Transmission Heat Losses - Summary

Transmission heat losses occur when heat flows through solid building components from inside to outside. They typically 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:

• Inside: 20°C (warm)
• Outside: -10°C (cold)
• Temperature difference: 30 K

The greater the temperature difference, the more heat flows outward.

The Calculation Formula

According to DIN EN 12831-1: ΦT = HT × (θi - θe)

Where:

• ΦT = Transmission heat loss (W)
• HT = Transmission heat loss coefficient (W/K)
• θi = Indoor temperature (°C)
• θe = Design outdoor temperature (°C)

Where Do the Greatest Heat Losses Occur?

Typical distribution of transmission heat losses:

• 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)

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