Building envelope · Control Layers
Structural Thermal Break System
This entry sets out the thermal break as a joint carrying two conflicting requirements at one point, and states plainly that every structural determination about it belongs to a qualified engineer.
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 structural thermal break is
Some elements have to cross the insulation line and carry load while doing it: a projecting slab, a canopy, a parapet upstand, a plant support, a column base, a shading bracket. Where such an element runs through unbroken, the conduction path runs through with it, and the interruption cannot simply be filled with insulation because the filling would have to carry the load.
A structural thermal break resolves that by placing a body of lower-conductivity material within the joint and passing through it only what has to transfer load. The connection then behaves as a designed joint rather than as a continuous member, with capacity, stiffness, movement and tolerance all determined for the specific case by the engineer responsible for it.
Everything about the detail follows from the conflict. The load path narrows to whatever crosses the break, so structural behaviour, movement, tolerance, corrosion protection and the continuity of the surrounding layers all have to be settled together at one point rather than resolved in sequence by consultants working separately.
The difference from thermal bridging control is what the interrupting element is being asked to do. Bridging control substitutes lower-conductivity material where the interruption carries little or no load, such as a closer at a reveal, a block below a wall plate or a differently made tie. Here the element must carry load across the line. The most familiar case, a projecting balcony, is not an instance of this entry but a structural connection that commonly contains one: it is published separately as the cantilevered balcony connection system, which owns that terminology.
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.
- load-bearing thermal break
- insulated structural connection
- thermal isolator connection
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.
- Allow a structural element to cross the insulation line without carrying the conduction path across it unbroken.
- Keep the load path continuous through a joint that has been deliberately interrupted for another purpose.
- Concentrate the transfer of load into components that can be designed, checked and recorded by an engineer.
- Let the insulation, air and water layers around the connection be closed rather than stopped short of it.
- Make the treatment of a projecting element a designed connection rather than a consequence of how the structure was framed.
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.
- Primary supportThe two elements being joined
The structure inboard of the insulation line and the element outboard of it. What each is made of, how each is framed and how much movement each undergoes all set what the joint between them has to accommodate.
- Thermal layerInsulating body within the joint
The lower-conductivity component placed in the load path, occupying the depth of the interruption and holding the two elements apart. It sits in a position where it is loaded, restrained and permanently inaccessible once the connection is complete.
- AttachmentElements crossing the break
The components that pass through the insulating body to transfer load between the two structural elements. Their arrangement, material and capacity are determined by the responsible engineer, and no schedule, spacing or size appears in this entry.
- Jointing and sealingMovement, tolerance and fit
The joint has to be assembled within the tolerances of two structural elements built by different trades, and then to accommodate the movement each undergoes afterwards. Both the erection tolerance and the in-service movement are engineering determinations.
- Control layerAir and water lines across the connection
The envelope layers running past the joint have to be carried across it without relying on the structure to close them. Where a projecting element passes through, the layers must be returned around it rather than simply stopped at its face.
- Edge and terminationInsulation continuity around the connection
The insulation adjoining the break has to close onto it rather than stop short. A joint designed to interrupt conduction still conducts around itself if the layers each side of it leave the surrounding zone open.
- ProtectionCorrosion protection of the crossing elements
The components passing through the break sit in a joint that may see moisture and cannot be inspected later. What protects them, and what that protection depends on remaining intact, is part of the engineering rather than a finishing matter.
- Finish surfaceInterior surface and soffit at the joint
The lining, plaster or soffit adjacent to the connection is where any residual effect becomes visible to occupants. Its temperature relative to the surrounding surfaces is a calculated matter that belongs with the professionals assessing the junction.
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 joint exists because two requirements collide at one point, and every component is a consequence of that collision. The insulating body is only useful if the number of load-transferring elements crossing it is limited, while the structure is only adequate if enough of them cross to carry what has to be carried. Neither requirement can be optimised alone, and the resolution is an engineering one rather than a detailing preference.
Load concentration changes the surrounding structure as well as the joint. Because the transfer narrows to whatever crosses the break, the elements either side receive load over a smaller area than a continuous member would deliver, and both the connection and the structure receiving it have to be considered together. Where a member was drawn as continuous and a break is introduced later, that change belongs to the responsible engineer rather than to the detailing of the envelope around it.
Tolerance and movement decide whether the assembled joint matches the designed one. The two elements are built by different trades to different tolerances, and a joint set out to the drawing may arrive with the insulating body compressed unevenly or the crossing elements out of alignment. What happens afterwards matters equally, since the two elements swing through different temperature ranges and the joint has to take up the difference between them rather than movement on one side alone.
The envelope layers depend on the joint being finished before they can close around it. Insulation, the air line and any water-shedding layer all have to return around a projecting element rather than stop at it, and the geometry available for that is fixed once the structure is set. A break that interrupts conduction while the surrounding insulation stops short simply relocates the path rather than removing it.
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
Structural families of these kinds are commonly encountered either side of a break of this type. Which is present, and what the connection between them requires, is determined entirely by the responsible structural engineer.
Thermal layer
Insulating families of these kinds are commonly encountered adjoining and within connections of this type. Whether any material can occupy a position inside a load path is a structural determination, not a property inferred from the family.
Attachment
Components of these families are commonly encountered as the elements crossing a break of this type. Their arrangement and capacity are determined by the engineer, and no sizes, numbers or spacings are stated anywhere in this entry.
Jointing and sealing
Sealing components of these kinds are commonly encountered closing the envelope layers around a projecting element. Whether any of them tolerates the movement expected at this joint is a matter for the product documentation and the designer.
Protection
Protective coating families of these kinds are commonly encountered on components in concealed connections. What protection a given component requires in a given exposure is determined by the engineer and by the relevant product documentation.
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.
Junctions that carry no load
Where an interruption carries little or no load, a closer or a differently made component can be substituted without any structural consequence. The boundary between the two systems is simply whether the crossing element has to transfer load.
Read about Thermal Bridging Control →Upstands and parapets
A parapet takes structure above the roof insulation line and often supports edge protection, so it can require a connection of this kind. The waterproofing and the insulation both have to close around whatever crosses at that point.
Read about Parapet System →Steel members crossing the line
Steel members supporting canopies, plant, walkways and shading pass through the envelope at discrete points, so the break is typically a plated connection. Fabrication tolerance and erection sequence then govern what can be assembled on site.
Read about Steel Frame →Concrete elements crossing the line
Where a concrete element continues outward, the break is formed within the pour and cannot be revisited afterwards. Everything about it, including how reinforcement continues across it, is settled by the engineer before any concrete is placed.
Read about Concrete Frame →Balconies carried on brackets
Supporting a balcony on brackets from the face of the structure replaces a continuous crossing with a set of discrete ones. That changes the connection but not the conflict, since each bracket still crosses the insulation line and carries load.
Read about Bracket Supported Balcony →
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.
- Movement
- The element outboard of the break changes temperature far more than the structure inboard of it, so the joint has to accommodate that difference rather than resist it, which is an engineering determination for the specific case.
- Thermal
- No thermal value, comparison or improvement is stated in this entry. What a particular connection achieves is calculated by qualified professionals for the actual geometry, materials and exposure.
- Durability
- The components crossing the break are concealed and cannot be inspected once the connection is complete, so their protection and the conditions they will sit in are part of the design rather than something to be reviewed later.
- Interfaces
- This connection sits precisely where structural design, facade design and envelope detailing meet, which makes it one of the few details that cannot be resolved within any single discipline.
- Documentation
- What was assumed about loading, movement, tolerance and exposure at this joint has to be recorded, because a later alteration to the element it supports cannot be judged without it.
- Maintenance and access
- Anything supported by the connection, such as drainage, a finish or a guarding arrangement, will eventually need attention, and the joint itself will not be reachable when it does.
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 structural design was drawn with a break at this point, rather than with a continuous member, is the question that governs everything after it.
- What is being carried across the insulation line at each location, and whether it truly has to be structural, is worth testing before the joint is designed.
- Whether an alternative arrangement avoids the crossing altogether, such as supporting the outboard element independently, deserves consideration early.
- How the insulation, air and water layers return around the projecting element is a geometry question fixed by the structure.
- Which discipline is appointed to resolve this junction, and whether that appointment covers the envelope layers as well, is worth establishing.
- What tolerance the assembled joint can accept, given that two trades build the elements either side, is an engineering matter to confirm.
- Whether the connection will ever have to carry a changed load, from an altered balcony or added plant, is worth recording at the outset.
- How the components crossing the break are protected, and what that protection assumes, should be documented while it is still visible.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A thermal break is imagined as insulation placed in a gap, when the point is that a designed structural connection passes through it.
- The connection is treated as a product selection, when capacity, movement and tolerance are determinations for the responsible engineer.
- The joint is thought to solve the junction on its own, when insulation stopping short around it simply moves the conduction path elsewhere.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Has the structural engineer designed this connection as a break, and what does that design assume about loading and movement?
- What crosses the insulating body at this joint, and how was that arrangement arrived at?
- How do the insulation, air and water layers return around the projecting element rather than stopping at its face?
- What tolerance is available when the two structural elements are built by different trades, and who verifies it before assembly?
- What protects the concealed components in this joint, and what condition does that protection assume?
- Which discipline is responsible for this junction as a whole, and does that responsibility include the envelope layers?
- How would a future change to the supported element, such as a heavier finish or added plant, be assessed?
- What record will exist of this connection once it is concealed, and where will it be kept?
What this page does not do
- Every structural aspect of a connection of this kind is determined by a qualified structural engineer, and no capacity, arrangement or dimension appears in this entry.
- Introducing a break into a structure originally designed as continuous changes how load is transferred and requires the structure either side to be reconsidered.
- No thermal outcome is stated here, since what a connection achieves depends on its geometry, its materials and the assembly around it.
- Concealed components in this joint cannot be inspected after completion, so decisions about their protection cannot be revisited later.
- Altering or extending an element supported by a connection of this kind is work for the engineer responsible rather than a facade or finishing decision.
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.
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.
Part of
The broader system this entry is a narrower form of.
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 →