Building envelope · External Wall Build-Ups
Light Steel Frame External Wall System
This entry follows the consequences of a conducting frame set inside a moving structure, so that the position of the insulation, the placement of the air and vapour control layer and the head detail at each floor are read as one connected problem.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What light steel frame wall is
A light steel frame external wall uses cold-formed sections as studs and tracks, sheathed and layered in the familiar framed order: insulation, a weather-resistive layer outside, an air and vapour control layer inside, a lining, and a cladding held off beyond a void. Its usual role is infill, built between the floors or beams of a concrete or steel primary structure rather than carrying that structure itself. What separates it from any other stud wall is that the frame conducts.
The test against timber-frame-external-wall-system can be settled on site with a magnet or by looking at any exposed stud. Where the frame is steel, the wall will normally show a continuous insulating layer outboard of the sheathing, because a section runs from the warm face to the cold face through whatever is packed between the studs, and the temperature of the sheathing and of the steel is therefore set by what lies outside the frame. It will also show a head detail at every floor left deliberately free rather than packed tight. A timber-framed wall needs neither of those things.
Being infill has consequences beyond structure. The wall stops at each floor edge, so the weather layer, the air line and the insulation all have to be carried across a band that belongs to the primary frame rather than to the wall. The head detail that permits movement is also a place where those layers must stay continuous while the gap they cross keeps changing dimension.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- cold-formed steel stud wall
- light gauge steel framed wall
- steel framing infill wall
- steel stud external wall
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Frame an external wall in light sections that can be formed off site and erected quickly at height.
- Fill between the floors or beams of a primary structure without taking vertical load from it.
- Carry a cladding system and pass its wind load back into the primary structure.
- Hold the thermal layer largely outboard of the frame so the sections are not the coldest surfaces in the wall.
- Accommodate movement of the structure above without that movement reaching the wall, its lining or its cladding.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 9. Order is meaningful.Primary supportCold-formed studs and tracks
Light sections rolled from strip and screwed into panels, spanning between floors and taking wind load. They are the wall's structure and, because they are metal, simultaneously a path for heat across whatever insulation sits between them.
- Layer 2 of 9. Order is meaningful.SubstrateExternal sheathing board
The board fixed over the outer flanges to stiffen the panel and give the weather layer a continuous background. It sits pressed against metal, so its behaviour when damp and its compatibility with steel fixings both matter here more than on a timber wall.
- Layer 3 of 9. Order is meaningful.Thermal layerContinuous insulation outboard of the frame
The layer running past the studs on the outside of the sheathing. On a wall of this kind it does most of the thermal work, because material between the sections is short-circuited by them, and it also raises the temperature of the steel behind it.
- Layer 4 of 9. Order is meaningful.Control layerWeather-resistive and breather layer
The membrane or applied coating on the sheathing. It sheds water arriving from behind the cladding, forms the outboard air line and stays open to vapour leaving the wall. On infill walls it also has to be joined to whatever the floor edge and the primary structure use, which is rarely the same material.
- Layer 5 of 9. Order is meaningful.Control layerInternal air and vapour control layer
The sealed layer on the room side of the frame. It resists warm humid internal air moving into the construction and slows vapour towards the colder outer parts of it, which is a different duty from the one carried on the sheathing, and its value comes from being continuous.
- Layer 6 of 9. Order is meaningful.Edge and terminationDeflection head and floor edge
The detail at the top of each panel that lets the structure above move without bearing on the studs, together with the band across the slab or beam edge. It is at once a structural allowance, an air seal and the place two insulation layers have to meet.
- Layer 7 of 9. Order is meaningful.AttachmentCladding brackets and rails
The supports that pass through the outboard insulation and fix back to the studs, carrying the facade. Each is a structural connection, a puncture in the weather layer and a metal path across the thermal layer at the same time.
- Layer 8 of 9. Order is meaningful.ProtectionCorrosion protection to the sections
The coating applied to the steel before it is formed, and the compatibility of every fastener, bracket and fixing that touches it. Because the frame is thin, its long-term condition depends on the coating staying intact where it was cut, drilled or bridged.
- Layer 9 of 9. Order is meaningful.Service zoneInternal lining and service zone
The framing and boards on the room face, with a zone for services in front of the internal air and vapour control layer. On infill walls this zone also conceals the head allowance, so the lining has to follow movement rather than restrain it.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Insulation between the studs and insulation outboard of them are not additive in the way a drawing makes them look. The between-stud layer is short-circuited by every section crossing it, so the sheathing and the room-side surface opposite each stud stay nearer to outside temperature than the average of the wall suggests. Material added outboard of the frame changes the temperature of the steel itself, and that is what decides whether the frame can become a condensing surface at all.
Where the internal control layer sits decides what the steel is exposed to. Placed on the warm face and properly closed, it keeps humid internal air away from the sections; run with services passing straight through it, with no zone in front to take them, it is perforated at every fitting and each of those perforations is a site-made seal that is never inspected again. Because the sections are metal and the sheathing is pressed against them, moisture that does arrive meets a surface which neither absorbs nor buffers it, so it stays as liquid exactly where it forms.
The head detail is where the structural and the envelope requirements pull against each other. The track has to let the floor above move without transferring load into the studs, which means a gap whose dimension changes; the air line, the weather layer and the insulation all have to remain continuous across that same gap. Any solution that closes the joint solidly in order to keep those layers continuous has reintroduced the load path the detail existed to prevent.
Cladding brackets tie the three problems together. Each one fixes through the outboard insulation into a stud, so it punctures the weather layer, crosses the thermal layer in metal and delivers a concentrated load into a thin section. Their number and arrangement follow from the facade and its wind loading, which is why changing the cladding material late in a design is never only a change of appearance.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Cold-formed sections in coated strip are the family commonly encountered forming these panels. Section shape, thickness and coating are set by the structural designer and the manufacturer's documentation against the loads and the environment involved.
Substrate
Mineral and wood-based sheathing boards are both commonly encountered on frames of this kind. How a board behaves against metal, when wetted and under the fixings involved is a matter for the board manufacturer and the designer.
Thermal layer
Board and slab insulation families are commonly encountered outboard of the sheathing, with further material sometimes packed between the sections. Which family belongs where in a given wall is decided by the designer against the whole build-up.
Control layer
Vapour-open sheet over the sheathing, vapour-resistant sheet on the room side and tapes closing the laps of each are the families commonly encountered here. Where either layer sits relative to the steel is a hygrothermal decision for a qualified professional rather than a product choice.
Attachment
Bracket and rail systems in corrosion-resisting metals are commonly encountered carrying the facade back to the frame. What any bracket can carry, and how it behaves in contact with the steel behind it, belongs with the structural designer and the supplier.
Protection
Protective coatings for cut edges, welds and site-formed connections are commonly encountered on frames of this kind. Whether a coating is called for at a given location is a durability matter for the designer and the frame supplier.
Edge and termination
Sliding head sections, fire-resisting sealants and compressible mineral materials are commonly encountered closing the movement gap at each floor. What belongs in a given head detail is set by the structural allowance and by the tested arrangement being followed.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Infill into a concrete structure
The wall is built between slab edges that deflect and shorten over time. The head allowance follows from the structural engineer's expectations for that movement, and the weather layer has to be carried across a concrete edge with a different surface and tolerance from the wall.
Read about Concrete Frame →Infill into a steel structure
Where the primary frame is steel, the wall spans between beams whose deflection and fire protection both affect the junction. Fixing back to a protected steel member is a detail that involves the fire protection system as well as the wall.
Read about Steel Frame →Facade carried off the frame
The rainscreen's brackets land on the studs through the outboard insulation, so the facade's weight, its fixing pattern and its wind loading are all transmitted through the wall. The two systems have to be designed together rather than in sequence.
Read about Rainscreen Facade →Openings framed in light sections
Openings are trimmed with additional sections that concentrate metal around the perimeter, which is the coldest part of the frame. The window is fixed into that zone, so the frame anchorage, the thermal detail and the seal to the air and vapour control layer are one problem.
Read about Window Opening Interface →Continuity across the floor edge
The wall's layers stop at the top and bottom of each panel, and the band across the slab or beam edge belongs to nobody by default. Who carries the air line and the insulation across that band, and with what material, is settled before the panels arrive.
Read about Control Layer Transition →Elements passing through the wall
Balcony arms, canopy supports and plant fixings pass through the outboard insulation and connect to the structure behind it. Each one is a designed penetration of both the thermal layer and the weather layer, not a fixing made after the wall is built.
Read about Structural Thermal Break →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Thermal
- Because the frame conducts across the wall, the ratio of insulation outboard of the studs to insulation between them governs the outcome far more than the total. Any thermal figure for such a wall is a calculation on the actual arrangement, made by a qualified professional.
- Moisture
- Metal does not absorb or buffer moisture, so anything that condenses on the sections stays where it forms. Whether that risk exists in a given build-up depends on the climate, the internal conditions and the position of the air and vapour control layer, and is assessed professionally.
- Movement
- The wall sits inside a structure that shortens, deflects and sways, and it is expected to follow rather than resist. The head allowance, the lining detail at that line and the joints in the cladding all have to be considered as one set rather than separately.
- Durability
- The frame is thin, so the loss of protective coating at a cut, a drilled hole or a dissimilar-metal contact matters more than it would on heavier steel. Conditions around the sections, and the metals allowed to touch them, are durability decisions taken at design stage.
- Fire
- Infill walls sit at the perimeter of every floor, and how the junction between wall and slab edge is treated is a designed matter rather than a site one. No fire performance is claimed anywhere in this entry, because it belongs to a tested assembly and to the authority having jurisdiction.
- Interfaces
- Almost everything difficult about this wall happens at its edges: the head, the floor band, the openings and the bracket penetrations. The field of the panel is comparatively simple, which is why review effort is better spent on the perimeter details.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- How is the insulation divided between the outboard layer and the space between the sections, and what drove that split?
- What movement is the head detail expected to accommodate, and who has stated the figure it is based on?
- Which layer carries the air line across the floor edge band, and which trade installs it there?
- How many bracket penetrations will the facade require through the outboard insulation and weather layer?
- What protects the frame at cut edges, drilled holes and points of contact with other metals?
- How is the lining detailed where it crosses the head allowance so that movement does not crack it?
- Where openings are trimmed with extra sections, how is that concentration of metal dealt with?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- That a steel stud wall is a timber wall in another material. The conducting frame moves the thermal work outboard and changes the layer positions.
- That insulation between the sections behaves like insulation between timber studs. It is short-circuited by every section that crosses it.
- That the head gap is a construction tolerance to be filled. It is a designed allowance, and packing it solid loads the wall from the structure above.
- That an infill wall is a self-contained assembly. Its layers stop at every floor, and the band between panels belongs to the interface, not to the wall.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How much movement has the structural engineer allowed for at each floor, and how does the head detail accommodate it?
- Where does the air and vapour control layer sit relative to the steel, and has that position been assessed for our climate?
- How is the air line taken across the slab or beam edge, and who is responsible for that band of work?
- What corrosion protection applies to cut edges and to fixings passing into the sections?
- How many facade brackets pass through the outboard insulation, and how is each one sealed to the weather layer?
- What happens at the perimeter of every floor where the wall meets the slab edge?
What this page does not do
- Packing the deflection head solid to make the layers continuous transfers structural movement into a wall that was never meant to carry it.
- Insulation counted as though the frame were not there will overstate what the wall does by a margin that only appears in service.
- This entry states no fire performance for the wall, the floor edge junction or any barrier within it; those belong to tested assemblies and the relevant authority.
- Contact between the sections and an incompatible metal fixing can shorten the life of a frame that will never be inspected again.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
External Wall Build-Up Systems
Complete external walls read as a layer order: where in the thickness of the wall water is stopped, and in which direction the wall is allowed to dry afterwards.
Browse all external wall build-ups entries →