Thermal Bridges: The Hidden Heat Losses
Thermal bridges are locations in the building envelope where more heat flows outward than through the adjacent building components. They not only increase the heating load but can also lead to moisture damage and mould.
What Is a Thermal Bridge?
A thermal bridge occurs when heat flow is concentrated or accelerated at a specific location. This happens through:
- Geometric effects: Corners, edges, protrusions
- Material changes: Highly conductive material penetrates insulation
- Constructive weak points: Missing or interrupted insulation
Analogy: Imagine a jumper with holes. More cold air streams through the holes than through the rest of the fabric – these are the "thermal bridges" in the jumper.
The Three Types of Thermal Bridges
1. Geometric Thermal Bridges
Created by the shape of the building:
| Location | Problem | Typical Loss |
|---|---|---|
| External corners | More external than internal surface | 5–15% more |
| Building edges | Intensified heat flow | 5–10% more |
| Attics | Large exposed surface | 10–20% more |
Compact buildings have fewer geometric thermal bridges. A cube has the most favourable ratio of surface area to volume.
2. Material-Related Thermal Bridges
Created by material changes in the construction:
| Location | Cause | Example |
|---|---|---|
| Steel beams | Steel conducts 50× better than insulation | Balconies, canopies |
| Ring beams | Concrete interrupts insulation plane | Floor connections |
| Window frames | Aluminium without thermal break | Old metal windows |
3. Constructive Thermal Bridges
Created by construction details:
| Location | Problem |
|---|---|
| Window connections | Insulation layer ends at frame |
| Roller shutter boxes | Often uninsulated or poorly insulated |
| Wall base | Transition wall/floor slab |
| Roof connection | Transition wall/roof |
| Balcony penetrations | Cantilevered reinforced concrete slabs |
Evaluating Thermal Bridges
The ψ-Value (Psi-Value)
The linear thermal transmittance ψ (Psi) describes the additional heat loss per metre of thermal bridge length:
Definition: ψ indicates the additional heat flow in watts lost per 1 metre of thermal bridge length at 1 Kelvin temperature difference.
Unit: W/(m·K)
| ψ-Value | Rating | Example |
|---|---|---|
| < 0.01 | Thermal bridge-free | Passive house detail |
| 0.01–0.05 | Very good | Optimised detail |
| 0.05–0.10 | Good | Standard new build |
| 0.10–0.20 | Medium | Simple new build |
| > 0.20 | Poor | Unoptimised connection |
The χ-Value (Chi-Value)
The point thermal transmittance χ (Chi) describes point thermal bridges like fixings:
Unit: W/K
Example: Fixing through thermal insulation
- 1 fixing with χ = 0.004 W/K
- With 100 fixings: 0.4 W/K additional heat loss
The Thermal Bridge Surcharge ΔUWB
For simplified calculations, a flat-rate thermal bridge surcharge is often used:
| Building Standard | ΔUWB | Application |
|---|---|---|
| Standard values | 0.10 W/m²K | Flat-rate surcharge on all components |
| Detailed proof | 0.05 W/m²K | Constructive optimisation |
| Thermal bridge-free | 0.00 W/m²K | All details proven ψ ≤ 0.01 |
| Unrenovated old building | 0.15 W/m²K | Many unoptimised details |
Caution: A thermal bridge surcharge of 0.10 W/m²K can increase transmission heat losses by 20–40%!
The Most Critical Thermal Bridges
1. Balconies and Loggias
The problem: Cantilevered reinforced concrete slabs completely penetrate the insulation layer.
| Situation | ψ-Value |
|---|---|
| Without thermal break | 0.5–1.0 W/mK |
| With thermal break element | 0.15–0.20 W/mK |
| Self-supporting balcony | 0.05 W/mK |
Solution:
- Thermally broken connection elements (Isokorb®, Schöck®)
- Self-supporting balconies on own supports
- Loggias instead of cantilevered balconies
2. Window Connections
The problem: The insulation layer ends at the window frame, the transition is critical.
| Installation Situation | ψ-Value |
|---|---|
| Window in reveal without insulation | 0.10–0.15 W/mK |
| Window with reveal insulation | 0.03–0.06 W/mK |
| Window in insulation plane (RAL installation) | 0.01–0.03 W/mK |
Solution:
- Install window in insulation plane
- Reveal insulation up to frame
- RAL installation with sealing tapes
3. Roller Shutter Boxes
The problem: Old roller shutter boxes are often uninsulated or have only thin polystyrene panels.
| Condition | Heat Loss |
|---|---|
| Uninsulated | 30–50 W per box (!) |
| Poorly insulated | 15–25 W per box |
| Well insulated | 5–10 W per box |
Solution:
- Retrofit roller shutter box insulation
- For renovation: On-top or external roller shutters
- Seals on strap guides and inspection covers
4. Building Base/Wall Foot
The problem: The transition from floor slab to external wall is constructively difficult.
| Execution | ψ-Value |
|---|---|
| Without perimeter insulation | 0.3–0.5 W/mK |
| With perimeter insulation | 0.1–0.2 W/mK |
| Optimised (e.g. thermal foundation) | 0.03–0.08 W/mK |
5. External Corners
The problem: Geometrically, more heat flows away in corners.
| Wall Thickness/Insulation | ψ-Value |
|---|---|
| Thin insulation | 0.05–0.10 W/mK |
| Thick insulation | 0.02–0.05 W/mK |
| Passive house | < 0.01 W/mK |
Thermal Bridges and Moisture Damage
Thermal bridges are not just an energy problem – they also lead to lower internal surface temperatures:
Critical point: When the internal surface temperature falls below the dew point of the room air, moisture condenses → Mould risk!
The fRsi Value
The temperature factor fRsi evaluates mould risk:
Formula: fRsi = (θsi - θe) / (θi - θe)
- θsi = Internal surface temperature
- θi = Room temperature
- θe = Outdoor temperature
| fRsi | Rating | Meaning |
|---|---|---|
| ≥ 0.70 | Critical | Mould risk! |
| ≥ 0.75 | Limit value per DIN 4108 | Minimum requirement |
| ≥ 0.85 | Good | Low risk |
| ≥ 0.95 | Very good | Practically no risk |
Example: External Corner
At 20°C inside, -10°C outside and fRsi = 0.70:
θsi = fRsi × (θi - θe) + θe = 0.70 × 30 + (-10) = 11°C
With typical room air (20°C, 50% relative humidity), the dew point is 9.3°C – just adequate!
At 60% humidity, the dew point rises to 12.0°C → Mould risk!
Calculation Example: Thermal Bridge Surcharge
A detached house with:
- External wall area: 150 m²
- Wall construction with U = 0.24 W/m²K
Without Thermal Bridge Surcharge
HT,wall = 150 × 0.24 = 36 W/K
With Standard Surcharge (ΔUWB = 0.10 W/m²K)
HT,wall = 150 × (0.24 + 0.10) = 150 × 0.34 = 51 W/K
Increase: +42%!
With Optimised Details (ΔUWB = 0.05 W/m²K)
HT,wall = 150 × (0.24 + 0.05) = 150 × 0.29 = 43.5 W/K
Measures to Minimise Thermal Bridges
For New Build
| Measure | Effect | Additional Cost |
|---|---|---|
| Compact building form | -5 to -15% TB | None |
| Thermally broken balconies | -70 to -80% TB | €150–300/m |
| Windows in insulation plane | -50 to -70% TB | €10–20/m |
| Continuous insulation layer | -30 to -50% TB | Planning |
For Renovation
| Measure | Effect | Cost |
|---|---|---|
| Roller shutter box insulation | -50 to -70% | €50–100/unit |
| Reveal insulation windows | -30 to -50% | €30–50/m |
| Internal insulation at reveals | -20 to -40% | €40–80/m² |
| Base/perimeter insulation | -30 to -50% | €80–120/m² |
Practical tip: When replacing windows, always pay attention to reveal insulation! Without insulation, the thermal bridge effect often worsens through the thicker insulation on the wall.
Thermal Bridges in the Heating Load Calculator
Our Heating Load Calculator accounts for thermal bridges:
- Flat-rate surcharge by building standard (0.05–0.15 W/m²K)
- Automatic assessment by building age
- Renovation suggestions for thermal bridge minimisation
Calculate now: See the impact of thermal bridges on your heating load with our Heating Load Calculator.
Further Reading
- Transmission Heat Losses – The main share of heating load
- The U-Value Explained – The component characteristic
- Renovation Recommendations – Measures against thermal bridges
- What Is Heating Load? – Back to the basics
Sources
- DIN EN ISO 10211 – Thermal bridges in building construction
- DIN 4108-2 – Minimum requirements for thermal protection
- DIN 4108 Supplement 2 – Thermal bridge catalogue
- Passive House Institute – Design recommendations