Humidity, Foggy Windows and Mold – Causes and Solutions for Homes with a Ventilation Unit
🍂 During autumn and winter, many residents notice that windows fog up 💧, mold spots appear on walls 🦠, and humidity lingers indoors even though the AHU/MVHR is running. This is common, especially in November, when outdoor air is humid and we spend more time at home.
⚙️ There can be several causes, but the most common is insufficient airflow or a sub-optimal ventilation unit setup.
💨 1. Insufficient Airflow Indoors
If ventilation filters or AHU/MVHR filters are clogged, the airflow decreases, preventing proper removal of indoor humidity. As a result, windows start fogging up 💧, condensation builds up, and conditions become ideal for mold growth 🦠.
🔍 Most Common Causes:
🧩 Clogged filters in the ventilation unit
This accounts for about 80% of cases. When filters get dirty, the ventilation unit can no longer maintain proper airflow, and humidity remains indoors. ✅ Solution: increase the airflow (fan speed) and replace the filters. Even in new homes, filters may already be contaminated if ducts were left open during installation or renovation.
⚙️ Other components may be dirty
If the filters have already been replaced, it’s worth checking:
- 🌬️ outdoor air grilles
- 🔁 heat exchanger – inspection and cleaning if needed (once per year)
- 📦 air ducts
- 🎯 diffusers
- 🧱 additional pre-filters or exhaust filters
⚖️ Filters that are too efficient or an undersized ventilation unit
In some cases, installers choose a ventilation unit with too low capacity, and when higher-class filters (e.g., F7 or F9) are installed, airflow drops even further. ✅ Solution: temporarily use a lower-class filter (e.g., M5) or increase the fan speed if the ventilation unit allows it.
📚 Read more about ventilation unit (AHU/MVHR) maintenance here: https://cleanfilter.eu/blog/post/is-your-ventilation-unit-ready-for-the-heating-season
🔗 Read the article🏠 2. New House and Enthalpy Heat Exchanger
If you’ve recently moved into a new home, increased humidity 💧 can be a natural process — during construction, the building materials absorb a large amount of moisture that evaporates over several months. Enthalpy or sorption heat exchangers return part of this moisture back into the rooms, so the relative humidity can reach as high as 80–85%.
✅ Solutions:
- 💨 Increase ventilation temporarily — raise the ventilation unit (AHU/MVHR) speed and enable the “by-pass” function if available.
- 🪟 Open windows slightly from time to time for natural air exchange.
- ⏳ After 3–6 months, the situation usually stabilizes naturally.
💎 Why Choose CleanFilter Ventilation Unit Filters?
CleanFilter offers high-quality filters for AHU/MVHR systems, compatible with most ventilation units used across Europe. They help maintain optimal airflow 💨 and clean indoor air 🌿.
✅ Why Choose CleanFilter:
- 🇩🇪 Certified German materials compliant with the VDI 6022 hygiene standard, free from PFAS compounds.
- 📊 Proven efficiency based on testing, not just theoretical data.
- 🧱 High sealing quality – the filter prevents leakage of contaminants.
- 🚚 Fast delivery across the Europe – within 3-5 business days.
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FLEXIT Nordic S2/S3 F7+F7 Filter set (Efficient)
Ex VAT: 12.36 €
| Manufacturer of MVHR | Flexit |
| Manufacturer | CleanFilter |
| Air handling unit |
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| Filter dimensions | 166x272x48 |
| Class EN779 | F7+F7 |
| Class ISO16890 | ePM1 70% |
| Protection level | High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust. |
| Quantity of filters | 2 |
| Advantages |
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| Manufacturer product number |
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Zehnder ComfoAir Q 350/450/600 G4+G4 Filter set (Standard)
Ex VAT: 13.23 €
| Manufacturer of MVHR | Zehnder |
| Manufacturer | CleanFilter |
| Air handling unit |
|
| Filter dimensions | 500x159x22 |
| Class EN779 | G4+G4 |
| Class ISO16890 | Coarse 75% |
| Protection level | Basic: larger solid particles, dust, fluff. |
| Quantity of filters | 2 |
| Advantages |
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| Manufacturer product number |
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| Remarks | ⚠️ Remarks: ComfoAir ≠ ComfoAir Q. Different dimensions, not interchangeable. Check your ventilation unit model/version before ordering. |
Brink Flair 325/400 1xF7 Filter (Smog&Pollen)
Ex VAT: 19.94 €
| Manufacturer of MVHR | Brink |
| Manufacturer | CleanFilter |
| Air handling unit |
|
| Filter dimensions | 245x510x22 |
| Class EN779 | F7 |
| Class ISO16890 | ePM1 70% |
| Protection level | High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust. |
| Quantity of filters | 1 |
| Advantages |
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| Manufacturer product number |
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| Remarks | ℹ️ This is a supply air filter. For extract air filtration, please order a separate G4/M5 filter (available in our shop). |
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Domekt R 450 V 1xF7/AC Activated carbon filter (Odor)
Ex VAT: 26.05 €
| Manufacturer of MVHR | Komfovent |
| Manufacturer | CleanFilter |
| Air handling unit | Domekt R 450 V (Rego 450 VE) |
| Filter dimensions | 470x240x46 |
| Class EN779 | F7/AC |
| Class ISO16890 | ePM1 70% |
| Protection level | Particles and odors: fine particles, dust, pollen, mold spores, combustion particles, volatile particles, NOx, O3 gases. |
| Quantity of filters | 1 |
| Advantages |
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| Remarks |
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NIBE ERS 10-400 / ERS 10-500 F7+G4 Filter set (Efficient)
Ex VAT: 15.64 €
| Manufacturer of MVHR | NIBE |
| Manufacturer | CleanFilter |
| Air handling unit |
|
| Filter dimensions | 248x226x48 |
| Class EN779 | F7+G4 |
| Class ISO16890 | ePM1 70% |
| Protection level | High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust. |
| Quantity of filters | 2 |
| Advantages |
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Paul Novus 300/450 F7+G4 Filter set (Efficient)
Ex VAT: 19.25 €
| Manufacturer of MVHR | Paul |
| Manufacturer | CleanFilter |
| Air handling unit |
|
| Filter dimensions | 180x475x50 |
| Class EN779 | F7+G4 |
| Class ISO16890 | ePM1 70% |
| Protection level | High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust. |
| Quantity of filters | 2 |
| Advantages |
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| Manufacturer product number | 527003440 |
SAVE VSR 300 (PF) F7+M5 Filter set (Efficient)
Ex VAT: 20.88 €
| Manufacturer of MVHR | Systemair |
| Manufacturer | CleanFilter |
| Air handling unit | SAVE VSR 300 (PF) |
| Filter dimensions |
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| Class EN779 | F7+M5 |
| Class ISO16890 | ePM1 70% |
| Protection level | High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust. |
| Quantity of filters | 2 |
| Advantages |
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| Manufacturer product number | 25211 |
| Remarks | ℹ️ Dimensions are provided according to the Systemair standard (with gasket). Actual filter dimensions without the gasket may differ slightly. The filter is matched to your unit model. |
🧾 Summary
If your ventilation unit (AHU/MVHR) is running but windows are fogging up 💧 and you feel humidity indoors, start by checking the filters. In most cases, a simple filter replacement and airflow adjustment ⚙️ solve the problem within a few days.
🌿 Properly selected and maintained ventilation filters ensure not only clean air, but also reliable protection against mold 🦠 and excess moisture in your home.