🌬️ In brief: After an energy-efficient renovation, a ventilation concept per DIN 1946-6 is often mandatory. Controlled residential ventilation (KWL) with heat recovery saves 15–25% heating energy, prevents mold, and is subsidized up to 20%.
Why Ventilation After Renovation Is So Important
New windows, an insulated facade, and a sealed roof — after renovation, the building envelope is significantly more airtight than before. That's great for energy consumption but bad for indoor air: the natural infiltration that worked through leaky windows and gaps in old buildings is gone.
The consequences: Moisture from cooking, showering, and breathing stays in the room. Without sufficient air exchange, relative humidity rises to levels where mold growth becomes inevitable — especially on cold exterior walls, window reveals, and behind furniture.
The German Building Energy Act (GEG) and DIN 1946-6 address this: Anyone who modifies the building envelope must ensure minimum air exchange. In practice, this is almost only achievable with controlled ventilation.
Ventilation Concept per DIN 1946-6: When Is It Required?
A ventilation concept is mandatory whenever you replace more than one-third of windows or seal more than one-third of the roof area in an existing building. The planner — typically your energy consultant — then calculates whether the minimum air exchange for moisture protection (LzF) can be achieved without technical measures.
The ventilation concept checks:
- ✓ Is infiltration (residual air leakage) sufficient for moisture protection?
- ✓ Is a ventilation measure necessary — and if so, which level?
- ✓ Four ventilation levels: moisture protection, reduced ventilation, nominal ventilation, intensive ventilation
- ✓ The concept is a mandatory document — but not yet a system solution.
If the concept determines that a ventilation measure is necessary, you must act. The simplest level would be window trickle vents — but in practice, we usually recommend a KWL with heat recovery because it delivers significantly more benefits.
Central vs. Decentralized Ventilation
Central Ventilation System
A central unit supplies all rooms with fresh air via a duct network and extracts stale air. Particularly efficient and quiet — but only practical when ducts can be installed (new builds, core renovations, suspended ceilings).
Decentralized Ventilation Units
Individual units in the exterior wall that work in pairs: while one supplies air, the other extracts, then they switch. Ideal for renovations since no duct network is needed. Each room is supplied individually.
💡 Rule of thumb: Decentralized units are the pragmatic solution for single-room renovations or when ductwork isn't feasible. For core renovations or when ceilings are being opened anyway, a central system is almost always worthwhile.
What Does Heat Recovery Achieve?
The core of every modern ventilation system is the heat exchanger: it extracts up to 90% of thermal energy from warm exhaust air and transfers it to cold supply air. The air is exchanged, but the heat stays in the house.
Calculation example — single-family home, 140 m²:
- ✓ Ventilation heat loss without HR: approx. 4,500 kWh/year
- ✓ With 85% heat recovery: only approx. 675 kWh/year
- ✓ Savings: approx. 3,825 kWh/year ≈ €500–600 with gas, €800–1,000 with oil
- ✓ System payback: 8–15 years (depending on system type and energy price)
Beyond energy savings, a KWL with heat recovery offers significant comfort benefits:
Subsidies for Ventilation Systems
Ventilation systems with heat recovery are subsidized as BEG individual measures via BAFA:
- ✓ 15% base subsidy on eligible costs
- ✓ +5% iSFP bonus when the measure is recommended in the renovation roadmap
- ✓ = up to 20% grant — on a €10,000 investment, that's €2,000 in subsidies
- ✓ Max. eligible costs: €30,000 per unit (€60,000 with iSFP)
- ✓ KfW supplementary loan (No. 358/359): low-interest loan up to €120,000 per unit
- ✓ ALWAYS apply before starting the work!
Important: The ventilation system must be installed by a certified contractor and meet certain minimum requirements (e.g., heat recovery rate ≥ 80% for central systems). We verify this as part of the energy consultation.
Common Planning Mistakes
💡 Our Tip: Consider Ventilation From the Start
You get the best results when ventilation is integrated into the renovation plan from the start — not as an afterthought. Planning insulation, windows, and ventilation together lets you incorporate ductwork into the construction schedule, size the heating system optimally, and maximize subsidies.
Especially in a complete renovation with iSFP, ventilation can be planned as an individual measure with the highest subsidy rate. This saves not only energy but also money on the investment.
Do You Need a Ventilation Concept?
We create the ventilation concept per DIN 1946-6, advise on system selection, and guide you through the subsidy process.