Aluminium Bifold Doors — Technical Glossary
This glossary defines the technical terms, regulatory standards, and product specifications used when researching, specifying, and buying aluminium bifold doors in the UK. Terms are organised by topic. For full detail on any subject, follow the links to the relevant section of our complete aluminium bifold doors handbook.
Written by the bifold-doors.com team — FENSA registered specialists with 25+ years of experience and 1000+ Installations. Company No. 13417467.
Table of Contents
Systems and Materials
Aluminium Extrusion
The manufacturing process that produces aluminium door frame profiles. Molten aluminium alloy is forced through a shaped die under pressure to create continuous lengths of precisely shaped cross-section. The process allows complex hollow profiles with consistent dimensions and tight tolerances. All three systems in our range — BSF70, F80 IconiK, and F82 Luminia — are manufactured from extruded aluminium profiles.
Thermal Break
A non-conductive barrier inserted between the inner and outer sections of an aluminium door frame. Without a thermal break, aluminium conducts heat directly from the warm interior to the cold exterior, causing heat loss and cold frame surfaces. A thermal break interrupts this conduction path. All quality modern aluminium bifold systems use thermally broken frames. See also: Polyamide Barrier.
Polyamide Barrier
The specific material used as the thermal break in aluminium bifold door frames. Polyamide is a glass-fibre reinforced plastic with very low thermal conductivity. It is mechanically pressed or poured between the inner and outer aluminium sections to create a continuous barrier across the full frame profile. The width and density of the polyamide barrier determines how effective the thermal break is at reducing heat conduction.
Powder Coating
The finishing process applied to aluminium door frames to provide colour and corrosion protection. Fine pigmented powder is applied to the aluminium profile electrostatically, then the coated profile is cured in an oven at approximately 200°C for 10 to 15 minutes. The heat causes the powder to flow and chemically bond to the aluminium surface, forming a hard, smooth finish. Powder coating can be applied in any RAL Classic colour and is more durable and colour-stable than wet paint.
RAL Colour System
The standard European colour reference system used to specify and match colours for aluminium door powder coating. The RAL Classic system contains 216 colours organised by number — for example, RAL 7016 Anthracite Grey and RAL 9005 Jet Black are the two most commonly specified colours for aluminium bifold doors. Custom RAL colours outside the Classic range are also available to order.
Dual Colour
A powder coating specification where the inner face of the door frame is finished in a different colour from the outer face. The most common combination in UK residential installations is RAL 7016 Anthracite Grey on the exterior and RAL 9016 Traffic White on the interior — matching external architectural language while maintaining a clean, light interior finish. Dual colour adds cost over a standard single-colour specification.
Performance Rating
U-value
The measure of how much heat passes through a building element — in this context, a door system — per second, per square metre, for each degree Celsius of temperature difference between inside and outside. Measured in W/m²K. Lower is better. Part L of the Building Regulations requires replacement door systems in the UK to achieve a maximum whole-door U-value of 1.4 W/m²K. The F80 IconiK and F82 Luminia in our range both achieve 1.3 W/m²K with standard double glazing.
Air Tightness Classification
The rating system for how well a sealed door system resists air infiltration under pressure. Classified from Class 1 (lowest, tested to 150Pa) to Class 4 (highest, tested to 600Pa) under the European standard EN 1026. A Class 4 rated system has been tested to resist air infiltration at 600 Pascals of pressure difference — equivalent to sustained strong wind conditions. The BSF70 and F82 Luminia in our range achieve Class 4 air tightness. The F80 IconiK achieves Class 2.
Water Tightness Classification
The rating system for how well a sealed door system resists water penetration under pressure. Classified from Class 1A (lowest) to Class 9A (highest, tested to 600Pa) under the European standard EN 12208. A Class 9A rating means the sealed system has been tested to resist water penetration at 600 Pascals — the standard benchmark for exposed and coastal locations in the UK. The BSF70 and F82 Luminia in our range both achieve Class 9A water tightness.
Double Glazed Unit (DGU)
A sealed glazing unit consisting of two panes of glass separated by a spacer bar, with the cavity between the panes filled with argon gas and sealed at the perimeter. The sealed cavity and argon fill reduce heat conduction through the glass significantly compared to single glazing. Standard double glazed units achieve a glass-centre U-value of approximately 1.0 W/m²K. All standard bifold door specifications use double glazed units as a minimum.
Triple Glazed Unit (TGU)
A sealed glazing unit with three panes of glass, two cavities, and two argon gas fills. Triple glazed units achieve glass-centre U-values of approximately 0.5–0.6 W/m²K — significantly better than double glazing. The F80 IconiK achieves a whole-door U-value of 1.0 W/m²K with triple glazing. The F82 Luminia achieves 0.9 W/m²K with triple glazing. Triple glazed units are heavier than double glazed units — confirm sash weight capacity before specifying triple glazing on large panels.
Argon Gas
Fill The inert gas used to fill the cavity between glass panes in a sealed double or triple glazed unit. Argon is denser than air and conducts heat less efficiently, improving the thermal performance of the glazed unit. Standard sealed double glazed units in aluminium bifold doors are filled with argon as standard. Krypton and xenon gas fills are used in some premium units for further thermal improvement but at significantly higher cost.
Laminated Glass
A safety glazing product consisting of two or more glass panes bonded together with a polyvinyl butyral (PVB) interlayer. If broken, the interlayer holds the glass fragments in place rather than allowing them to fall. Laminated glass is significantly more resistant to sustained manual attack than toughened glass alone, making it the preferred glazing specification for high-security applications. It is available as an upgrade option on all systems in our range.
Toughened Glass
Also called tempered glass. Standard safety glass used in all residential glazing. Toughened glass is approximately four times stronger than standard float glass and, when broken, shatters into small blunt pieces rather than sharp shards. All glass used in our bifold door systems is toughened as a minimum.
Regulations and Standards
Part L (Building Regulations)
The energy efficiency section of UK Building Regulations. Part L requires replacement door systems to achieve a maximum whole-door U-value of 1.4 W/m²K. This requirement came into force on 15 June 2022. A FENSA-registered installer self-certifies compliance with Part L automatically as part of every installation.
Approved Document Q (Document Q)
The security section of UK Building Regulations, covering security of dwellings. Approved Document Q requires accessible doors — including bifold doors at ground level — in new-build properties, extensions requiring Building Regulations approval, and buildings converted to residential use to meet PAS 24 security testing. It came into force in England on 1 October 2015 and in Wales on 1 November 2018.
PAS 24
The British Standards Institution specification for enhanced security performance of door sets. PAS 24 subjects a door system to controlled attack testing — physical attacks on the frame, locks, hinges, and glazing using commonly available tools. A door certified to PAS 24 has demonstrated meaningful resistance to forced entry. The current version is PAS 24:2022, updated September 2024 with amendment A1:2024. The F82 Luminia in our range carries full PAS 24 certification.
FENSA
The Fenestration Self-Assessment scheme. FENSA is a government-authorised scheme that allows registered window and door installation companies to self-certify that their work complies with Building Regulations for energy efficiency. When a FENSA-registered installer fits your bifold doors, they issue a FENSA certificate confirming Part L compliance. This certificate is required when selling the property. You can verify any company’s FENSA registration on the FENSA website before appointing them.
Permitted Development (PD) Rights
The legal right in England and Wales for homeowners to make certain alterations to their property without applying for planning permission. Replacing an existing door opening with a bifold system is almost always covered by Permitted Development rights. Single-storey rear extensions within 4 metres depth for detached houses and 3 metres for semi-detached and terraced properties are also typically permitted. PD rights do not apply to listed buildings, flats, or properties where PD rights have been removed by an Article 4 Direction.
Conservation Area
A designated area of special architectural or historic interest where planning controls are tighter than standard. In a conservation area, external alterations that affect the character of the area — particularly those visible from a public road — may require Conservation Area Consent in addition to or instead of standard planning permission. Rear elevations not visible from the street are generally less restricted.
Listed Building Consent
A statutory consent required for any works that affect the character of a listed building — either internally or externally. Listed Building Consent is separate from and in addition to any planning permission required. It is required regardless of whether the works are visible from a public road. For listed buildings, consult a conservation architect before specifying any door system.
Door Components & Configurations
Traffic Door
A single panel within a bifold door system that opens independently on its own hinges, like a standard hinged door, without folding or stacking any other panels. Also called a lead door. Traffic doors are available in odd-numbered panel configurations — 3-panel, 5-panel, 7-panel. For households using the door multiple times daily, a traffic door is a practical necessity. Without one, the entire bifold system must be opened and closed for every single access.
Threshold
The section at the base of the door opening where the door frame meets the floor level. The threshold type determines the step height between interior and exterior floor levels, the weather sealing performance, and whether the installation is accessible under Part M of the Building Regulations. The four main threshold types are: rebated (standard, small step), low-rise (reduced step), level (no step, Part M compliant), and weathered (deeper rebate for exposed locations). See the threshold section of our handbook for full detail on choosing the right threshold.
Sightlines
The visible width of the aluminium frame when the bifold door is closed — specifically the frame profiles visible between glass panes. Measured in millimetres. Narrower sightlines mean more glass and less aluminium visible, improving natural light and visual minimalism. The F80 IconiK achieves meeting sightlines of 87mm using the slim sash option. Standard systems achieve meeting sightlines of 100–140mm. See the sightlines section of our handbook for a full comparison.
Bottom Rolling
The operating mechanism where bifold door panels travel on rollers fitted to the base of each panel, running in a track recessed into or sitting on the threshold at floor level. The weight of each panel is carried by the bottom rollers. All three systems in our range — BSF70, F80 IconiK, and F82 Luminia — are bottom rolling. Bottom rolling is the standard for heavy aluminium bifold systems.
Top Hung
An alternative operating mechanism where bifold door panels hang from and travel on a track fitted at the head of the opening. Panel weight is carried at the top rather than the bottom. Top hung systems can achieve cleaner threshold details but are mechanically more demanding for heavy aluminium panels and are not available in the systems we supply.
Shoot Bolt
A locking pin that projects from the top or bottom of a door panel into the head track or threshold when the door is locked. Shoot bolts provide secondary retention points supplementing the main multi-point lock. They prevent the panel from being levered open at the head or foot of the door.
Multi-Point Locking
A locking system where operating the door handle or key engages multiple locking points simultaneously at different heights up the door panel. A door secured at only one point can be forced open at the unsecured ends. Multi-point locking requires simultaneous force at all points to defeat — significantly more difficult than a single-point lock. Multi-point locking is standard on all quality aluminium bifold systems.
Anti-Lift Device
A mechanical feature preventing bifold door panels from being lifted off their bottom track and removed from the outside. Anti-lift protection is a standard feature on quality aluminium bifold systems. Confirm it is included when comparing systems.
Floating Corner
A configuration connecting two bifold door systems meeting at a right angle with no structural corner post between them. When both systems are fully open, the corner is completely clear — maximum connection between inside and outside. The F82 Luminia system supports floating corner configurations. Structural feasibility must be confirmed with a builder or structural engineer before specifying.
Stacking
The arrangement of folded bifold door panels when the system is open. Master stacking sends all panels to one end of the opening. Split stacking divides panels equally to both ends. The stacking direction is specified at order stage and built into the system — it cannot be changed after manufacture. See the configurations section of our handbook for full detail.
Panel Configuration
The number and arrangement of panels in a bifold door system. Common configurations range from 2-panel to 6-panel for standard residential installations. Odd-numbered configurations (3-panel, 5-panel) allow a traffic door. Even-numbered configurations (2-panel, 4-panel, 6-panel) can split stack symmetrically. See our configurations guide for detail on choosing the right panel count.