System Windows & Doors

What is a System Window

A system window (and the matching system door and curtain wall) is a complete, tested fenestration system in which one organization, the system house, takes responsibility for every component that touches performance. The system house designs the extruded profile cross-section, selects the polyamide thermal-break bars, defines the EPDM or TPE gasket lines, fixes the chamber and drainage geometry, lists the compatible glass build-ups, and approves a specific catalogue of hardware. It then runs initial type tests and publishes the results so that any licensed fabricator who follows the manual reproduces the same thermal, air, water, and wind numbers. The fabricator is contractually bound to that manual.

This contrasts with a generic or "open" window, where a workshop buys loose aluminium profiles, off-the-shelf gaskets, and whatever hardware is cheapest, then combines them on its own judgement. Such a window may use a thermal break and look identical, but its real-world air tightness, water resistance, and frame thermal transmittance are unknown because the specific combination was never tested as a unit. The system approach trades some sourcing freedom for repeatable, certifiable, and warrantable performance, which is exactly what a building code, a facade consultant, or a procurement audit needs.

The commercial backbone of this model is the European harmonized product standard EN 14351-1, which obliges any manufacturer placing windows or external pedestrian doors on the EU market to apply CE marking and issue a Declaration of Performance (DoP). The DoP states assessed values for the essential characteristics: thermal transmittance, air permeability, watertightness, resistance to wind load, load-bearing capacity of safety devices, acoustic performance where claimed, impact resistance, and more. For a procurement engineer the DoP is the single most valuable document, because each declared value is anchored to a test report rather than a sales brochure.

Historically, aluminium windows began as simple single-skin extrusions with no thermal separation, which conducted heat aggressively and condensed water on the inner face. The polyamide thermal-break bar, introduced and refined from the 1970s onward, split the profile into outer and inner shells joined by low-conductivity polyamide reinforced with glass fibre, cutting frame heat transfer dramatically. System houses such as Schuco (Germany, founded 1951), Reynaers (Belgium), AluK, and Kommerling for PVC then industrialized the concept into documented, license-controlled systems, which is the form the market buys today.

Scale matters in selection. The same nominal "tilt-turn aluminium window" spans an enormous performance range: a basic system might declare a whole-window Uw near 2.0 W/m2K, while a triple-glazed passive-house variant of the same opening type declares Uw below 0.8 W/m2K. A "good system window" in the abstract does not exist; the engineering task is to map a building's climate, exposure, acoustic, and security requirements onto a specific system, profile depth, glass build-up, and hardware set whose declared numbers satisfy the brief.

Opening Types and Configurations

System houses offer the same engineered profile family in several opening types, because the way a sash moves changes the sealing strategy, the hardware load, and therefore the achievable air and water classes. Choosing the wrong opening type for the exposure is a common and expensive error: a side-hung sash that suits a sheltered facade can leak badly on a high-rise corner subject to driving rain. The table below summarizes the mainstream configurations and their typical performance character.

Tilt-turn is the workhorse of European system windows. The sash either tilts inward at the top for secure trickle ventilation or swings fully inward for cleaning and egress, and a single handle drives a perimeter multi-point lock that compresses the sash against continuous EPDM gaskets. Because the seal is compression-based all around the perimeter, tilt-turn routinely reaches the highest declared air and water classes. The trade-off is that the sash swings into the room, consuming interior space and conflicting with internal blinds.

Casement and awning windows hinge outward, which keeps interior space clear and suits markets where outward opening is the norm. Their weather performance is good but more sensitive to exposure and hardware quality than tilt-turn, since wind pressure can act to peel the sash off its seal. Lift-and-slide and sliding doors solve the large-aperture problem: a lift-and-slide mechanism raises the heavy sash onto its gaskets before sliding, achieving far better tightness than a plain slider, which relies on brush seals and is therefore the leakiest configuration in the family. Match the opening type to both the user need and the facade exposure, not to aesthetics alone.

System doors follow the same logic with heavier sections. Entrance doors carry thicker thermal breaks and reinforced corners to handle the weight of insulated panels and burglar-resistant hardware, while folding (bi-fold) door systems multiply the seal length and hinge count, which is why their declared air and water classes are usually a step below an equivalent fixed or tilt-turn unit. Every opening type, however, is only as good as the gasket and hardware the system house has tested with it.

Frame Materials and Thermal Break

The frame contributes a large share of a window's heat loss, so the frame material and the design of its thermal separation are decisive. Four frame families dominate: thermally broken aluminium, PVC-U (uPVC), timber, and aluminium-clad timber composites. The table below compares their engineering character; the figures are typical ranges for the frame contribution (Uf) and should always be confirmed against a system's published values.

Thermally broken aluminium is the system-window default for commercial and high-end residential work. Raw aluminium conducts heat several hundred times faster than the glass-fibre-reinforced polyamide that interrupts it (aluminium near 200 to 237 W/mK against roughly 0.3 W/mK for PA66 GF25), so without a thermal break an aluminium frame would be a severe thermal bridge and a condensation risk. The remedy is a continuous bar of glass-fibre-reinforced polyamide (commonly PA66 GF25) crimped between the outer and inner aluminium shells, mechanically locking them while thermally isolating them. System houses widen and shape these bars, and add foam inserts in the cavity, to push the frame value down. A representative example is the Schuco AWS 75.SI+ tilt-turn system, which uses a 75 mm frame depth with polyamide insulating bars and declares a Uf of about 1.2 W/m2K; the Reynaers MasterLine 8 family uses a 77 mm fixed-frame depth (87 mm at the vent) to similar effect.

The depth of the polyamide bar and the size of the insulation zone are what differentiate performance grades within a single system. A wider thermal break, foam-filled chambers, and insulated glazing-rebate inserts can drop a frame from roughly Uf 2.0 to well under Uf 1.4 W/m2K. This is why two windows with the same outward appearance and the same brand can carry very different declared numbers: the difference lives inside the chamber, not on the surface, and only the system manual and DoP reveal it.

PVC-U profiles are inherently low-conductivity, so a multi-chamber PVC frame achieves excellent frame insulation cheaply, but the profiles are bulkier and must be steel-reinforced internally to carry large spans and heavy glass. Timber offers naturally warm frames and a renewable material story but needs periodic refinishing; aluminium-clad timber resolves that by putting weatherproof aluminium on the outside and warm timber on the inside, at a premium price. For procurement, the key point is that material choice sets the realistic floor for Uf, while the system design within that material sets where on the range a given product actually lands.

Glazing, Gas Fills, and Spacers

Glass is the largest area of a window, so the insulated glass unit (IGU) usually dominates the whole-window Uw. An IGU is two or three panes sealed around a perimeter spacer, with the cavities filled with a dry, low-conductivity gas. Three levers set its thermal behaviour: the number of panes, the low-emissivity (low-E) coating, and the gas fill plus spacer type. Understanding how they interact is essential, because over-specifying glass can negate the cost of a premium frame and vice versa.

Low-E coatings are microscopically thin metallic-oxide layers, usually on a cavity-facing surface, that reflect long-wave heat back into the room while letting visible light through. A single soft-coat low-E surface transforms a plain double unit's center-of-glass value from roughly Ug 2.7 W/m2K (uncoated air-filled) to around Ug 1.0 to 1.2 W/m2K. Coatings also tune the solar heat gain coefficient (SHGC): a high-gain coating suits cold climates that want free solar heat, while a solar-control coating suits hot climates that need to reject it. In much of the world the SHGC matters as much as the U-value, which is why both appear on an NFRC label.

Gas fills and spacers finish the job. Argon, denser than air, slows convection in the cavity and is the standard economical fill; krypton performs better in narrow cavities but costs far more. The optimum cavity width for argon double glazing is about 12 to 16 mm. The perimeter spacer that holds the panes apart used to be conductive aluminium, creating a cold edge and condensation line; modern warm-edge spacers made of stainless steel, polymer, or composite cut that linear thermal bridge (the Psi value) significantly, improving both the whole-window Uw and edge condensation resistance. The durability of the IGU seal itself is declared under EN 1279, which governs gas-loss rate and moisture ingress over the unit's life.

The table below shows representative center-of-glass Ug values for common build-ups. These are typical published figures, not guarantees; the achieved Uw of the finished window will be higher than Ug once frame and edge effects are included, so always design to the whole-window value.

Note the weight column: triple glazing at roughly 30 kg/m2 demands deeper frames and heavier hinges, and a system house will only warrant a glass build-up that its tested hardware can carry. Specifying triple glazing into a frame and hardware set rated for double units is a classic field failure, producing sagging sashes and lost air-tightness within a year.

Key Performance Parameters

A system-window quotation or DoP lists many figures, but six families of parameter drive the selection decision: thermal transmittance (Uw), air permeability, watertightness, wind-load resistance, acoustic reduction, and security class. Reading them correctly, and knowing which standard each is tied to, is the core procurement skill for this category.

Thermal transmittance (Uw) is the whole-window heat loss in W/m2K, computed by weighting the glass value Ug, the frame value Uf, and the linear edge bridge Psi by their respective areas and lengths. The lower the better. Insist on the Uw value for the actual window size and glass build-up being quoted, because Uw varies with the glass-to-frame ratio; a small window with a large frame fraction performs worse than a large pane in the same system. In North America the equivalent is the NFRC U-factor in Btu/h.ft2.F, convertible to W/m2K by multiplying by roughly 5.678.

Air permeability is graded by EN 12207 in classes 1 to 4, tested at pressures up to 150, 300, and 600 Pa, with class 4 the tightest. Watertightness is graded by EN 12208 in classes 0 to 9 with an A or B exposure suffix, where the top classes resist 600 Pa of driving-rain pressure for 55 minutes without leakage. Wind-load resistance is graded by EN 12210 with a numeric pressure class (1 to 5) and a letter frame-deflection class (A, B, C); class 5 corresponds to roughly 2000 Pa design pressure. A high-performance system window commonly declares class 4 air, class 9A water, and class C5 wind. The table below decodes these classes side by side.

Acoustic performance is declared as the weighted sound reduction index Rw in decibels under EN 14351-1; laminated glass with an acoustic interlayer and asymmetric pane thicknesses pushes a window from a basic 30 dB toward 45 dB or more. Security is graded by EN 1627 to EN 1630 from RC1 (opportunistic) to RC6 (sustained power-tool attack), and depends on the full system of glass, hardware, and anchoring, not the glass alone. The decisive caution across all six parameters is that they are declared for a tested configuration; change the glass, gasket, or hardware and the declared class no longer applies.

In China the parallel framework is GB/T 7106, which grades air permeability, watertightness, and wind-load resistance for building external windows and doors; GB/T 8484, which classifies thermal-insulating performance; and GB/T 8478, the product standard for aluminium windows and doors. When sourcing across regions, confirm which national framework the project specification cites, because an EN class and a GB class are not interchangeable even when they describe the same physical property.

Selection Decision Factors

To turn the preceding five chapters into a defensible specification, follow the decision sequence below. Most selection failures come not from a single wrong number but from deciding glass or hardware before the building's exposure and climate have been quantified. These eight steps double as an RFQ template a buyer can paste into an enquiry.

1. Quantify exposure and wind load first: establish the building height, terrain category, and design wind pressure, then translate them into a required EN 12210 wind class (and the matching frame-deflection letter). Everything downstream depends on this floor.
2. Set the weather classes: from the local driving-rain severity choose the minimum EN 12207 air class (commonly class 3 or 4) and EN 12208 water class (commonly class 8A or 9A). Coastal and high-rise corner positions need the top classes.
3. Set the thermal target: derive the required whole-window Uw (or NFRC U-factor and SHGC by climate zone) from the energy code or passive-house brief, and specify it at the actual window size, not just center-of-glass Ug.
4. Choose opening type per opening: match tilt-turn, casement, awning, lift-and-slide, or fixed to both the user need and the exposure, accepting that sliders trade tightness for space.
5. Choose frame family and system: select thermally broken aluminium, PVC-U, timber, or aluminium-clad timber to meet the Uf implied by your Uw target, then name a specific system and profile depth whose published values clear the brief.
6. Specify glazing as a system, not a wish: double or triple, low-E surface count, gas fill, spacer type, laminated or acoustic interlayer, and confirm the system house warrants that build-up and weight in the chosen frame and hardware.
7. Lock hardware, security, and safety standards: reference EN 13126 hardware, the EN 1627 RC class, EN 1935 hinges, EN 12209 locks, and EN 179 / EN 1125 for escape doors, and require the approved components from the system manual, no equivalents.
8. Demand the evidence and total cost of ownership: require the CE Declaration of Performance (EN 14351-1) or the NFRC label, the EN 1279 IGU durability declaration, and weigh purchase price against installation quality, air-tightness longevity, glass-seal life, and hardware serviceability.

One frequently overlooked dimension is serviceability and installation. A system window only delivers its declared classes if it is installed into a correctly sealed, square, and properly anchored opening with continuous interior air seals and exterior weather seals; a class 9A window in a badly sealed reveal leaks like a class 1. Confirm that the fabricator is a licensed partner of the named system house, that gaskets and hardware are stocked for the building's expected life, and that the installer follows a documented sealing detail. These factors determine whether the performance you paid for survives the first decade of service.