²·K) corresponds to roughly U-factor 0.14.
- Uw value
- The heat transfer coefficient of the complete window — frame, glazing and spacer combined — stated in W/(m²·K). This is the figure that describes what is actually installed, and the one that belongs in a specification.
- Uf value
- The heat transfer coefficient of the frame alone, stated in W/(m²·K). Manufacturers publish Uf per profile system, which makes systems comparable — but a good Uf does not guarantee a good Uw, because glazing and unit size also count.
- Ug value
- The heat transfer coefficient of the glazing alone, stated in W/(m²·K). Modern triple glazing reaches roughly 0.5 W/(m²·K).
- SHGC
- Solar Heat Gain Coefficient — the fraction of solar radiation admitted through a window, between 0 and 1. A lower SHGC reduces summer cooling load; a higher one is desirable where passive solar gain is wanted. Required alongside U-factor on NFRC labels.
- Thermal break
- An insulating barrier, normally polyamide, that separates the inner and outer aluminum shells of a profile so heat cannot pass straight through the metal. It is what makes aluminum windows thermally viable, and its depth largely determines the Uf value.
- Warm edge spacer
- The spacer bar that holds insulating glass panes apart at their edge, made of a low-conductivity material instead of aluminum. It reduces heat loss at the glass edge and lowers the risk of condensation on the inner pane.
- Triple glazing
- An insulating glass unit with three panes and two gas-filled cavities, standard on high-performance European windows and the basis for the lowest published Uw values.
10 terms
Window construction
- Sash
- The movable part of a window that carries the glazing and swings, tilts or slides. In German technical documents this is the Flügel.
- Outer frame
- The fixed frame anchored into the wall opening, into which the sash closes. The German term is Blendrahmen.
- Sightline
- The visible width of frame and sash seen from the front, stated in millimetres. Narrower sightlines mean more glass and a slimmer appearance. Also called face width.
- Concealed sash
- A construction in which the sash profile is hidden behind the outer frame, so only frame and glass are visible from outside. Schüco AWS 75.BS.SI+ reaches an exterior face width of 66 mm this way.
- Mullion
- A vertical structural member dividing a window or façade into fields.
- Transom
- A horizontal structural member dividing a window or façade into fields.
- Mullion-transom façade
- A curtain wall built as a grid of vertical mullions and horizontal transoms, with glass or panels infilled between them. The German term is Pfosten-Riegel-Fassade; it is the most common form of stick-built curtain wall.
- Glazing bead
- The removable profile that secures the glass in the sash or frame and can be taken out for reglazing. In German, Glasleiste.
- Glazing rebate
- The recess in the profile that receives the edge of the insulating glass unit. Its insulation materially affects the thermal performance of the finished window.
- Sealing plane
- A continuous line of gasket between sash and frame. High-performance European windows typically use two or three sealing planes, which is what produces their air-tightness ratings.
6 terms
Opening types
- Tilt and turn
- A European opening type in which one handle position swings the sash fully inward and another tilts it inward at the top for ventilation. It gives a large clear opening for cleaning and escape, and allows secure ventilation. The German term is Dreh-Kipp.
- Casement
- A sash hinged at one side that swings open. Common in both markets; European casements normally open inward, US casements normally outward.
- Fixed glazing
- A non-opening unit. Because there is no sash, fixed glazing achieves the best thermal values of any configuration in the same system and allows the largest panes.
- Lift-and-slide
- A large sliding door in which the panel is lifted slightly off its seals before it slides, then lowered to compress them when closed. This allows very heavy panels to move easily while keeping the closed unit tightly sealed. In German, Hebe-Schiebe.
- Bifold door
- A door of several panels that fold back against one another to open almost the entire width of the opening. Also called a folding glass wall; in German, Falttür or Glasfaltwand.
- Pivot door
- An entrance door that rotates on a pivot point set inside the leaf rather than on side hinges, allowing very large and heavy door leaves.
8 terms
Security
- RC classes
- Burglar resistance classes to EN 1627, from RC1 N to RC6. Each class defines a tool set and a resistance time under test. RC2 and RC3 are the classes relevant to most buildings; German police guidance names RC2 as the minimum for easily accessible windows.
- RC2
- Resists an attempt with simple tools such as a screwdriver, pliers and wedge for three minutes under test. Requires a lockable handle and, in the full RC2 class, laminated safety glass to P4A.
- RC2 N
- The same mechanical resistance as RC2 but without the requirement for attack-resistant glazing. Used where the glass is specified separately or supplied by others.
- RC3
- Resists an attempt with an additional crowbar for five minutes under test, with laminated safety glass to P5A.
- Multi-point locking
- Hardware that engages the sash into the frame at several points around its perimeter simultaneously, instead of at a single latch. It is the mechanical basis of every RC class.
- Mushroom-head cam
- A locking pin with a mushroom-shaped head that draws behind a hardened keeper when the handle is turned, resisting attempts to lever the sash out of the frame.
- Laminated safety glass
- Two or more panes bonded by a tough interlayer that holds fragments together when broken and resists penetration. Classified P1A to P8B; German term VSG. Required for RC2 and above, and for fall protection.
- Toughened safety glass
- Glass heat-treated to several times the strength of ordinary float glass, breaking into blunt granules rather than shards. German term ESG; it adds strength but, unlike laminated glass, does not resist penetration once broken.
7 terms
Standards and finishes
- EN 14351-1
- The European product standard for windows and external pedestrian doors. It defines how performance characteristics are declared and is the basis for CE marking in the EU. It is not a US product approval.
- CE marking
- The manufacturer's declaration that a product meets the applicable European requirements. Mandatory in the EU and useful evidence of tested performance — but not accepted in place of a US approval.
- NFRC
- The National Fenestration Rating Council, whose certified ratings for U-factor, SHGC and visible transmittance are used in US energy codes and for ENERGY STAR eligibility.
- Performance grade
- The rating from AAMA/WDMA/CSA 101, combining design pressure with air, water and structural performance, expressed for example as LC-PG50.
- Anodizing
- An electrochemical process that converts the aluminum surface into a hard oxide layer, giving a durable metallic finish in natural, bronze or black tones.
- Powder coating
- A dry finish applied electrostatically and cured under heat. Fine-texture powder coating produces a thicker, more impact- and scratch-resistant surface than smooth coatings and is available in any RAL color.
- RAL color
- The European color reference system used to specify architectural finishes, for example RAL 9016 Traffic White or RAL 7016 Anthracite Grey. Inside and outside faces can be specified in different colors.
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