A heating load calculation is more than just a figure for heating system sizing – it is an X-ray of your building. The detailed results show exactly where heat is lost and where renovation measures have the greatest effect. In this article, you will learn how to use this information for targeted renovation planning.

Understanding the Building Envelope

Heat Escapes in Three Ways

Loss Path	Typical Share	Main Causes
Transmission	70–85%	External walls, windows, roof, floor
Ventilation	15–25%	Air exchange, leaks
Thermal bridges	5–15%	Connections, penetrations

The heating load calculation breaks down these losses in detail – the foundation for any renovation planning.

The U-Value as Key Figure

The thermal transmittance (U-value) describes how much heat escapes through a component:

Q = U × A × ΔT

Symbol	Meaning	Unit
Q	Heat loss	Watts (W)
U	U-value	W/(m²·K)
A	Area	m²
ΔT	Temperature difference	Kelvin (K)

U-Values of Typical Components

Component	Unrenovated	After GEG 2024	Passive House
External wall	1.0–1.8	≤ 0.24	≤ 0.15
Roof/top floor ceiling	0.6–1.2	≤ 0.20	≤ 0.15
Basement ceiling	0.8–1.5	≤ 0.25	≤ 0.15
Windows	2.5–3.5	≤ 1.10	≤ 0.80
External door	2.5–4.0	≤ 1.80	≤ 0.80

Identifying Weak Points

The Heating Load Calculation as Diagnostic Tool

From the component-by-component breakdown, you can directly read:

1. Which components cause the highest losses?
2. How far do the U-values deviate from the GEG standard?
3. Where is the savings potential greatest?

Priorities by Loss Share

Loss Share	Priority	Typical Components
> 30%	Very high	External walls (large area)
15–30%	High	Windows, roof
5–15%	Medium	Basement ceiling, doors
< 5%	Low	Individual thermal bridges

Example: Loss Distribution in an Old Building

Component	Area	U-Value	Loss	Share
External wall	150 m²	1.2	6,300 W	42%
Windows	30 m²	2.8	2,940 W	20%
Roof	80 m²	0.8	2,240 W	15%
Basement ceiling	80 m²	0.9	2,160 W	14%
Ventilation	-	-	1,350 W	9%
Total	-	-	15,000 W	100%

Renovation Suggestions in the PV-Calor Heating Load Calculator

Our heating load calculator automatically analyses the optimisation potential of each component group based on GEG 2024:

Automatic analysis of savings potential according to GEG standards

The Analysis in Detail

For each component group, the calculator shows:

Key Figure	Description
Area	Total area of the component group
U-value ACTUAL (Ø)	Current average U-value
U-value TARGET (GEG)	GEG requirement for component renovation
Energy savings	Annual savings in kWh/a
Heating load reduction	Reduction of design heating load in kW

Overall Potential

The header summarises the total savings potential:

• Total energy savings: Possible annual savings if all measures are implemented
• Total heating load reduction: Possible reduction of heating load
• Heating degree days (Kd): Climate basis of the calculation

The Most Important Renovation Measures

1. External Wall Insulation

The external wall is often the largest cause of heat losses.

Insulation systems compared:

System	Insulation Thickness	U-value after	Cost/m²
ETICS (External thermal insulation composite system)	14–20 cm	0.18–0.24	€120–180
Ventilated facade	16–24 cm	0.15–0.20	€180–280
Internal insulation	6–10 cm	0.35–0.50	€80–130
Cavity wall insulation	4–8 cm	0.30–0.40	€25–50

Advantages of external wall insulation:

• Highest savings potential (often 25–35% of heating energy)
• Improved living comfort (warm interior walls)
• Protection of building fabric

To consider:

• Plan window connections carefully
• Choose adequate insulation thickness (one-time costs!)
• Check heritage protection and design regulations

2. Roof Insulation / Top Floor Ceiling

Warm air rises – an uninsulated roof is a huge energy waster.

Options:

Variant	Application	U-value	Cost
Between-rafter insulation	Occupied attic	0.18–0.24	€50–80/m²
Above-rafter insulation	During roof renovation	0.14–0.18	€150–250/m²
Top floor ceiling insulation	Unheated attic	0.14–0.20	€30–60/m²

Special case top floor ceiling:

• Cheapest measure with very high benefit
• Often feasible as DIY project
• Mandatory under GEG upon change of ownership

3. Basement Ceiling Insulation

Cold feet on the ground floor? The basement ceiling is often the culprit.

Variant	Insulation Thickness	U-value	Cost
Underside insulation	8–12 cm	0.25–0.30	€35–55/m²
Topside insulation	3–6 cm	0.40–0.50	€50–90/m²

Advantages:

• Easy implementation with accessible basement ceiling
• Noticeable comfort improvement (warm feet!)
• Relatively inexpensive

4. Window Replacement

Old windows are often the largest individual weak points:

Window Type	Uw-Value	g-Value	Cost
Single glazing	5.0–5.5	0.85	-
Double glazing (old)	2.5–3.0	0.75	-
Double low-e	1.1–1.3	0.60	€300–450/m²
Triple low-e	0.6–0.9	0.50	€400–600/m²

Important when replacing windows:

• Consider g-value (solar gains)
• Account for frame proportion and installation situation
• Consider facade insulation at the same time

5. Airtightness and Thermal Bridges

Often underestimated, but important:

Measure	Savings Potential	Cost
Roller shutter box insulation	1–3%	€50–150/unit
Window/door sealing	1–2%	€10–30/window
Radiator niche insulation	0.5–1%	€30–60/niche
Pipe insulation	2–5%	€10–20/m

Economic Viability and Prioritisation

Cost-Benefit Ratio

Measure	Savings	Cost	Payback
Top floor ceiling insulation	10–15%	€30–60/m²	3–6 years
Basement ceiling insulation	5–10%	€35–55/m²	5–8 years
External wall insulation	20–35%	€120–180/m²	12–20 years
Window replacement	10–15%	€400–600/m²	15–25 years
Roof insulation	15–25%	€100–200/m²	10–15 years

Prioritisation Matrix

Criterion	Highest Priority	Medium Priority	Low Priority
Savings potential	> 20%	10–20%	< 10%
Payback	< 8 years	8–15 years	> 15 years
Comfort improvement	High	Medium	Low
Due anyway	Yes	Partly	No

The Right Sequence

Recommended renovation sequence:

1. Top floor ceiling / Roof

   • Inexpensive, high impact, fast payback

2. Basement ceiling

   • Inexpensive, noticeable comfort improvement

3. External wall + Windows (together!)

   • Greatest potential, but expensive
   • Only sensible when facade renovation is planned anyway

4. Ventilation system

   • Important after improving airtightness
   • Heat recovery saves additional energy

GEG Requirements for Renovation

Conditional Requirements

The GEG 2024 only prescribes U-values when you renovate anyway:

Component	Required U-value	Trigger
External wall	≤ 0.24 W/(m²·K)	Renewal of > 10% of area
Roof	≤ 0.20 W/(m²·K)	Renewal of roof covering
Top floor ceiling	≤ 0.24 W/(m²·K)	Mandatory upon change of ownership
Windows	≤ 1.10 W/(m²·K)	Window replacement
External door	≤ 1.80 W/(m²·K)	Door replacement

Exceptions

• Listed buildings
• Economic non-viability (to be proven case by case)
• Technical impossibility

Subsidy Opportunities 2025

Federal Funding for Efficient Buildings (BEG)

Measure	Subsidy Rate	Max. Subsidy Amount
Individual envelope measures	15%	€30,000/dwelling
+ iSFP bonus	+5%	-
Heating replacement (HP)	30–70%	€30,000/dwelling
Complete renovation EH	5–25%	€150,000/dwelling

Tax Incentive

Variant	Rate	Period
§ 35c EStG	20%	Spread over 3 years

Important Prerequisites

1. Involve energy efficiency expert (for BEG)
2. Submit application before starting the measure
3. Meet technical minimum requirements
4. Engage qualified contractors

From Calculation to Implementation

Step-by-Step Approach

1. Carry out heating load calculation

   • Component-by-component analysis
   • Document U-values

2. Identify weak points

   • Components with highest losses
   • Deviation from GEG values

3. Create renovation roadmap

   • Prioritise measures
   • Plan timing

4. Check economic viability

   • Determine costs
   • Factor in subsidies
   • Calculate payback

5. Apply for subsidies

   • Involve energy consultant
   • Submit applications

6. Implementation

   • Engage specialist contractors
   • Quality control

After Renovation

• New heating load calculation
• Adjust heating system (often smaller possible)
• New hydraulic balancing
• Monitor success (consumption monitoring)

Conclusion

Determine your potential now: Go to the Heating Load Calculator with Renovation Analysis

Further Reading

• Understanding Heating Load Results
• Radiator Optimisation for Heat Pumps
• Guide to Heating Load Calculation

Sources

• Building Energy Act (GEG) 2024
• DIN EN 12831-1: Heating load calculation
• DIN 4108: Thermal insulation in buildings
• BAFA: Federal funding for efficient buildings
• dena: Renovation roadmap for residential buildings