Thermal Bridging: Why Heat Leaks Through Structure

Insulation only works where it is continuous. Wherever a more conductive material punches through it — a concrete slab edge, a steel bracket, a balcony cantilever, an aluminium window frame — heat takes the short cut. That short cut is thermal bridging, and it is one of the most common reasons a building with good specifications on paper performs worse than expected, or develops damp and mould in strangely specific places.

This article explains the physics in plain terms, the details where thermal bridges usually occur, why they matter in a hot, humid climate like Singapore’s as much as in a cold one, and what designers and engineers can realistically do about them. The emphasis is on judgement rather than numbers, because the size of any bridge depends entirely on the detail.

The physics: heat follows the easiest path

Heat flows from warm to cold, and it flows faster through materials that conduct well. Mineral wool or PIR board conducts heat poorly; dense concrete conducts it many times better; steel and aluminium conduct it far better still. A wall can therefore be thought of as a set of parallel routes. If ninety-five per cent of the area is well insulated but a narrow band of concrete or steel runs straight through, a disproportionate share of the heat crosses at that band.

Engineers separate two effects. A repeating thermal bridge occurs at regular intervals — studs in a lightweight wall, wall ties in cavity masonry, fixings for cladding rails — and is usually folded into the wall’s overall U-value. A non-repeating or linear bridge occurs at junctions: where the floor meets the façade, around window reveals, at the roof-to-wall junction, at the base of the wall. These are quantified separately as a heat loss per metre of junction, commonly called a psi-value, while point bridges such as a single steel bracket are described by a chi-value. The useful intuition is simple: the more often and the more bluntly structure crosses the insulation line, the worse the envelope performs.

There is also a geometric effect. At an external corner the outside surface area is larger than the inside, so even a perfectly homogeneous wall loses more heat per unit of inside area there. That is why corners are often the first place condensation appears.

Where thermal bridges occur in real buildings

From a structural point of view, most serious thermal bridges are places where the frame has to reach the outside. The usual suspects are:

Slab edges — a reinforced-concrete floor running through to the façade, often with only a thin band of finish over it.
Balconies and canopies — a cantilevered slab continuous with the internal floor acts like a cooling or heating fin.
Columns and beams at the perimeter — especially where the insulation stops at the column face instead of wrapping it.
Window and door frames — metal frames without a thermal break, and poorly detailed reveals where the frame sits outside the insulation line.
Cladding brackets and fixings — steel or aluminium brackets that pass through the insulation layer of a rainscreen façade.
Parapets and roof junctions — where the wall continues above roof level and the roof insulation does not meet the wall insulation.

Many of these are decided early, when the structural grid and façade line are set. A building envelope drawn as a continuous line on plan becomes a series of interruptions once the frame, the windows and the fixings are added.

Why thermal bridging matters in the tropics

Much of the published guidance on thermal bridging comes from cold climates, where the concern is heat escaping in winter and cold internal surfaces attracting condensation. In Singapore the direction is reversed but the problem survives. Outdoor air is warm and very humid for most of the year; indoor spaces are air-conditioned, often well below outdoor temperature. Heat flows inward through the bridges, adding to the cooling load. More importantly, the chilled side of the bridge can sit below the dew point of humid air somewhere in the construction.

Where that happens depends on the detail. A cold internal slab soffit on a balcony, a chilled-water pipe bracket fixed through a wall, or an air-conditioned room against an un-air-conditioned void can all produce surfaces that sweat. Moisture can also condense inside a wall or roof build-up if warm, moist outdoor air reaches a cold layer — which is why the placement of any vapour barrier in a tropical building needs more thought than copying a temperate-climate detail. The visible results are familiar: stained ceilings, peeling paint, and mould on walls in tidy lines that trace the structure behind.

So in the tropics thermal bridging is less about winter heating bills and more about cooling energy, condensation and durability. Persistent moisture at a slab edge or bracket is not only a finishing problem; over long periods it can contribute to corrosion of embedded steel.

How engineers assess thermal bridging

For simple, repeating situations, hand calculations and standard tables are adequate. For junctions, the reliable approach is two- or three-dimensional heat-flow modelling, following methods such as those in ISO 10211, which produces the linear and point transmittances and the minimum internal surface temperature. That surface temperature is the number that matters for condensation: if it falls below the dew point of the adjacent air under realistic conditions, the detail will eventually show it.

In Singapore, envelope performance for many building types is assessed through BCA’s Envelope Thermal Transfer Value framework and the energy requirements of the Green Mark scheme. These look at the whole envelope’s heat gain rather than individual junctions, so a project can comply on paper while still containing local bridges that cause condensation. Good practice treats the compliance calculation and the junction review as two separate checks.

Designing thermal bridges out — and the structural trade-offs

The most effective strategy is to decide where the insulation line runs and then keep structure from crossing it wherever possible. In practice that means:

Wrapping the frame — placing insulation outside the slab edges and columns so the structure sits on the conditioned side.
Structural thermal breaks — proprietary load-bearing insulated connectors at balcony and canopy cantilevers, which transfer moment and shear through stainless steel bars and compression modules while interrupting the concrete.
Separate supports — making balconies self-supporting on their own columns or hangers so they need not cantilever from the floor slab.
Thermally broken frames and brackets — window systems with polyamide breaks, and cladding brackets with isolating pads or lower-conductivity materials.

Each of these has a structural consequence. A thermal-break connector is weaker and more flexible than a continuous slab, so the cantilever must be checked for deflection, vibration, fire performance and the reduced moment capacity at the joint. Isolating pads under brackets must not creep under sustained load. Wrapping a frame in insulation affects how the façade is fixed. This is where the architect, façade consultant and structural engineer need to work through the junctions together rather than in sequence, and where façade engineering earns its place on a project.

Thermal bridging in renovation and existing buildings

In an existing building, the structure is fixed, so the options narrow. External insulation can sometimes wrap slab edges; internal insulation tends to leave the floor junctions untreated, which can concentrate condensation where the insulation stops. Before adding insulation or air-conditioning to a previously naturally ventilated space, it is worth checking which surfaces will become cold and whether the moisture has anywhere to go. A short thermal-imaging survey on a humid day often reveals the bridges clearly, and is a sensible step before committing to remedial work.

Where damp patterns follow beams or slab edges, treat it as a building-physics question first and a painting question second. Repainting a condensing surface simply resets the clock.

Frequently asked questions

What is thermal bridging in simple terms?

Thermal bridging is heat taking a short cut through a building’s envelope where a conductive material, such as concrete, steel or aluminium, interrupts the insulation. The bridge lets heat flow faster than through the surrounding wall or roof, increasing energy use and creating local cold or warm spots on surfaces.

Is thermal bridging a problem in Singapore’s climate?

Yes, although the effect differs from cold climates. In Singapore heat flows inward through bridges, adding to the cooling load, and chilled surfaces near the bridge can fall below the dew point of humid air. That leads to condensation, staining and mould, typically along slab edges, beams, balconies and around windows.

How do you fix thermal bridging at a balcony slab?

In new construction the usual solutions are a structural thermal-break connector at the slab edge or a balcony supported independently so it does not cantilever from the internal floor. Both need structural design for strength, deflection, vibration and fire. In existing buildings, options are more limited and usually involve insulating the soffit and edges where practical.

What is the difference between a U-value and a psi-value?

A U-value describes heat flow through a uniform area of wall, roof or floor, per square metre. A psi-value describes the additional heat flow along a linear junction, such as a slab edge or window reveal, per metre of length. Together they give a more honest picture of envelope performance than U-values alone.

Can thermal bridging cause structural damage?

Not directly, but persistent condensation at a thermal bridge can keep concrete and fixings damp for long periods, which may contribute to corrosion of embedded steel and deterioration of finishes over time. That is why repeated damp patterns along structural elements deserve investigation rather than repeated repainting.

Seeing condensation along slabs or beams, or detailing balconies and façade fixings on a new project? Talk to our consultancy or get in touch.