Thermal Bridge

What is a thermal bridge?

A thermal bridge is a section of a building envelope where heat flows significantly faster than through the surrounding insulation. Unlike the well-insulated wall or roof, a thermal bridge creates a direct path for heat to escape. This happens because the thermal bridge material conducts heat more readily than insulation, or the insulation itself is interrupted, compressed, or bypassed. Thermal bridges are a leading cause of heat loss in buildings, measurable at the whole-building scale through energy calculations and sometimes visible on thermal images as cold spots on interior surfaces.

How do geometric and material thermal bridges differ?

Thermal bridges fall into two categories. A geometric thermal bridge arises from the shape of the building envelope itself. When a wall meets a corner, or a roof plane meets an exterior wall, the interior surface area is smaller than the exterior area. This geometry concentrates heat flow. The material is uniform, but the geometry forces heat through a narrower inside path. A material thermal bridge occurs when a high-conductivity material penetrates the insulation layer. Common examples include steel anchors securing prefabricated panels, concrete balcony slabs bearing through the façade, metal window frames without thermal breaks, and roof ties. Even a small cross-section of conductive material can conduct enough heat to create surface condensation.

Both types contribute to total heat loss. In passive-house design and deep-energy renovations, both must be addressed. Energy-performance calculations account for them through linear thermal transmittance (ψ-value, psi) and point thermal transmittance (χ-value, chi) of specific junctions.

Where do thermal bridges commonly occur in Slovak residential construction?

Slovakia's dominant building practice involves thick cavity-wall construction with mineral-wool insulation, which creates thermal bridges at nearly every construction joint. The most common locations are:

Location	Typical Cause	Remedy
Balcony slab junction	Concrete slab bearing through insulation; steel reinforcement conducts heat	Thermal-break balcony or recessed balcony outside the insulation plane
Window sill and reveal	Window frame not recessed into insulation; poor sealant and framing placement	Sill and reveal insulation; frame positioned at insulation's outer face
Wall-to-foundation junction	Foundation concrete exposed above grade; insulation stops at soil line	Foundation insulation below and above grade; continuous coverage
Roof eaves and gutters	Gutter brackets and roof-edge details; insulation ends at roof plane	Continuous roof insulation; thermally-isolated gutter brackets
Integrated garage junction	Garage slab and walls not thermally separated from living areas	Garage isolated as separate thermal zone; insulated party wall
Rolling-shutter or blind box	Metal or plastic box penetrating insulation; poor airtightness	Insulated shutter box recessed fully inside or outside insulation plane

What damage can thermal bridges cause?

The immediate consequence is increased heat loss and higher heating costs. A single uninsulated balcony junction can lose as much heat as a 2 m² window. At larger scales, thermal bridges across the whole building envelope degrade the effective U-value by 10–30%, depending on design. Beyond energy consumption, thermal bridges create a secondary and sometimes more serious problem: surface condensation. Where a thermal bridge brings the interior surface below the indoor dew point, moisture condenses directly on that surface. In Slovak climates, this occurs most often in winter at corners, window sills, and poorly-detailed junctions. Persistent condensation leads to mould and mildew, which damage finishes, create health risks, and may eventually rot timber elements. Building users often respond by opening windows for ventilation, further increasing heating demand.

How does thermal-bridge-free detailing work?

Avoiding thermal bridges requires continuous insulation and interruption of conductive paths. The foundational principle is the insulation-outside-the-thermal-mass approach. Insulation should form an unbroken envelope; wherever structural or mechanical elements penetrate this envelope, they must be interrupted by a thermal break. For balconies, this means either a separate insulated bracket system, or setting the balcony on the outside of the insulation plane so the slab does not conduct through the wall. For windows, the frame is positioned as close as possible to the outer surface of the insulation, and reveals are filled with continuous insulation. For junctions with cold zones (basements, garages), thermal breaks or fully insulated transition walls separate heated from unheated spaces. Roof eaves avoid thermal bridges by ensuring insulation continues to the building edge and by using thermally-broken gutter brackets.

ETICS (External Thermal Insulation Composite Systems) are inherently superior to cavity-wall construction for avoiding thermal bridges, because insulation is applied continuously to the outside of the structural shell. No structural elements pierce the insulation layer. Passive-house construction standards require ψ-values (linear thermal transmittance at junctions) below 0.01 W/(m·K) and often elimination of significant point thermal bridges. This is possible only with careful detailing: thermally-isolated balcony systems, insulated shutter boxes, and continuous exterior insulation.

How are thermal bridges accounted for in energy calculations?

Building-energy standards and certification schemes (such as the Slovak projektové energetické hodnotenie, the EU EPBD, and passive-house standards) explicitly model thermal bridges as additional heat loss beyond the basic U-values of walls, roofs, and windows. Each significant junction is assigned a linear thermal transmittance (ψ, psi) in units of W/(m·K). For a 1-metre length of a wall-to-roof junction with ψ = 0.1 W/(m·K), if that junction runs around a building 30 m in perimeter, the annual heat loss is equivalent to losing an extra 3 m² of wall area with U = 0.2 W/(m²·K). Energy modelling software sums the ψ-values of all junctions and the χ-values of point bridges (such as corner pillars) to calculate total additional heat loss. Buildings with poor thermal-bridge detail may show ψ-values of 0.15–0.30 W/(m·K) per junction; passive-house designs typically achieve 0.01–0.05 W/(m·K).

This is why airtightness and thermal-bridge detail are not separate concerns; both reduce unwanted heat loss. A building that is airtight but riddled with thermal bridges still wastes energy and suffers condensation. Conversely, excellent insulation with poor air-tightness allows wind-driven infiltration to bypass the insulation entirely. The two must be solved together.

What common misconceptions surround thermal bridges?

A widespread belief in Slovak practice is that a small material thermal bridge (such as a single steel anchor) is negligible and need not be addressed. In isolation, a single anchor may seem trivial, but a prefabricated panel façade may have dozens of anchors, and their collective effect is substantial. Another misconception is that interior-insulated buildings have fewer thermal-bridge problems than exterior-insulated buildings; the opposite is true. Interior insulation cannot provide continuous coverage at junctions and corners. A final myth is that thermal bridges disappear if the building is warm enough on average. Surface condensation depends on local surface temperature and humidity, not bulk conditions; a single cold corner will condense moisture regardless of average room temperature.

Misconception	Reality
Small material thermal bridges are negligible	Cumulative effect across a building is significant; each should be minimised
Interior insulation avoids thermal bridges	Interior insulation worsens geometric bridges and cannot cover junctions continuously
Thermal bridges don't matter if the building is warm	Condensation depends on local surface temperature, not average warmth
Passive-house allows significant thermal bridges	Passive-house tolerates only small ψ-values (< 0.01 W/(m·K) per junction)