Building envelope · Control Layers
Thermal Bridging Control
This entry describes bridging as the behaviour of the insulation line wherever something crosses it, so a reader can see why a small conductive element matters beyond its area and why more insulation elsewhere does not answer it.
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 thermal bridging control is
Heat crosses a construction by whatever route offers it least resistance. Where an insulating layer is interrupted, by a structural member passing through it, by a fixing, by a frame set out of line with it, by a slab running to the outer face, or simply by the geometry of a corner, the interruption carries a share of the flow out of all proportion to the area it occupies.
That disproportion is the whole point. Insulation added to the field of a wall acts on the area where heat was already moving slowly, and does nothing to the path running around it. An assembly can therefore be improved substantially in the field and left almost unchanged in behaviour, because the junctions were never part of the exercise.
The consequence usually appears as moisture rather than as energy. The interior surface at a bridge is colder than the surfaces around it, which is where surface condensation and mould appear first and where an occupant notices something long before any measurement is taken. That is what ties this system to the vapour control and ventilation strategies.
Its nearest relative, thermal bypass and air loop control, is separated by a test anyone can apply. If closing gaps and stopping air moving around the insulation resolves the problem, it was a bypass. Bridging is conduction through solid material and remains after every gap has been closed. This entry states no values of any kind and defers all assessment to qualified professionals.
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 bridge control
- thermal bridge mitigation
- conduction path control
- heat-flow path management
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.
- Keep the insulating layer continuous wherever the construction allows, and interrupt the conduction path where it does not.
- Reduce the extent to which junctions and fixings carry heat around insulation that is otherwise complete.
- Limit the formation of colder interior surfaces at junctions, where condensation and mould tend to appear first.
- Make the treatment of junctions a designed part of the envelope rather than a consequence of how elements happen to meet.
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.
This system is itself the junction between two others, so there is no build-up to read. The roles below are the conditions that have to be resolved where those systems meet, and no order is implied between them.
- Thermal layerContinuous insulation line
The layer whose continuity the whole system is about. Drawn as an unbroken line through a section, it shows immediately where the construction forces it to stop, step, thin or hand over to another element.
- Primary supportInterrupting structural element
A beam, column, slab edge, lintel, wall plate or sill that crosses or touches the insulation line because the structure requires it there. What it is made of matters, since families of structural material differ greatly in how readily they conduct.
- AttachmentFixings, ties and support brackets
The small components that pass through insulation to hold cladding, rails and leaves back to the structure. They are individually minor and collectively repetitive, which is what makes their material and arrangement a design decision rather than a site one.
- Edge and terminationReveal, sill and cavity closers
The pieces that close the insulation line where it runs into an opening or a change of construction. They occupy the position where the frame, the wall and the insulation all arrive, and where the line most often thins to almost nothing.
- Finish surfaceInterior surface at the junction
The plaster, lining or reveal where the effect of a bridge becomes visible to an occupant. Its temperature relative to the surrounding surfaces, and the humidity of the air meeting it, are what decide whether anything appears there.
- Service zoneRecesses cut into the insulation line
Sockets, recessed luminaires, extract terminals, meter boxes and chases cut for services all remove insulation locally after the line has been designed. Their positions are set by a different drawing, which is why they appear as bridges nobody intended.
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 insulation line and the interrupting element behave as a pair rather than as an obstacle and a victim. The element concentrates flow because the insulation around it is resisting; increase the resistance of the field alone and the concentration at the interruption becomes more pronounced rather than less. Improving the field and improving the junction are therefore separate exercises that have to be done together.
Geometry acts as an interruption in its own right, without any material changing. At an external corner, at a parapet, at a projecting slab or at a recessed opening, the surface losing heat is larger than the surface receiving it from the room, so the interior surface runs colder even where the line is unbroken. This is why bridging cannot be read off a specification and has to be read off a section.
Fixings and closers work against each other. Brackets, ties and rails have to reach the structure, and every route they take crosses the line; closers, reveals and sills have to fill the gaps left where openings interrupt it. Each is decided by a different trade at a different time, so an insulation line that is continuous on the architect's section is routinely crossed by the cladding setting-out and thinned by the window schedule.
The consequence lands on the internal finish, which connects this system to the vapour and ventilation strategies. A colder interior surface only produces condensation if the air meeting it carries enough moisture, so bridging, internal humidity and ventilation act together. Treating any one of them alone can move the visible problem elsewhere rather than remove it, which is why assessment belongs with a qualified professional considering them together.
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.
Thermal layer
Insulating families of these kinds are commonly encountered forming the line whose continuity is at issue, including in the constrained positions found at junctions. Whether any of them fits a given detail is a matter for the manufacturer's documentation and the designer.
Primary support
These families are commonly encountered as the element that crosses or touches the insulation line, and they differ substantially in how readily they conduct. No values are stated here and the comparison belongs with a qualified professional.
Attachment
Fixing and support families of these kinds are commonly encountered crossing insulation to reach the structure behind it. Which arrangement is appropriate is determined by structural and facade design rather than by the role described here.
Edge and termination
Components of these families are commonly encountered where the insulation line runs into an opening or changes construction. Whether any of them belongs in a particular reveal or closer detail is a question for the designer and the product documentation.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Edge and termination
- Threshold →A trafficked structural element at floor level where insulation continuity is hardest.
- Eaves →One of the junctions where insulation continuity between two planes is most commonly lost.
- Head →A structural element crossing the wall at the head is a bridge that has to be designed for.
- Jamb →Insulation continuity around openings is one of the principal bridging conditions.
- Parapet →One of the most significant bridges in a flat-roofed building, where the wall passes the roof insulation line.
- Slab Edge →One of the principal linear bridges, running continuously around every floor.
Finish surface
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Frame position relative to the line
Where a frame sits in the depth of a wall determines whether the insulation can be carried onto it or has to stop short. The same window in the same opening produces a different junction depending on that position, and the choice is made when the wall is designed.
Read about Window Opening Interface →Interruptions that carry load
Where the interrupting element has to transfer load across the insulation line, substituting a lower-conductivity closer is not available and the joint becomes a structural one. That case is a separate system with its own engineering requirements.
Read about Structural Thermal Break →Insulation outboard of the structure
Carrying insulation across the outside of a structure removes many junction interruptions at once, and replaces them with a smaller set at openings, at the base, at the eaves and wherever brackets pass through to the wall behind.
Read about Rendered External Insulation →Insulation inboard of the structure
Lining the inside face leaves every intermediate floor, party wall and internal partition crossing the line. Those junctions cannot all be treated, so the assessment has to address returns, flanking construction and the surfaces that will now run colder.
Read about Internal Wall Insulation →Wall insulation meeting roof insulation
At the eaves the wall line and the roof line have to meet in a space also occupied by the wall plate, the ventilation path and the gutter support. Whether the two lines actually join there is decided by the depth available above the wall head.
Read about Eaves System →Floor perimeter and slab edge
The junction between floor insulation and wall insulation sits at the point where the slab, the barrier, the wall base and often a threshold all meet. It is built early, buried immediately and cannot be revisited after completion.
Read about Ground-Bearing Slab →Strip Foundation
The base of the wall is where the insulation lines of floor and wall have to be reconciled around a structural element running straight through them. What that junction should look like belongs with the designer and depends on the whole build-up.
Read about Strip Foundation →Raft Foundation
The edge of a raft is where the plate, the insulation beneath it and the wall insulation have to be reconciled around a continuous structural element. What that junction should be is a whole-envelope question rather than a foundation one.
Read about Raft Foundation →Cold-Formed Steel Frame
Every stud, every track and every bracket crossing the insulated line is a potential path, and the frame fixes where those paths are before it reaches site. The control strategy therefore has to be given the fixing arrangement for the cladding at the same time as the framing layout.
Read about Cold-Formed Steel Frame →Control Layer Transition
The geometry that makes a control layer transition awkward is usually the geometry that thins or interrupts the insulation. Resolving the barrier without looking at the thermal layer can move the coldest internal surface to exactly the place where the band was fixed.
Read about Control Layer Transition →Door Threshold Interface
Meets the insulation line within the width of one doorway, where the floor edge, the wall base and the frame each interrupt it and the return has to be made in a very short length.
Read about Door Threshold Interface →Parapet System
A parapet places a connected area of wall outside the insulated volume while leaving it structurally continuous with what is inside. How far the insulation is returned, and what that means for the construction behind it, is assessed by a qualified professional.
Read about Parapet System →Rainscreen Facade
Support brackets pass through the insulation by design, so the insulation line is repeatedly interrupted at points chosen for structural reasons. How that is treated, whether by isolating components or by bracket arrangement, is settled by the thermal and facade designers together.
Read about Rainscreen Facade →Precast Facade Panels
The support and restraint layout is fixed for structural reasons and then has to be reconciled with a continuous insulation line. Because the fixings are buried in the joint zone, the reconciliation happens on paper long before anything is lifted.
Read about Precast Facade Panels →Warm Pitched Roof
Rafters, purlins, dormer cheeks and steelwork all cross the thermal line in this assembly. How the layer is carried past each, and what happens where it cannot be, is treated as a continuity question rather than as a matter of adding more insulation.
Read about Warm Pitched Roof →Twin-Skin Metal Roof
Every spacer and bracket forms a repeated crossing of the thermal layer, and the wall build-up meets this roof along the same lines. What that means for the assembly is analysed by a qualified professional and is not a property of any single component.
Read about Twin-Skin Metal Roof →Mass Timber Wall Build-Up
The thermal layer here is genuinely continuous until something crosses it, so the bridging question moves entirely onto the cladding support. How much any given bracket pattern matters for a particular wall is assessed rather than assumed from the arrangement.
Read about Mass Timber Wall Build-Up →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Brick Support Angle to Outer Leaf
Where a masonry veneer stops carrying itself and hands its weight back to the structure, at a line that also interrupts the cavity.
Structural transfer · Movement · Drainage · Thermal continuity · Buildability
Cantilever Member to Thermal Break Pad
Where a projecting member crosses the insulation line through a connection that has to carry its bending while interrupting the path through the metal.
Structural transfer · Thermal continuity · Movement · Drainage
Cavity Closer to Frame
Where the element closing the cavity meets the frame, and where the opening's thermal perimeter is either continuous or not.
Thermal continuity · Moisture · Air leakage · Fire continuity
Facade Bracket Through Insulation
Where a facade support crosses the insulation to reach the backing wall, forming both the load path and the main thermal bridge.
Thermal continuity · Structural transfer · Moisture · Buildability
Frame to Jamb
Where a frame meets the vertical edge of an opening, and where the wall's every layer must be closed onto it.
Thermal continuity · Air leakage · Moisture · Movement
Frame to Opening Head
Where a frame meets the top of an opening, beneath a structural element that deflects and above which water collects.
Structural transfer · Moisture · Thermal continuity · Movement
Inner Leaf to Wall Plate
Where a bedded wall plate meets the top of the inner leaf, spreading roof and floor bearings and tying the wall head to the structure above.
Structural transfer · Thermal continuity · Air leakage · Moisture · Buildability
Insulation Layer to Parapet
Where the thermal layer of the roof and the thermal layer of the wall arrive at a wall that continues past the roof, and have to meet round it.
Thermal continuity · Moisture · Air leakage · Buildability
Parapet Base to Roof Upstand
Where a flat roof meets the wall continuing above it, and where three exposed surfaces converge on one waterproofed corner.
Moisture · Thermal continuity · Movement · Weathering
Reveal to Frame
Where the internal finish returns into an opening and meets the frame, on the coldest internal surface near a window.
Thermal continuity · Moisture · Movement · Air leakage
Slab Edge to External Wall
Where a floor meets the external wall, forming a line that runs around the whole building at every level.
Thermal continuity · Moisture · Fire continuity · Structural transfer
Slab Edge to Sole Plate
Where a framed wall is stood and secured on the perimeter of the floor slab, at the point its load path, its insulation and its moisture separation all have to be resolved together.
Structural transfer · Moisture · Thermal continuity · Air leakage · Buildability
Slab Edge to Spandrel Panel
Where the edge of a floor meets the opaque panel concealing it, forming a continuous slot at every storey that several separate planes have to cross.
Fire continuity · Thermal continuity · Acoustic continuity · Air leakage · Movement
Threshold to External Ground
Where a door opening meets the ground outside, and where keeping water out and letting people in are in direct conflict.
Moisture · Drainage · Thermal continuity · Buildability
Wall Head to Roof Insulation Continuity
Where the wall insulation and the roof insulation must meet at the eaves, in a space that narrows exactly where they need to join.
Thermal continuity · Moisture · Air leakage · Buildability
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
- The effect of any junction is a calculated matter, and no value, comparison or improvement is stated here. Junction assessment is carried out by a qualified professional using the actual geometry and materials.
- Moisture
- Colder interior surfaces at junctions are where surface condensation and mould are most likely to appear, so this system is assessed alongside internal humidity and the ventilation strategy rather than on its own.
- Interfaces
- Junctions between elements are where bridging exists by definition, which means the drawings that matter are the ones showing two elements meeting rather than the ones showing either element alone.
- Buildability
- Closers, reveals and continuity pieces sit in the tightest parts of the construction and are installed by whichever trade reaches them, so a detail that cannot physically be built will be improvised on site.
- Documentation
- Where junction treatments have been assessed, recording which junctions were considered and what was assumed makes later alteration possible without repeating the whole exercise.
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.
- Whether the insulation line has been drawn as a continuous line through every section, and where it visibly stops, is the starting point.
- Which junctions repeat through this building, and over what extent each of them runs, is what an assessment needs before extent and severity can be weighed together.
- Where each window and door frame sits within the depth of the wall determines whether insulation can be carried onto it.
- Whether the cladding support arrangement has been coordinated with the insulation line, rather than set out afterwards, changes how often it is crossed.
- Whether socket, luminaire and service positions have been checked against the insulation line is worth doing before linings are fixed.
- Whether junction assessment is within somebody's appointed scope is worth establishing rather than assuming.
- How the treatment of junctions in an existing building is decided, given that many cannot be reached, is a question for the survey stage.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Bridging is thought of as a fault in workmanship, when most bridges are consequences of geometry and structure that were designed in.
- Adding more insulation to the field is assumed to compensate for junctions, when it acts only on the path that was already resisting flow.
- A bridge is imagined as something that must pass right through the wall, when a step, a thinning or a corner produces the same concentration.
- Bridging and air movement around insulation are treated as one problem, when only one of them is resolved by closing gaps.
- The effect is thought of as an energy matter alone, when the visible consequence for occupants is usually a cold surface and what forms on it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which junctions in this design have been assessed for their effect, and by whom?
- How is the insulation line carried past the intermediate floors, the party walls and the eaves?
- Where does each frame sit in the depth of the wall, and can the insulation be carried onto the frame there?
- What arrangement of ties, brackets and rails crosses the insulation, and how was that arrangement decided?
- Which interior surfaces are expected to run coldest, and how does the ventilation strategy address them?
What this page does not do
- No thermal values, comparisons or improvements are stated in this entry, and junction assessment is carried out by qualified professionals using the real geometry.
- Treating a cold surface without addressing internal humidity and ventilation can move a condensation problem rather than resolve it.
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.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Narrower entries
Systems published as variants of this one
Each of these is a case of the system above, narrowed by one defining property. They inherit what is described here rather than restating it, so this page remains the general account.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Building Envelope · Envelope
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.
Envelope Control Layers and Continuity
The continuous planes running around an enclosure — air, vapour, water and heat — and the interruptions that decide whether that continuity actually exists.
Browse all control layers entries →