Building envelope · External Wall Build-Ups
Timber Frame External Wall System
This entry sets out the timber frame wall as two opposed control strategies running at once, open outward and sealed inward, so that the consequence of moving, puncturing or omitting any single layer can be traced through the assembly.
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 timber frame wall is
A timber frame external wall is a layered assembly built around a stud frame. Insulation occupies the space the studs create and frequently continues outboard of them; a sheathing board stiffens the frame and gives the outer layers a background; a breather membrane covers that sheathing; and a cladding stands off beyond a drained and ventilated void. On the room side, an air and vapour control layer is fixed across the studs, with a service void and a lining in front of it.
The order of those layers is not arbitrary. Resistance to moisture moving as vapour rises towards the inside and falls towards the outside, so that whatever does reach the frame has an easier route out than in. The cladding and the void keep liquid water off the membrane, the membrane keeps it off the sheathing, and the control layer inside keeps warm humid indoor air away from the colder outer parts of the insulation.
The test against light-steel-frame-external-wall-system is simply what the studs are made of. Both walls are sheathed, membraned and clad in the same order, but a timber stud is a modest path through the insulation while a steel one is a strong path. Where the frame is steel, material outboard of it stops being an enhancement and becomes the layer carrying most of the thermal role, and the control layer has to be positioned so the metal is not the coldest surface in the build-up.
Racking resistance, the platform each storey is built from, holding-down and the load path are owned by timber-platform-frame-system. This entry is concerned with the wall as a control-layer assembly, with how the cladding is carried off it, and with what happens when one of its layers is relocated or interrupted.
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.
- timber stud external wall
- framed wall build-up
- timber frame wall panel
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.
- Carry the enclosure on a stud frame while the insulation occupies the depth that frame creates.
- Hold the cladding away from the structure across a void that interrupts liquid water passing the cladding, gives it a route back out and lets both its faces dry.
- Allow the frame and sheathing to dry outward through a layer that is open to vapour but closed to liquid water.
- Limit the moist internal air that reaches cold surfaces within the insulation by sealing a control layer on the warm side.
- Give services a route that does not require the sealed layer to be cut open after it is complete.
- Provide a background to which linings, fixings and cladding supports can all be fixed in known positions.
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 supportStud frame
Vertical members with head and sole plates, forming the depth of the wall and carrying load down through each storey. The frame is also the fixing background for almost everything else, which is why its layout governs where other layers can be interrupted.
- Layer 2 of 9. Order is meaningful.SubstrateSheathing board
The board fixed over the studs that stiffens the frame against racking and gives the membrane a continuous, flat background. Because it sits at the cold side of the between-stud insulation, its tolerance of occasional wetting matters as much as its strength.
- Layer 3 of 9. Order is meaningful.Thermal layerInsulation between and outboard of the studs
Material filling the stud depth, and often a further layer running past the frame outside the sheathing. The split between those positions determines both how much of the wall is interrupted by framing and how warm the sheathing runs.
- Layer 4 of 9. Order is meaningful.Control layerBreather membrane
The layer over the sheathing that sheds water arriving in the void while remaining open to vapour leaving the wall. It is also the surface that wind pressure acts on before the cladding battens transfer that load back to the frame.
- Layer 5 of 9. Order is meaningful.Control layerAir and vapour control layer
The sealed layer on the warm side of the insulation, resisting air movement into the construction and reducing vapour diffusion towards the colder parts of it. Its value comes entirely from continuity, so its joints, edges and penetrations are the layer itself.
- Layer 6 of 9. Order is meaningful.Cavity or voidDrained void behind the cladding
The space between membrane and cladding, kept open so water can run down and out and so air can move through to dry both surfaces. It also relieves the pressure difference that would otherwise drive water through the cladding joints.
- Layer 7 of 9. Order is meaningful.AttachmentBattens and cladding support
The members fixed through the membrane into the studs that hold the cladding off, form the void and carry wind load back to the frame. Their direction decides whether the void can drain, so they are a drainage component as much as a fixing one.
- Layer 8 of 9. Order is meaningful.Service zoneService void
The battened or framed space between the control layer and the lining where cables, boxes and pipes run. It exists so that everyday electrical and plumbing work never needs to cut the sealed layer behind it.
- Layer 9 of 9. Order is meaningful.Finish surfaceInternal lining
The board or panel finish on the room side, fixed to the service zone framing. On many walls it is also asked to protect the control layer behind it from damage during the rest of the build and during occupation.
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.
The breather membrane and the inner control layer are usually described as opposites, but they only work as a pair. The outer layer is asked to be open to vapour and closed to liquid water; the inner one to be closed to air and more resistant to vapour. Leave the inner layer unsealed and warm humid air moves bodily into the insulation, depositing far more moisture than diffusion alone ever would, at which point the outer membrane's openness is the only thing keeping the sheathing dry.
The service void exists because of what happens to the layer behind it. Every socket box, cable and pipe that crosses the control layer becomes a hole to be sealed individually and never inspected again. Moving those services in front of the layer turns many small site-made seals into a handful of designed ones, which is why deleting the void to gain floor area quietly transfers the airtightness of the whole wall onto workmanship at each fitting.
Insulation between the studs and insulation outboard of them are not interchangeable quantities. The between-stud layer is interrupted by every stud, plate, lintel and noggin, so what it achieves depends on how much of the wall is actually frame. The outer layer runs past all of them and also raises the temperature of the sheathing, which is what governs whether moisture arriving there can be tolerated. Shifting material between the positions moves the coldest surface in the wall.
The battens do three jobs at once: they hold the cladding off, they keep the void open for drainage and ventilation, and they carry wind load through the membrane into the frame. Fixings therefore puncture the membrane at every batten, which is why the membrane is expected to close around a compressed fixing rather than remain unpierced, and why battens run so that they never dam the void they were installed to create.
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
Graded softwood, treated sections and engineered members are commonly encountered forming studs, plates and lintels. What grade, treatment and section belong in a given wall is set by the structural designer and the treatment supplier's documentation.
Substrate
Wood-based and mineral sheathing boards are commonly encountered stiffening frames of this kind. How a given board behaves if it is wetted, and what it contributes to racking, are matters for the board manufacturer and the structural designer.
Thermal layer
Batts, boards and blown fibres from these families are commonly encountered both between and outboard of the studs. Which position and which family belong in a particular wall is a design decision taken against the whole build-up rather than layer by layer.
Control layer
Vapour-open sheet outside and vapour-resistant sheet inside are the families commonly encountered performing these opposed roles. Whether a particular pairing works in a given climate and build-up is assessed by a qualified professional, not chosen by product name.
Jointing and sealing
Tapes, flexible sealants and gap fillers are commonly encountered closing the laps, edges and penetrations of the sealed layer. Compatibility with the sheet and with the background being bonded to is a matter for the manufacturer's documentation.
Protection
Boarded and panelled cladding families are commonly encountered as the outer screen beyond the void. Which of them belongs on a given elevation, and how it is fixed and supported, is set by the cladding manufacturer and the designer.
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.
Cladding carried off the wall
The cladding is a separate system whose fixings, support spacing and wind loading all land back on the studs. Changing the cladding therefore changes the number of penetrations through the membrane and the depth of the void the wall has to accommodate.
Read about Rainscreen Facade →Openings through the layered wall
An opening cuts every layer at once. The sill has to drain outward into the void, the membrane has to be dressed into the reveal and returned, and the inner control layer has to be sealed to the frame - three separate continuities meeting at one hole.
Read about Window Opening Interface →The sealed line around the enclosure
The wall's control layer is one length of a line that also runs through floors, the roof and the ground floor construction. Where that line changes plane, the wall's layer has to be joined to something else, and those changeovers are outside the wall's own scope.
Read about Air Barrier System →Intermediate floor zone
At each floor the wall is interrupted by a rim member and by joists bearing on the frame. Both the insulation and the sealed layer have to be carried across that band, and the timber there will shrink as it dries, which the layers have to tolerate.
Read about Timber Joisted Upper Floor →Sole plate at the base
The sole plate sits close to the ground floor construction and to whatever is outside, so it needs separating from moisture below it while the wall's sealed layer is joined to the floor's. Both jobs are done at the same line, often by different trades.
Read about Damp-Proof Continuity →Wall head into the roof
At the head, the wall's insulation has to meet the roof's over the plate without a gap while the void behind the cladding is closed against birds and driven rain. Ventilation of the roof space and drying of the wall void are separate paths that meet here.
Read about Cold Pitched Roof →Services in front of the sealed layer
The zone in front of the control layer is a system in its own right, with rules about what may run in it and how deep it must be to take fittings. Where its depth is reduced, services begin to press against or pass through the layer behind.
Read about Service Void →
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.
- Moisture
- The wall relies on being able to dry outward faster than it wets inward. Whether interstitial condensation is a risk in a given build-up depends on the internal conditions, the climate and every layer's vapour behaviour together, and it is assessed by a qualified professional.
- Thermal
- The framing interrupts the between-stud layer wherever it occurs, so the thermal outcome depends on how much of the wall is timber and how much material runs past it. Any figure for that outcome is a calculation on the actual construction, and none is offered here.
- Acoustic
- Lightweight framed walls behave quite differently from mass construction against outside noise, and the cladding, void and lining all take part. Acoustic behaviour is a property of the complete tested assembly and the flanking construction, and belongs with a qualified acoustic professional.
- Fire
- The void behind the cladding is a continuous vertical path running the height of the wall, and where and how it is interrupted is a designed matter that belongs with the tested arrangement and the relevant authority. Fire performance is a property of a tested assembly; this entry states none.
- Movement
- Timber changes dimension as it dries and with seasonal humidity, mostly across the grain, so the plates and rim members at each floor move more than the studs themselves. Cladding fixings, sealed joints and window surrounds all have to tolerate that without opening.
- Buildability
- The frame is often erected before it is weathered, so the sheathing and any exposed timber can be wetted during construction. Whether the wall is closed while that moisture is still in it is a sequencing decision with consequences that only appear later.
- Documentation
- The sealed layer, the insulation arrangement and the membrane laps disappear behind linings and cladding. A photographic record taken before closing, keyed to each elevation, is what a later air test result or damp investigation can be interpreted against.
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.
- Which layer in this wall is nominated as the air control layer, and can it be traced without a break on section and on plan?
- How is the insulation divided between the stud depth and the space outboard of the frame, and why that way?
- Is there a service zone in front of the sealed layer, and is it deep enough for the fittings actually planned?
- Which way do the battens run, and does the void behind the cladding drain freely to an outlet?
- How is the sealed layer carried across the intermediate floor zone where the joists interrupt it?
- What happens to the membrane at reveals, at the sill and at the head of every opening?
- How will the frame be protected from wetting between erection and closing in?
- Where the wall meets the roof and the ground floor, which trade joins the wall's layers to theirs?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- That the breather membrane and the inner control layer do the same job in different places. One sheds water while passing vapour, the other stops air.
- That insulation between the studs and insulation outside the frame are interchangeable. They are interrupted differently and they cool different surfaces.
- That the service void is a luxury. Without it every socket and pipe becomes a hole in the layer the wall's airtightness depends on.
- That the cladding keeps the wall dry. It is a screen, and the void and membrane behind it deal with what passes through.
- That a timber frame wall and a light steel frame wall are the same build-up in different metal. The frame material changes where the thermal layer has to sit.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which surface is the air barrier in this wall, and how is it joined to the floor and roof constructions?
- What led you to this split of insulation between the stud zone and the outboard layer?
- Has the vapour behaviour of this build-up been assessed for our climate and our expected internal conditions?
- How deep is the service zone, and what happens if a fitting needs more depth than it offers?
- How is the membrane detailed at the openings, and who installs it relative to the window fitter?
- What protection is planned for the frame between erection and the cladding going on?
- Where are the cavity barriers in the void behind the cladding, and who has specified them?
- What record of the sealed layer will exist before the linings are fixed?
What this page does not do
- No single layer here resists air movement on its own; the result depends on every joint, edge and penetration in the nominated layer being closed.
- Battens laid across the drainage direction convert the void from a drained space into a series of shelves holding water against the membrane.
- Building a frame in while it still carries construction moisture puts that moisture inside the completed wall with nowhere quick to go.
- This entry states no fire or acoustic performance for any arrangement described in it; both belong to tested assemblies.
- Later work that chases into the lining will reach the sealed layer if the service zone was omitted, and the damage will not be visible.
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 →