"Sleek" is the most overused word in a window brochure and the least useful one on a drawing. Every factory claims it, almost no factory defines it, and the gap between the two is where elevations go wrong. On an ALPES aluminum casement window the word has to survive contact with four ordinary things that photographs never show: the insect screen nobody counted, the handle that projects into the room, the glazing bead that outlines every single pane, and the section depth that no one ever measures from the street. This article is about turning an adjective into numbers you can put into a request for quotation, and about the three places where a slim frame quietly charges you for the privilege.
Every aluminium system publishes more than one "width", and the three get mixed up in quotes, comparison charts and showroom conversations with remarkable consistency. Understanding which one you are being sold is the whole first half of the job.
| Type of width | What it actually is | What it decides |
|---|---|---|
| Body width | The full extrusion across the member, most of it buried in the reveal or hidden behind the sash | Nothing you can see once installed |
| Visible face | The strip of aluminium your eye reads from inside or out | Whether the window looks slim |
| Section depth | The front-to-back dimension of the profile | How much thermal break and how thick a glass unit will fit |
ALPES publishes mullions in a two-number form, and the second figure is the one that matters visually: 75/31 mm on the C120, 58/22 mm on the C116, 75/21 mm on the C113. Read the other way around, the frame wall width figures across the outward-opening range (113 mm, 116 mm and 119 mm) are depths, not sightlines, and the glass sash thickness figures (57 mm, 62 mm and 81 mm) are glass pockets. Putting a 22 mm visible mullion and a 116 mm frame depth into the same comparison is exactly how a window that looked slim in a sample case arrives on site reading heavy.
Before you compare anything, ask casement window suppliers to state the convention they are quoting in. If the answer is a single millimetre number with no drawing reference behind it, you are not specifying, you are shopping.
This is the single most common reason a slim casement stops being slim. In most of the markets ALPES exports to, the window is delivered with an insect screen, and in a great many specifications that screen is drawn in late, quoted separately, and never measured. A 22 mm mullion sitting behind a chunky screen frame is a chunky window wearing a slim badge.
Ask for the combined frame-plus-screen sightline at the point where people actually look through the window, which is normally the centre of the opening at seated eye height. If the screen cannot be dimensioned, it cannot be compared, and any claim about slimness is only half a claim.
The glazing bead, or press line, is the visible border around every pane. It repeats across the entire elevation, so it has more influence on how the facade reads than the outer frame does. The C116 uses an oblique press line to bring that border down to a 28 mm frame line. Ask what the bead dimension is before you ask what the perimeter is.
Two thin members offset from each other read as one thick member. The C116 aligns the glass sash and the mesh screen so the two sit on the same plane, which is where the 14 mm interior profile figure comes from. The AW90 takes the same logic further on the tilt-and-turn platform, where the mullion is published as 72/4 mm - a figure low enough that you should confirm the exact visible condition on the section drawing, because measurement conventions can differ between series.
Handles are the one part of a window that the owner sees at eye level and touches constantly. The C116 offers two options: a baseless handle with a 90 degree design, and a side-mount handle. A slim frame carrying a lever that projects deep into the room is not a slim window from the sofa, whatever the section drawing says. Ask for the projection dimension, not just the finish.
The C113 uses full glue injection with integrated glued corner brackets to produce a monolithic structure. The C120 seals frame, sash and mesh with injection-moulded structural adhesive. The C116 runs seamless welded corners on its triple sealing strips. Narrower profiles leave less metal to absorb a poor joint, so corner construction is not a footnote on a slim system, it is the reason the system survives being slim.
Taking millimetres out of the visible face does not remove load from the window. It relocates that load into four places, and a serious supplier can name all four.
There is a second, quieter trade-off. A slimmer frame means a higher glass fraction, which usually means a larger and heavier insulating glass unit per opening. That weight goes straight into the hinges and the stays. The C120 carries a fall rope safety mechanism and a roller locking device for exactly this reason.
An honest specification names what the client is not getting.
Stop asking "what is your sightline". Ask instead: in the assembly I have drawn, which is the widest visible member, and how many millimetres is it? Then ask for a scaled section at that point. A capable factory answers with a drawing reference. A hopeful one answers with the narrowest number in its catalogue.
| Ask this | A useful answer | A red flag |
|---|---|---|
| Visible face of every member, from both sides | A marked section drawing with dimensions | One millimetre figure, no drawing |
| Is that figure the face or the body | "The face, measured here on drawing X" | "Our system is a slim system" |
| Combined frame plus screen sightline | A dimensioned screen section | "The screen is optional" |
| Handle projection into the room | A dimensioned handle drawing | No figure available |
| Widest visible member in my elevation | Names it and gives the number | Restates the narrowest figure |
| Whole-window U-value for my size and glass | A calculated value plus the method used | Only the centre-of-glass value |
| Wind, water and air test report for this build | A report with class and pressure figures | "Complies with all standards" |
| Series | Mullion (body / visible) | Frame depth | Glass sash depth | Slimming detail | Published K-value |
|---|---|---|---|---|---|
| C120 Outward Opening | 75 / 31 mm | 119 mm | 62 mm | Nail-free rolled press mesh, flush and fixing-free | 1.6 |
| C116 Outward-Open | 58 / 22 mm | 116 mm | 81 mm | Oblique press line at 28 mm, sash and screen flush, 14 mm interior profile, welded sealing corners | 1.7 |
| C113 Outward-Open | 75 / 21 mm | 113 mm | 57 mm | Full glue injection with integrated corner brackets | Not published |
| AW90 Tilt-and-Turn / Inward | 72 / 4 mm | 90 mm | 90 mm | 360 degree locking point distribution, 240 h salt spray, 5,000 N shear | 1.3 |
Figures as published in the ALPES product specifications for each series. Confirm the visible condition on the section drawing for your own configuration, and treat any figure quoted without a drawing reference as provisional.
Dark, matte finishes recede; pale, glossy finishes advance. That is a design observation rather than a test result, but on a facade it is worth several millimetres of extrusion. ALPES lists Ice Cream White, Aurora Gray, Starry Sky Gray and Brown Coffee as standard, with custom colour matching available. If you want the frame to disappear against the street while sitting quietly against a pale interior, ask about a china double colour casement window option, with a different finish on each face. ALPES does list custom colour matching, so confirm dual-face capability against your specific order rather than assuming it comes as standard.
No. Slimmer is better only where the view and the daylight are the point, and only when the section still carries its glass, its hardware and its thermal break. On a wind-exposed upper floor, or on an opening that needs a heavy acoustic glass build-up, the deeper section is usually the right answer even if it looks heavier in the sample case.
Usually because they are measuring different things. One is quoting the visible face of the mullion, the other the body width, and a third may be quoting a fixed-light sightline that no operable sash can match. Ask each of them to dimension the same point on your own elevation.
Wind resistance is a property of the whole assembly: profile cross-section, wall thickness, corner construction, glass build-up, hardware and how the unit is fixed to the building. A slim section can pass the same test as a deep one if it is engineered for the load, which is why the request should always be for a test report on your configuration rather than an assurance.
The widest visible member in the assembly, named and dimensioned, including the insect screen and the handle. It is the only sightline question that cannot be answered with a marketing number.
It will if it is specified late. Choose a screen that mounts flush with the glass sash, as the C116 does, and dimension the combined sightline before the order is placed. A screen added on site by a different trade is the most common way a slim specification is lost.
Partly. Within the ALPES range the AW90 carries the lowest published K-value at 1.3 and the lowest published mullion figure, but it is an inward-opening tilt-and-turn, which is a different decision about ventilation, furniture clearance and internal sill space. Among the outward-opening casements, the C120 publishes 1.6 and the C116 publishes 1.7. If thermal performance is the governing constraint, ask for whole-window figures for your sizes before you fall in love with a sightline.
None of this needs a new product. It needs a specification that asks for the right four things: the visible face of every member, the widest member in the assembly, the screen and handle dimensions, and the whole-window thermal figure for the actual opening. Ask those four and "sleek" stops being a claim you have to take on trust and becomes a set of dimensions you can check against the delivery. ALPES publishes most of the underlying numbers for its casement range, and the ones that are not published are worth asking for directly, because on a slim frame the numbers you were not given are usually the ones that decide how it looks in ten years.
If you are working on an elevation now, send the opening schedule and the section details to the ALPES team and ask for a dimensioned sightline drawing for each window type. You can start from the ALPES casement window range, or reach the team through the ALPES contact page with your drawings attached.
