Transmission Heat Losses: When Heat Passes Through Walls
Transmission heat losses occur when heat "flows" through solid building components from inside to outside. In most buildings, they account for 60–80% of the total heating load and are therefore the most important starting point for energy renovations.
The Physical Principle
Heat always flows from warm to cold – this is the second law of thermodynamics. In a heated building, this means:
- Inside: 20°C (warm)
- Outside: -10°C (cold)
- Temperature difference: 30 K (Kelvin)
The greater the temperature difference, the more heat flows outward.
Analogy: Imagine a hot cup of coffee. The colder the surroundings, the faster it cools down. The cup is like the building envelope – the better insulated (thermos flask), the slower the heat loss.
The Three Modes of Heat Transfer
Heat can be transferred in three ways:
| Mode | Description | Example in Buildings |
|---|---|---|
| Conduction | Through solid materials | Through the wall |
| Convection | Through moving air | Air movement at surfaces |
| Radiation | Through electromagnetic waves | Heat radiation from the wall |
In transmission, all three work together:
- Heat is transferred from room air to the inner wall (convection + radiation)
- Heat conducts through the wall (conduction)
- Heat is released from the outer wall to outside air (convection + radiation)
The Calculation Formula
Transmission heat losses are calculated according to DIN EN 12831-1:
Formula: ΦT = HT × (θi - θe)
with HT = Σ (U × A × fT) + ΔUWB × A
- ΦT = Transmission heat loss (W)
- HT = Transmission heat loss coefficient (W/K)
- θi = Indoor temperature (°C)
- θe = Design outdoor temperature (°C)
- U = U-value of the component (W/m²K)
- A = Area of the component (m²)
- fT = Temperature correction factor
- ΔUWB = Thermal bridge surcharge (W/m²K)
The Temperature Correction Factor fT
Not all components border directly on outside air. The factor fT accounts for this:
| Adjacent to | fT | Explanation |
|---|---|---|
| Outside air | 1.0 | Full temperature difference |
| Unheated basement | 0.5 | Basement is warmer than outside |
| Ground | 0.3–0.6 | Ground temperature relatively constant |
| Unheated garage | 0.8 | Garage somewhat warmer than outside |
| Heated neighbour | 0.0 | No temperature difference |
Example: A basement ceiling with U = 0.5 W/m²K and fT = 0.5 has the same effective heat loss as an external wall with U = 0.25 W/m²K.
Building Components and Their U-Values
The most important components and typical U-values compared:
External Walls
| Construction Period | Wall Type | U-Value | Rating |
|---|---|---|---|
| before 1970 | Solid brick 36cm | 1.4–1.6 W/m²K | Poor |
| 1970–1990 | Hollow block 30cm | 0.8–1.2 W/m²K | Moderate |
| 1990–2010 | Aerated concrete 30cm | 0.4–0.6 W/m²K | Medium |
| after 2010 | ETICS 16cm | 0.2–0.3 W/m²K | Good |
| Passive house | ETICS 30cm+ | < 0.15 W/m²K | Very good |
Windows
| Generation | Glazing | U-Value (Uw) | Rating |
|---|---|---|---|
| before 1980 | Single glazing | 5.0–5.8 W/m²K | Very poor |
| 1980–1995 | Double without Low-E | 2.7–3.0 W/m²K | Poor |
| 1995–2010 | Double with Low-E | 1.3–1.6 W/m²K | Medium |
| after 2010 | Triple with Low-E | 0.7–1.0 W/m²K | Good |
| Passive house | Triple special | < 0.8 W/m²K | Very good |
Roof
| Insulation | U-Value | Rating |
|---|---|---|
| Uninsulated | 2.0–3.0 W/m²K | Very poor |
| 8cm insulation | 0.4–0.5 W/m²K | Moderate |
| 16cm insulation | 0.2–0.25 W/m²K | Good |
| 24cm+ insulation | < 0.15 W/m²K | Very good |
Calculation Example
A room with the following components at θi = 20°C and θe = -12°C:
| Component | Area | U-Value | fT | HT Contribution |
|---|---|---|---|---|
| External wall | 15 m² | 0.28 W/m²K | 1.0 | 4.2 W/K |
| Window | 4 m² | 1.3 W/m²K | 1.0 | 5.2 W/K |
| Roof | 12 m² | 0.20 W/m²K | 1.0 | 2.4 W/K |
| Basement ceiling | 20 m² | 0.35 W/m²K | 0.5 | 3.5 W/K |
| Total | 15.3 W/K |
Calculation: ΦT = 15.3 W/K × (20°C - (-12°C)) = 15.3 × 32 = 489.6 W
The room loses almost 490 watts through transmission at -12°C outdoor temperature.
Where Do the Greatest Heat Losses Occur?
Typical distribution of transmission heat losses:
| Component | Share | Why? |
|---|---|---|
| Windows | 25–35% | High U-values despite small area |
| External walls | 25–30% | Large area |
| Roof | 15–25% | Warm air rises |
| Basement/floor | 10–15% | Partially buffered by ground |
| Thermal bridges | 5–15% | Often underestimated |
Note: Windows may have the smallest area but often the highest U-value. A 2 m² window with U = 1.3 W/m²K loses as much heat as 10 m² of well-insulated wall with U = 0.26 W/m²K!
Measures for Reduction
1. External Wall Insulation
| Measure | Investment | U-Value Improvement |
|---|---|---|
| ETICS 12cm | £70–100/m² | from 1.4 to 0.28 W/m²K |
| Cavity insulation | £15–35/m² | from 1.0 to 0.4 W/m²K |
| Internal insulation | £40–70/m² | from 1.4 to 0.5 W/m²K |
2. Window Replacement
| Measure | Investment | U-Value Improvement |
|---|---|---|
| Double → Triple | £250–400/m² | from 1.4 to 0.9 W/m²K |
| Complete new (triple) | £350–500/m² | depending on starting condition |
3. Roof Insulation
| Measure | Investment | U-Value Improvement |
|---|---|---|
| Between rafters 16cm | £35–50/m² | from 0.5 to 0.22 W/m²K |
| Above rafters 20cm | £85–130/m² | from 0.5 to 0.16 W/m²K |
| Top floor ceiling | £15–35/m² | from 0.8 to 0.18 W/m²K |
The Heating Load Calculator
Our Heating Load Calculator calculates transmission heat losses automatically:
- Component catalogue with over 150 typical constructions
- Automatic U-value determination by construction period
- Correction factors for ground contact, unheated spaces etc.
- Thermal bridge surcharges by construction standard
Calculate now: Determine the transmission heat losses of your building with our Heating Load Calculator.
Further Reading
- The U-Value Explained – The most important component parameter in detail
- Thermal Bridges – The hidden heat losses
- Ventilation Heat Losses – The other component of heating load
- What is Heating Load? – Back to overview
Sources
- DIN EN 12831-1:2017-09 – Method for calculation of the design heat load
- DIN 4108-2 – Thermal protection in buildings
- DIN EN ISO 6946 – Building components – Thermal resistance and U-value