Thermal bridges: how to identify and treat them in your Luxembourg home?
A thermal bridge is a weak point in a building’s insulation through which heat escapes at an accelerated rate. In Luxembourg homes built before 1990, they are invisible to the naked eye but responsible for 5 to 25% of total heat losses, depending on the existing insulation level. They cause not only higher heating costs, but also condensation problems, mould and building fabric deterioration. Knowing how to identify them is the first step in an effective energy renovation — and the 2026 Klimabonus directly subsidises their calculation and treatment via an accredited energy advisor.
What is a thermal bridge? Definition and physical mechanism
The term « thermal bridge » refers to any area of a building’s thermal envelope where resistance to heat transfer is significantly lower than in adjacent areas. In other words, it is a preferential pathway through which heat escapes outward (in winter) or penetrates inward (in summer).
The phenomenon is governed by the laws of thermodynamics: heat spontaneously moves from warm areas to cold areas. Where insulation is continuous and homogeneous, this transfer is slowed. Where it is interrupted or reduced — at a junction between a floor slab and a wall, a concrete window lintel, or a cantilevered balcony — heat transfer accelerates. This weak point is what we call a thermal bridge.
The thermal loss of a thermal bridge is measured in W/(m·K). The higher the Ψ coefficient, the more severe the thermal bridge. For reference:
• Untreated slab/wall junction in an old building: Ψ ≈ 0.5 to 1.0 W/(m·K)
• Slab/wall junction with thermal break connector: Ψ ≈ 0.05 to 0.15 W/(m·K)
• Wall/window junction with metal threshold profile: Ψ ≈ 0.2 to 0.6 W/(m·K)
In Luxembourg, the Grand Ducal regulation for new construction sets a maximum average Ψ for the building envelope. In older buildings, these values are often much higher.
The 3 types of thermal bridges to know
Not all thermal bridges are identical in nature or treatment. Three fundamental categories can be distinguished:
1. Junction thermal bridges (or structural bridges)
These are the most common and the most significant in terms of heat losses. They appear where two walls meet and where insulation continuity is interrupted by a conductive structural element. In typical Luxembourg masonry or reinforced concrete construction, they are found systematically at:
- Junctions between floor slabs and external walls (each intermediate floor creates a thermal bridge along the full length of the building)
- Junctions between walls and the roof (wall plate, parapet)
- Junctions between the external wall and the ground floor slab above a basement
- Re-entrant and projecting angles of buildings
2. Geometric thermal bridges
These result from the building’s geometry itself, independently of the materials used. Angles (wall corners, roof-wall junctions) generate a larger external exchange surface than internal: interior heat disperses faster than in flat sections. A projecting corner of a house loses significantly more heat per linear metre than a straight wall section, even with identical insulation.
3. Material thermal bridges (or conductivity bridges)
These appear when a highly conductive material passes through the insulation layer. Typical examples include steel beams, steel fixings, balcony brackets, through-bolts and metal window mullions. In Luxembourg homes from the 1960s–1980s, concrete balconies poured continuously with the interior floor slab are the most common and problematic example of this type.
Where are thermal bridges found in a Luxembourg home?
Here is a systematic map of the most frequent thermal bridges in the Luxembourg residential building stock, from the bottom to the top of the building.
Ground floor slab / external wall junction (base)
The ground floor slab abuts against the external wall, creating a linear thermal bridge around the entire building perimeter. In uninsulated homes, this is often where the first signs of moisture and mould appear, at the junction between the bottom of the wall and the interior floor.
Window lintels and sills
The lintel (load-bearing element above an opening) is often made of reinforced concrete or special bricks, materials far more conductive than the surrounding insulation. The window sill, in natural stone or concrete, creates a similar thermal bridge from below. This is why dark stains frequently appear above windows in older homes.
Intermediate floor slabs
Each intermediate floor slab that is anchored into the external wall creates a thermal bridge along the full length of the building, at each storey. In a two-storey house, this represents two continuous lines of thermal bridge around the entire perimeter — often more than 40 cumulative linear metres.
Cantilevered balconies and terraces
When the balcony slab is poured continuously with the interior floor slab (common construction until the 1980s), concrete penetrates the thermal envelope across the full balcony width. This thermal bridge can account for 10 to 20% of total losses on an exposed floor. Visible condensation on the ceiling or interior wall adjacent to the balcony is the most telling sign.
Roller shutter boxes
Roller shutter boxes integrated into the window reveal are an often-overlooked thermal bridge. Uninsulated or poorly insulated, they form a cold box in direct contact with indoor air. In homes from the 1970s–1990s, this detail can account for up to 5–8% of losses through the joinery.
Metal fixings and penetrating structures
Roof eave brackets, balustrades fixed into the facade, rainwater downpipes penetrating the insulation, facade bolts and ties — any metal element penetrating the thermal envelope conducts heat very effectively (the thermal conductivity of steel is approximately 1,000 times higher than that of common insulation materials).
Wall / roof junction (wall plate, parapet, overhang)
The connection between walls and the roof structure is a constructively complex point where thermal bridges are almost inevitable without specific design. In houses with pitched roofs, the wall plate (the lowest roof beam resting on top of the wall) creates a continuous thermal bridge at the roof edge.
Building corners and angles
Each external angle (projecting corner) of the house is a geometric thermal bridge. The exposed external surface is larger than the corresponding internal surface, increasing losses compared to a flat wall section. Re-entrant angles (inner courtyards, bay windows) present the inverse phenomenon.
Consequences of thermal bridges: far more than heat losses
Thermal bridges have cascading effects that go well beyond simple thermal comfort or energy bills.
Heat losses and higher heating costs
In an uninsulated home built before 1974, thermal bridges account for approximately 5% of total heat losses — a modest fraction compared to massive losses through walls and the roof. But in a home whose envelope has been partially renovated (walls insulated, roof insulated, but junctions untreated), thermal bridges can represent up to 25 to 30% of residual losses. This is why treating thermal bridges is a priority in the final stage of any ambitious renovation.
Surface condensation and mould: a health risk
This is the most visible and most concerning effect of thermal bridges. The mechanism is as follows: the interior surface of a wall at a thermal bridge is colder than the rest of the partition. When the surface temperature drops below the dew point — the temperature at which ambient air becomes saturated with water vapour — condensation forms on the wall. This moisture creates conditions favourable to mould growth (Aspergillus, Cladosporium, Stachybotrys), which degrades finishing materials and constitutes a proven health risk for occupants, particularly for asthma and allergy sufferers.
Building fabric degradation and interstitial condensation
Condensation does not only occur on visible surfaces: it can appear inside the walls themselves (interstitial condensation), where water vapour migrating from interior to exterior encounters a cold zone. This invisible moisture progressively degrades insulation materials (loss of efficiency), can corrode the steel reinforcement in concrete structures, and causes rotting of timber elements in roof structures and floors.
Asymmetric thermal discomfort
Thermal bridges create localised cold zones in a home (cold walls, cold floors, cold corners) that generate a perceived discomfort far greater than air temperature alone would suggest. An occupant feels cold not only from air temperature but also from radiation: being close to a cold surface gives a sensation of cold even if the room is at 21 °C. This is the « cold wall » effect characteristic of homes with untreated thermal bridges.
How to detect thermal bridges in your home?
There are several levels of diagnosis, from the simplest to the most precise. The choice of method depends on the objective: simple confirmation of a problem or comprehensive mapping for a renovation project.
Level 1 — Visual inspection (free, accessible to all)
The most common visual signs of a thermal bridge are:
- Black or grey marks in room corners, under windows or along floors: these are dust and mould deposits on cold, damp areas
- Peeling wallpaper or paint at corners or floor/wall junctions
- Persistent morning condensation on certain interior wall surfaces (distinct from normal condensation on glazing)
- Moisture stains on ceilings in corners, indicating a thermal bridge at the wall plate or parapet
- Sensation of cold draught at floor level along external walls
Level 2 — Infrared surface thermometer (€15 to €80)
A laser thermometer measures the surface temperature of a wall without contact. A difference of 3 °C or more between the tested area and a reference surface (normal wall section) confirms the presence of a thermal bridge. This consumer tool quickly locates cold spots and helps prioritise intervention.
Level 3 — Professional infrared thermography (€200 to €600)
A thermal imaging camera visualises the complete thermal map of a building’s entire envelope in real time and with precision. Heat loss areas appear in warm colours (red/yellow) on the images. It is the reference tool for a comprehensive diagnosis before renovation.
• Indoor/outdoor temperature difference of at least 10 °C
• Heating running for at least 48 hours
• No direct solar radiation on the facades being analysed (carry out at night or in overcast conditions)
• Doors and windows closed for at least 12 hours before the diagnosis
• Ideal periods: November to March in Luxembourg
In Luxembourg, several engineering firms carry out thermographic diagnostics compliant with the Grand Ducal regulation. These assessments are particularly useful in the context of a Klimabonus energy audit, as they precisely identify which thermal bridges should be treated as a priority to maximise energy performance gain.
Level 4 — Numerical calculation (thermal engineering firm)
For ambitious renovation projects or construction disputes, thermal bridge calculation by numerical simulation (2D or 3D software) allows precise quantification of the Ψ coefficients of each junction and sizing of treatment solutions. In Luxembourg, this calculation is subsidised by the 2026 Klimabonus within the energy advisory framework (see dedicated section).
Level 5 — Blower Door airtightness test
The pressurisation or depressurisation test of the house measures the airtightness of the envelope. Combined with thermography, it distinguishes losses through thermal transmission (pure thermal bridges) from losses through air infiltration (defective seals, insufficient joinery draught-proofing). Both phenomena often coexist in the same locations.
Treatment solutions for thermal bridges by configuration
There is no universal solution. Treating a thermal bridge depends on its type, location and the overall renovation context. Here are the approaches available for the most common configurations.
| Thermal bridge | Solution with external insulation (ETI) | Solution with internal insulation (ITI) | Comparative effectiveness |
|---|---|---|---|
| Slab/external wall junction | Naturally treated by insulation continuity | Wrapping: 50 cm insulation return on the floor + ceiling | ETI: significantly superior |
| Window lintel | Treated by continuous insulation in the reveal | Insulation of the lintel from the inside (reveal lining) | ETI: slightly superior |
| Window sill | Replacement or insulation of the existing sill | Insulation under the sill on the interior side | Comparable depending on technique |
| Cantilevered balcony | Insulation of reveals + balcony soffit | Thermal break connector (retrofit is difficult in renovation) | Partial treatment in both cases |
| Roller shutter box | Replacement with external insulated box | Internal insulation of the box + seals | Comparable |
| Projecting corner | Automatically treated by insulation continuity | Difficult treatment, reduced interior corner | ETI: significantly superior |
| Wall/roof junction | Continuous insulation up to the roof upstand | Insulation return under the roof slope + wrapping | ETI: slightly superior |
Wrapping: the key technique for internal insulation
ETI vs internal insulation: which one really eliminates thermal bridges?
✅ External thermal insulation (ETI)
Principle: insulation is applied continuously to all facades, wrapping the building like a coat. The load-bearing structure (slabs, columns, cross-walls) is entirely included in the heated zone.
- Almost totally treats junction thermal bridges
- Treats geometric thermal bridges (corners)
- Preserves the building’s thermal inertia
- No reduction in living area
- Higher cost (€130–280/m²)
- Change to exterior appearance (permit required)
⚠️ Internal insulation (ITI)
Principle: insulation is applied on the interior side of walls. The load-bearing structure remains on the exterior side (cold zone). Slab/wall junctions remain weak points if untreated.
- Partially treats thermal bridges (if wrapping is carried out)
- Does not treat geometric thermal bridges
- Structure remains cold: increased interstitial condensation risk
- Reduces living area (8–12 cm per wall)
- Lower cost (€30–80/m²)
- Interior works, no permit required (except co-ownership)
Thermal break connectors: the targeted technical device
A thermal break connector is a product specifically designed to locally interrupt thermal conductivity in a constructive junction. It is inserted between two structural elements (slab and external wall, for example) to reduce thermal transfer between them while maintaining the mechanical continuity of the structure.
Application areas for thermal break connectors
Slab/external wall junctions (insulating formwork blocks)
In new construction or during major renovation with structural intervention, insulating blocks (in high-density EPS or composite material) are placed at the slab edge, between the slab and the external wall. In standard renovation, these devices cannot be retrofitted without major structural works.
Balcony brackets (balcony thermal break connectors)
Products such as Schöck Isokorb connectors or equivalents allow a balcony to be mechanically connected to the interior floor with an integrated thermal break. In renovation, their implementation is complex (cutting the existing structure, reinforcement). They are mainly used in new construction or when a deteriorated balcony is removed and rebuilt.
Window thresholds and reveals
Insulating threshold profiles treat the thermal bridge at the bottom of the window opening when joinery is replaced. Combined with frames incorporating thermal breaks, they significantly reduce losses through reveals and thresholds.
Thermal break fixings and anchors
For facade fixings (balustrades, brackets, panels) passing through insulation, thermal break anchors limit the point thermal bridge created by each metal fixing. Their Ψ coefficient is 10 to 20 times lower than that of a conventional steel anchor.
Klimabonus 2026 and thermal bridges: a specific financial grant
The Klimabonus Wunnen 2026 includes a specific financial grant for the calculation of thermal bridges and treatment proposals, as part of an energy audit carried out by a Klima-Agence accredited energy advisor.
Grant for thermal bridge calculation (Klimabonus 2026)
Conditions:
1. The calculation must be part of an energy advisory report carried out by an accredited energy advisor
2. The corresponding renovation measures must have been carried out
3. The building’s heating energy consumption index after works must reach at least efficiency class C
Source: Guichet.lu — Financial aid for residential energy renovation based on an energy advisory report (Klimabonus 2026 scheme)
This bonus is in addition to the base grant for the energy advisory report (up to €1,600 for a detached house) and grants for the insulation works themselves. It acknowledges that treating thermal bridges is a specialised intellectual service — numerically calculating a thermal bridge by an engineering firm requires several hours of work — and encourages homeowners not to settle for « surface » insulation but to address critical junctions.
What the Klimabonus energy advisory covers for thermal bridges
A Klima-Agence accredited energy advisor commissioned within the Klimabonus framework can:
Carry out a thermal audit of the home including identification of the main thermal bridges
Calculate the Ψ coefficients of critical junctions by 2D numerical simulation
Recommend suitable treatment solutions (ETI, ITI wrapping, thermal break connectors) and include them in the advisory report
Verify the compliance of contractor quotes against the recommendations (Klimabonus: €100 grant for this verification)
Inspect works on site to ensure that junction details have been properly treated (Klimabonus: €400 grant for this inspection)
Stackable grants for a project incorporating thermal bridge treatment
| Energy advisory grant with report (detached house) | up to €1,600 |
| Thermal bridge calculation bonus (e.g. 3 junctions × €160) | €480 |
| Quote compliance verification grant | €100 |
| Works inspection grant | €400 |
| Klimabonus ETI external walls (e.g. €60/m² × 200 m² facade) | €12,000 |
| 3% VAT instead of 17% (included in invoiced amount) | included |
| Total grants linked to thermal bridges + advisory | ≈ €14,580 |
Indicative simulation. Amounts vary by actual floor areas, performance levels and household composition (Social Topup for low-income households = potential doubling of the Klimabonus grant). Use the official simulator at aides.klima-agence.lu.
Have your thermal bridges calculated under the Klimabonus
An accredited energy advisor can map your thermal bridges, calculate the expected gains from their treatment and estimate your 2026 Klimabonus grants. Request a quote or simulate your grants now.
Frequently asked questions about thermal bridges in Luxembourg
What is the difference between a thermal bridge and an air infiltration?
These are two distinct problems, although they often coexist in the same locations. A thermal bridge is an area of low thermal resistance in the wall through which heat escapes by conduction through the materials. An air infiltration is an airtightness defect (defective seal, crack, unsealed cable penetration) through which cold air physically enters the home. Both can be detected by thermography, but the Blower Door test is the only tool that quantifies infiltration losses. In renovation, it is recommended to treat both simultaneously.
Can thermal bridges cause structural damage in the long term?
Yes, in some cases. Repeated interstitial condensation can corrode steel reinforcement in reinforced concrete structures, causing it to expand and crack the concrete (accelerated carbonation). In timber frame structures, moisture from an untreated thermal bridge can cause structural timber elements to rot. These structural damages develop over decades but are irreversible and very expensive to repair. Prevention — treating thermal bridges during renovation — is therefore much cheaper than the remedy.
My home is well insulated but I still have mould in a corner. Is this a thermal bridge?
Very likely yes. In a home where the main wall surfaces have been well insulated but junctions have not been treated (no wrapping, intermediate slabs not insulated), residual thermal bridges become the only cold spots in the envelope. Interior moisture condenses precisely there. If mould consistently appears in the same room corner, at the base of a wall or under a window, it is a very strong indication of an untreated junction thermal bridge. A €15 surface thermometer is often sufficient to confirm the diagnosis.
Can I receive the Klimabonus solely for having my thermal bridges calculated, without other works?
No. The Klimabonus grant for thermal bridge calculation (€160 per calculation, €800 maximum) is conditional on the actual completion of the corresponding renovation measures AND on reaching energy class C after works. Calculation alone, without works, is not subsidisable via the Klimabonus. However, it can form part of a global energy advisory, itself partially subsidised if at least one remediation measure is carried out.
My concrete balcony creates a major thermal bridge. What can I realistically do in renovation?
Fully correcting a cantilevered balcony poured continuously with the slab is indeed difficult and costly in renovation. The available partial solutions are: insulating the soffit (underside) of the balcony with a waterproof insulation fixed under the slab, insulating the balcony reveals (sides), applying insulation on the interior face of the adjacent wall (with wrapping), and removing the balcony if it is deteriorated and replacing it with a structure incorporating an integrated thermal break. These measures reduce the thermal bridge’s impact without eliminating it entirely. Your energy advisor can quantify the expected gain for each option.
What is the ideal season for infrared thermography in Luxembourg?
Infrared thermography requires a temperature difference of at least 10 °C between interior and exterior to be reliable. In Luxembourg, this corresponds to the November–March period. December, January and February are ideal. Thermography cannot be carried out when facades being analysed are subject to direct solar radiation (which falsifies results). Night-time or overcast condition surveys are therefore preferable. Also avoid rainy periods, which cool facades uniformly and mask thermal gradients.
Is external insulation sufficient to treat all thermal bridges in my home?
ETI treats the vast majority of junction (slabs, cross-walls) and geometric (corners) thermal bridges. But it does not treat « internal » thermal bridges related to cantilevered balconies (the slab remains integral with the interior structure), unreplaced roller shutter boxes, or joinery with conductive metal mullions. A well-designed ETI typically reduces thermal bridge losses by 70 to 90%, which is sufficient to reach energy classes A or B in most Luxembourg cases.
Are thermal bridges taken into account in the Luxembourg energy passport?
Yes. The Luxembourg Energy Performance Certificate (CPE), established by an accredited expert, takes into account the linear thermal transmission coefficients (Ψ) of the main envelope junctions in the calculation of the heating energy consumption index (kWh/m²/year). When the precise thermal bridge values are not known, the method applies conventional flat-rate values that tend to penalise the overall score. A precise thermal bridge calculation — subsidised by the Klimabonus — can therefore improve the calculated CPE score and consequently the displayed energy class.
Treat your thermal bridges, maximise your 2026 Klimabonus grants
Simulate your grants in 2 minutes and request multiple free quotes from certified tradespeople to treat your thermal bridges as part of a global energy renovation in Luxembourg.
Related guides — Energy renovation Luxembourg