Minimal-frame sliding doors: the technical guide

A sightline is the width of aluminium you can see when you stand inside the room and look at the door. It is measured in millimetres, and on a minimal-frame system it is the number that decides how much of the view the frame takes up. Published figures across the systems we install run from 18 mm on MFP18 and Vitrocsa 3001 to 34 mm on Solarlux cero III.

The measurement is not always taken at the same place, which is why two systems can both advertise a slim frame and still feel different. A frame sightline is the fixed frame and sash you see around the perimeter: on MFP18 that is 18 mm at the frame, reducing to just 8 mm of visible aluminium once the surround is embedded in the wall. A panel-joint sightline is the line of metal where two panels meet or overlap, and it is usually the widest line in a sliding door. Solarlux Ecoline S and Highline S publish a uniform 55 mm at the panel joint, deliberately matching connected Solarlux windows and bi-folding doors so nothing steps out of line; MFP 25 publishes a 25 mm interlock and up to 94% glass surface.

If you are comparing systems, compare the panel joint first. It is the line you will notice every day, and a frame-only figure will flatter a system that meets in the middle with something much wider.

Two numbers get quoted, and they are not the same. Ug is the thermal transmittance of the glass alone — W/m²K, lower is better. Uw is the whole door: glass, frame, spacer bar, seals and threshold together. Uw is always the higher and more honest figure, because it includes the frame you have chosen. A door with excellent glass and a poorly insulated frame will show a good Ug and a mediocre Uw.

Published examples put that in perspective. MFP18-H30 quotes Uw 1.3 W/m²K with Ug 0.7; MFP18-H50, on 48 mm triple glazing, quotes Uw 1.05 with Ug 0.5. Solarlux Highline S quotes Uw from 0.9 W/m²K and Ug 0.6 with a warm edge. Solarlux cero III is the outlier: Uw ≤ 0.80 W/m²K, which is passive house certified.

• →Warm-edge spacer — the bar between the panes. A standard aluminium spacer conducts heat and cold around the edge of the glass, which makes the inner edge colder, invites condensation and drags the Uw down. A warm-edge spacer breaks that bridge; it is the single cheapest upgrade to a triple-glazed specification and Solarlux publishes it as part of the Highline S figure.
• →Passive house certification — awarded by the Passive House Institute. For a sliding door it means a certified whole-door Uw of 0.80 W/m²K or better, tested rather than calculated. Solarlux cero III holds it.
• →Thermal break — a non-conducting element, normally glass-fibre reinforced polyamide, separating the inside face of the frame from the outside face so heat does not travel straight through the metal. Every system we install is thermally broken.

The threshold is where a sliding door lives or dies. A track set into the floor with nowhere for water to go will pool at the head of the opening; too much upstand and you lose the seamless threshold that makes large glass worthwhile. The detail is designed at survey stage, because it decides the floor build-up and the drainage route.

On MFP18 the track carries an integrated stainless-steel gutter and drainage, with an integrated flush floor threshold. MFP 25 offers a continuous flush threshold with optional ACO drainage. Solarlux systems use level-access tracks with cascading drainage, and Highline S integrates a brush that cleans the rail as the panel moves. Vitrocsa runs on thermally broken tracks with stainless-steel 4×4 floor bearings and is Part M compliant with a flush level-access floor.

Part M is the part of the Building Regulations that deals with access to and use of buildings, and in practice it is what people mean by level access. Barrier-free thresholds are standard on what we install, and where an accessible route crosses the opening, the threshold has to be flush and drained rather than dressed with a raised sill.

Each system is engineered around a range of glazing builds, and the build you choose affects weight, panel size, thermal performance and acoustics. MFP18 runs 28 mm double, 38 mm double and 48 mm triple; MFP 20 uses 26–30 mm and MFP 25 up to 56 mm; Solarlux Highline S accepts 22–60 mm with 50 mm heat-absorbing triple as standard; Vitrocsa runs from 12–26 mm on the hot-climate 3001 up to 56 mm on the V56.

Beyond the build, the glass itself changes the appearance of the frame. Low-iron glass removes the green edge tint of standard float glass, which matters most where panels stack or overlap and you see the glass edge repeatedly. Solar-control coatings reduce heat gain on south and west facing glass in large openings. Self-cleaning coatings cut maintenance on tall or hard-to-reach panels. Laminated make-ups add security and acoustic performance, and are common on accessible ground floors. The exact make-up is agreed with your quotation so nothing is left to chance.

Finishes are where a project becomes personal. MFP 25 is available in 450+ finishes including 150+ RAL colours, and Solarlux Ecoline S in RAL and NCS with 30 standard shades, alongside anodised options. Powder-coated aluminium, anodising and special finishes are quoted per project because the finish affects lead time as well as price.

Two openings the same size can price very differently, and it is rarely the glass alone. In our experience the cost of a minimal-frame sliding door project is driven by:

• →The system family. Engineering an 18 mm frame that carries 1,000 kg panels costs more than a 55 mm lift-and-slide frame carrying 400 kg. Slimmer and larger is more expensive to engineer, to make and to install.
• →Panel size and weight. Glass is priced by area and thickness, and weight decides carriages, lifting equipment and how many people are needed on site.
• →Glazing build. Double or triple, low-iron, solar-control, self-cleaning and laminated layers each add cost, and each changes the frame depth required.
• →Motorisation and automation. HYMOTOR on MFP18, and motorised opening on MFP 25 and Vitrocsa, add hardware, controls and an electrical supply that somebody has to provide.
• →Threshold and drainage works. Flush tracks, gutters and drainage runs affect the floor build-up and the groundwork, and are often the hidden variable in a renovation.
• →Structural work to the opening. Lintels, head heights, forming the aperture and making good are usually by others, but they are part of the project budget and they gate the programme.
• →Access and lifting. Upper floors, restricted gardens, glass that will not pass through the house, cranes, hoists and road permissions all add cost.
• →Finish, configuration and quantity. Special colours, anodising, pocket modules, corner units and the number of panels all move the price.

None of that is a reason to accept a vague quotation. Because every figure is published, we can tell you which system meets your opening, what it will achieve thermally and what it cannot do — before you commit to anything.