Indoor Humidity in Winter: Why It Drops to 20–30 RH and How to Control It (Heat Exchanger Comparison + Practical Solutions)

date_range 18 January 2026

Indoor Humidity Balance During the Heating Season: Real Situations, Practical Solutions and Common Questions

Air that is too dry or too humid – these are two common problems affecting thousands of homes during the heating season.

This is especially relevant in new or renovated homes with mechanical ventilation systems.

In some homes, relative humidity drops to 15–20 %, while in others windows fog up and RH rises above 60 %.

This article is not about theory for its own sake, but about real-life situations:

✅ Why this happens.
✅ What you can do about it.
✅ How to choose the right solution for your home and climate.

Why This Topic Matters So Much

Cold outdoor air contains very little absolute moisture
Ventilation “carries” that moisture out of the home
New and renovated homes are very airtight
Different heat exchanger types behave very differently

👉 That’s why indoor humidity isn’t random — it’s the result of balance between ventilation, people and the building itself.

How Humidity Changes When Outdoor Air Is Heated to 22 °C

Below are typical, realistic cold-season scenarios. We assume the outdoor air is heated to 22 °C without adding or removing moisture (so the relative humidity drops).

Very dry <20% Dry 20–30% Comfort zone 30–50%

Outdoor conditions	Typical outdoor RH	RH after heating to 22 °C
+10 °C, overcast	~85%	~39–40%
+5 °C, rainy	~95%	~30–32%
0 °C, sleet	~90%	~20–22%
–5 °C, snowing	~80%	~12–14%
–10 °C, clear skies	~60%	~6–8%

👉 Even “humid” outdoor air can become very dry once heated to 22 °C indoors. Ventilation without moisture recovery can amplify this effect even further.

Heat Exchanger Comparison: Supply-Air RH After Heating to 22 °C

Assumptions for this comparison: indoor air is 22 °C and ~40% RH (return air), outdoor RH reflects a typical cold-season scenario, and the supply air leaving the heat exchanger is heated to 22 °C. The table shows the supply-air RH after heating.

Very dry <20% Dry 20–30% Acceptable in winter 30–40% Comfort zone 40–55% High >55%

Note: “comfort” is often quoted as 40–60% RH, but in winter (especially in airtight homes), 30–40% is often a realistic and still acceptable range (particularly during cold spells). Above 55–60% in winter, the risk of window condensation and mould increases, so such values should be treated with caution.

Outdoor conditions	Plate (condensing) moisture recovery 0%	Rotary (condensing) up to 20%	Plate (enthalpy) 60–73%	Sorption rotor 70–83%
+10 °C, cloudy (typically ~85% RH)	39.5%	39.5–41.0%	44.0–46.0%	45.0–47.0%
+5 °C, rainy (typically ~95% RH)	31.4%	31.4–33.1%	36.6–37.7%	37.4–38.5%
0 °C, sleet (typically ~90% RH)	20.9%	20.9–24.7%	32.3–34.8%	34.3–36.7%
–5 °C, snowing (typically ~80% RH)	12.8%	12.8–18.2%	29.1–32.7%	31.8–35.4%
–10 °C, clear skies (typically ~60% RH)	6.5%	6.5–13.2%	26.6–31.0%	30.0–34.3%

✅ Without moisture recovery, winter supply air can become extremely dry after heating (often 6–13% RH during cold spells).
✅ Enthalpy and sorption solutions usually raise supply-air RH to a more acceptable level (often around 30–40% RH, and higher in milder conditions).
⚙️ In real systems, defrost cycles, airflow balance and filter condition can shift the numbers — so correct setup and control still matter.

FAQ: Common Situations and Practical Fixes

Rotary or plate (condensing) heat exchanger, indoor RH ~20%

What’s happening

Moisture recovery is too limited for the season, so once the supply air is warmed up it becomes very dry. Ventilation airflow is not “one number for everyone” — it depends on the building, the number of occupants, and the operating limits of the unit itself.

What to do

reduce ventilation intensity during cold nights, if your controls and comfort allow it (for some units, you may already be close to the minimum safe airflow)
enable an “anti-dry” / humidity protection mode, if your unit offers one
check for airflow imbalance between supply and extract
consider upgrades: an enthalpy heat exchanger or an additional humidifier

⚙️ reduce airflow at night ✅ anti-dry mode ⏳ check airflow balance

👉 Living at ~20% RH long-term is discomfort — not “normal”.

Enthalpy heat exchanger, but indoor RH still dropped to ~30%

What’s happening

This can be normal during the heating season, especially during cold spells (around –10 °C and below). Even with moisture recovery, outdoor air contains very little moisture, so indoor RH may still drop to ~30%.

What ~30% RH means

often still considered the lower edge of comfort, especially in winter
many people already feel dryness (skin, eyes, throat)

Tips

avoid overheating your rooms (higher temperature often lowers RH even more)
reduce ventilation during cold nights or enable an “anti-dry” mode, if your unit has one
temporarily raise RH with a humidifier to ~35–40%

⚙️ reduce airflow at night ⏳ anti-dry mode ✅ humidifier to 35–40%

👉 An enthalpy exchanger doesn’t perform miracles — it reduces moisture loss, but in cold weather RH can still drop.

Sorption rotor (high moisture recovery), but windows fog up — or moisture builds up

What’s happening

A sorption (enthalpy) rotor returns a lot of moisture (typically ~70–83%), so indoor humidity can stay higher in winter.
If the home produces more moisture (more people, long showers, cooking, drying laundry), RH can rise further.
When cold surfaces (windows, corners) reach the dew point, condensation appears — and in some setups, moisture may also accumulate inside the unit during certain operating conditions.

What to do

check whether ventilation is too low at night and after showers / cooking; increase airflow when moisture spikes
reduce moisture sources: shorter showers, use kitchen extraction, avoid drying laundry indoors when possible
improve air movement near windows (open curtains/blinds, don’t block radiators, guide supply air)
if condensation happens only in specific spots, check thermal bridges and window installation; also verify airflow balance (supply vs extract) and defrost settings

⚙️ adjust airflow when needed ✅ reduce moisture sources ⏳ check cold spots / balance

👉 High moisture recovery is great for comfort — but if your home generates a lot of moisture, you may need smarter airflow control to avoid condensation on cold surfaces.

New home, RH ~70%: enthalpy plate or sorption rotor

What’s happening

This is very common in new-build homes. Building materials may not be fully dry yet, so a large amount of additional moisture is continuously released indoors.

a home can take roughly 6–24 months to dry out
a lot of construction moisture is released during this period

Recommendations

temporarily increase ventilation to remove construction moisture more effectively
avoid maximum moisture recovery (limit enthalpy/sorption recovery if your unit allows it)
during the first year, it’s often better to use lower moisture recovery (for example, a standard plate exchanger), or run an enthalpy/sorption unit in a mode that prioritises drying

⚙️ temporary setup ⏳ drying phase ✅ more ventilation

👉 A new home doesn’t always need “maximum humidity retention” — early on, removing construction moisture efficiently is usually the priority.

When Do You Need a Humidifier?

Consider a humidifier if

wintertime RH is consistently <25–30%
you experience discomfort such as dry skin, dry eyes or irritated airways
reducing ventilation is not possible (or indoor air quality suffers when you do)

Important

a humidifier usually treats the symptom
but it doesn’t remove the root cause (very dry outdoor air + the drying effect of ventilation)

✅ aim for 35–40% RH ⏳ ongoing maintenance ⚙️ coordinate with ventilation

👉 The ideal setup is moisture recovery + a small humidifier (used only when it’s truly needed).

Simple Ways to Increase Indoor Humidity

add more houseplants (the effect is more noticeable in smaller rooms)
steam from cooking (don’t extract everything immediately, but ventilate afterwards if windows start fogging)
temporarily raise RH with a humidifier (for example, up to ~35–40% RH)
dry laundry indoors (in moderation, while watching for window condensation)
lower ventilation at night or use an “anti-dry” mode, if available

✅ Practical winter target: in most homes, keeping around 35–45% RH is enough for comfort without increasing the risk of window condensation.

Simple Ways to Reduce Indoor Humidity

increase ventilation (especially for short periods after showers or cooking)
use your kitchen hood and bathroom extraction as intended
avoid drying laundry indoors, or do it only briefly
reduce the number of plants if you have many and RH stays consistently high
temporarily bypass moisture recovery (bypass mode), if your unit offers it
temporarily switch from an enthalpy exchanger to a standard plate (sensible) exchanger (if your unit allows it), to reduce moisture recovery so RH drops faster

👉 If wintertime RH stays above 55–60%, it’s worth reacting early — this reduces the risk of window condensation and mould.

Is it worth opening windows in winter?

Short answer: opening windows does not increase humidity.

In winter, outdoor air contains very little absolute moisture. When you open windows, the air entering the home is usually even “drier”, so indoor RH tends to drop rather than rise.

When opening windows can make sense

when indoor humidity is too high (for example, when windows are fogging up)
when using an enthalpy or sorption heat exchanger and you want to quickly bring RH down (short, intensive airing)

✅ when it’s too humid ⚙️ enthalpy / sorption ⏳ short-term effect

👉 If your goal is to reduce humidity, opening windows can help temporarily. If your goal is to increase humidity, windows won’t help.

Very Important: Ventilation Control

Regardless of the technology used:

reduce ventilation during cold nights (if your controls allow it and indoor air quality is not compromised)
use “anti-dry” or humidity control modes, if available
regularly replace filters to keep airflow stable and system behaviour predictable

👉 Even the best system won’t work properly if it’s out of balance.

Final takeaway — in one sentence

Indoor humidity is neither “good” nor “bad” by itself. It needs to be balanced for the building, the people living in it, and the climate. When that balance is right, comfort follows naturally.

🧠 Recommendations by Climate Zone: choosing the right heat exchanger

Indoor humidity depends strongly on climate, outdoor temperatures and the length of the heating season. Below are practical, experience-based recommendations by region — focused on balancing comfort, moisture and energy efficiency.

❄️ Scandinavia (NO, SE, FI)

Long, cold winters with very low absolute outdoor humidity.

✅ Best choice: enthalpy plate or sorption rotor
Helps prevent extremely dry indoor air during prolonged cold periods.

❄️ Baltic States (LT, LV, EE)

Cold winters with frequent sub-zero days and long heating seasons.

✅ Best balance for most homes: enthalpy plate

⏳ Sorption rotors suit very airtight A/A+ homes, but require good control to avoid excess humidity in milder weather.

🌬️ Central Europe (SK, SI, CZ, RO, HU)

Mixed winters: cold spells alternate with milder periods.

✅ Recommended: enthalpy plate for stable comfort

⚙️ If indoor moisture is already high (or the home is drying out after construction), a standard plate exchanger may be sufficient.

🌧️ Western coastal (DK, BE, NL)

Mild but humid winters, frequent rain, fewer extreme cold days.

✅ Often suitable: standard plate or rotary with low moisture recovery

⏳ Enthalpy/sorption can work, but monitor RH to avoid window condensation.

🌦️ France & Luxembourg

Transitional climate with regional differences.

✅ Colder inland areas: enthalpy plate often works best

⚙️ Milder/humid regions: standard plate + good airflow control is usually sufficient.

🌤️ Southern zones (HR, N-IT, N/central ES)

Shorter heating season and moderate winter temperatures (varies by area).

✅ In many cases: standard plate exchanger is enough

⚙️ Enthalpy/sorption helps mainly in very airtight homes or when occupants feel winter dryness.

🏔️ Mountain regions (Alps, Carpathians, Tatras, Pyrenees)

Cold winters, strong winds and very low absolute outdoor humidity — even in Southern Europe.

✅ Recommended baseline: enthalpy plate exchanger

⏳ Very cold/exposed or ultra-airtight homes: sorption rotor can reduce moisture loss further, but needs careful control in milder periods.

🌬️ Ireland & United Kingdom

Mild, humid winters with limited frost.

✅ Usually best: standard plate or rotary with low moisture recovery

⏳ High moisture recovery is rarely needed and may increase condensation risk if ventilation is insufficient.

🌦️ Southern Australia

Many homes have a cooler winter season, while summer often brings air-conditioning. Conditions vary widely by location and building type.

✅ Solid baseline: standard plate exchanger + good control

⚙️ If winter heating causes noticeable dryness, an enthalpy plate exchanger can improve comfort. Very high moisture recovery is rarely necessary.

👉 There is no single “best” solution for every country. The right heat exchanger depends on climate, building airtightness, moisture sources and — most importantly — proper ventilation control.

Why choose CleanFilter?

Find your filter → cleanfilter.eu

✅ Verified performance. Our filters are tested to the ISO 16890 standard and reach the equivalent of F7-class performance — typically ePM1 70%. Some filters on the market only declare efficiency without providing real test results (or are not tested at all).
✅ Extra airtight construction. The filter media is bonded across the full frame width, helping prevent air and pollutants from bypassing through gaps. This reduces the risk of unfiltered air entering your ventilation system.
✅ A hygienic choice. Our filter media meets VDI 6022 requirements, helping reduce the risk of microbial growth and mould and supporting cleaner air at home.

Click to choose a filter for your ventilation unit model: cleanfilter.eu

Categories: AHU maintenance Indoor air quality

Manufacturer of MVHR	Zehnder
Manufacturer	CleanFilter
Air handling unit	ComfoAir Q 450 ComfoAir Q 350 ComfoAir Q 600
Filter dimensions	500x159x22
Class EN779	F7+G4
Class ISO16890	ePM1 65%
Protection level	High: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters	2
Advantages	Cleaner indoor air: mechanical filtration helps capture combustion (soot) and traffic-related particles without relying only on electrostatic charge. Easier airflow: up to 3× lower initial resistance compared to the previous material (internal lab tests). Proven performance: tested according to ISO 16890, made in the EU. Better protection from fine particles: ePM1 65% filtration (many original filters are typically ~ePM1 50–55%).
Manufacturer product number	400100118 400502015 400502013
Remarks	⚠️ Remarks: ComfoAir ≠ ComfoAir Q. Different dimensions, not interchangeable. Check your ventilation unit model/version before ordering.

Manufacturer of MVHR	Komfovent
Manufacturer	CleanFilter
Air handling unit	Domekt R 300 V
Filter dimensions	290x205x46
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	German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Absorbs volatile particles (odors) and harmful gases as NOx, O3, etc. Made in the EU – reliable quality and strict safety standards
Remarks	🌟 High-quality replacement filter: made from certified materials (Germany, SANDLER AG) with tested performance. Provides reliable air filtration and optimal service life.

Manufacturer of MVHR	Brink
Manufacturer	CleanFilter
Air handling unit	Brink Flair 325 Brink Flair 400
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	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Maximum protection – ePM1 70% removes up to 40% more smog, pollen and fine dust than standard filters (50–60%).
Manufacturer product number	532702 580616 532703
Remarks	ℹ️ This is a supply air filter. For extract air filtration, please order a separate G4/M5 filter (available in our shop).

Manufacturer of MVHR	Flexit
Manufacturer	CleanFilter
Air handling unit	FLEXIT Nordic S2 FLEXIT Nordic S3
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	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Maximum protection – ePM1 70% removes up to 40% more smog, pollen and fine dust than standard filters (50–60%).
Manufacturer product number	116597 116597

Manufacturer of MVHR	Paul
Manufacturer	CleanFilter
Air handling unit	Paul Climos 200 Paul Climos F 200 ECO Paul Climos F 200 Enthalpy
Filter dimensions	168x174x45
Class ISO16890	ePM10 55%
Protection level	Average: medium-sized solid particles, dust, fluff, pollen, mold spores.
Quantity of filters	2
Advantages	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Lower running costs – lower filter price, lower energy use, and usually longer time between replacements than higher-efficiency filters.
Manufacturer product number	5270004270

Indoor Humidity in Winter: Why It Drops to 20–30 RH and How to Control It (Heat Exchanger Comparison + Practical Solutions)

Indoor Humidity Balance During the Heating Season: Real Situations, Practical Solutions and Common Questions

Why This Topic Matters So Much

How Humidity Changes When Outdoor Air Is Heated to 22 °C

Heat Exchanger Comparison: Supply-Air RH After Heating to 22 °C

FAQ: Common Situations and Practical Fixes

Rotary or plate (condensing) heat exchanger, indoor RH ~20%

Enthalpy heat exchanger, but indoor RH still dropped to ~30%

Sorption rotor (high moisture recovery), but windows fog up — or moisture builds up

New home, RH ~70%: enthalpy plate or sorption rotor

When Do You Need a Humidifier?

Simple Ways to Increase Indoor Humidity

Simple Ways to Reduce Indoor Humidity

Is it worth opening windows in winter?

Very Important: Ventilation Control

Final takeaway — in one sentence

🧠 Recommendations by Climate Zone: choosing the right heat exchanger

Why choose CleanFilter?

Zehnder ComfoAir Q 350/450/600 F7+G4 Filter set (Efficient)

Domekt R 300 V 1xF7/AC Activated carbon filter (Odor)

Brink Flair 325/400 1xF7 Filter (Smog&Pollen)

FLEXIT Nordic S2/S3 F7+F7 Filter set (Efficient)

Paul Climos 200 M5+M5 Filter set (Standard)

Oxygen X-Air C180,C200,C250 / Easy C150,C200,C250 M5+M5 Filter set...

SAVE VTR 150 (PF) F7+M5 Filter set (Efficient)

Brofer RDCD25SK G4+G4 Filter set (Standard)

Questions? We’re here to help!

Information

List links

My account

Contacts

Klimtera JSC

Manufacturer of MVHR	Oxygen
Manufacturer	CleanFilter
Air handling unit	Oxygen X-Air C180 Oxygen X-Air C200 Oxygen X-Air C250 Oxygen Easy C150 Oxygen Easy C200 Oxygen Easy C250
Filter dimensions	205x205x46
Class EN779	M5+M5
Class ISO16890	ePM10 55%
Protection level	Average: medium-sized solid particles, dust, fluff, pollen, mold spores.
Quantity of filters	2
Advantages	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Lower running costs – lower filter price, lower energy use, and usually longer time between replacements than higher-efficiency filters.
Remarks	🌟 Only certified premium-class filtration material (SANDLER AG, Germany) is used in our filters. 🌟 High-quality replacement filter: made from certified materials (Germany, SANDLER AG) with tested performance. Provides reliable air filtration and optimal service life.

Manufacturer of MVHR	Systemair
Manufacturer	CleanFilter
Air handling unit	SAVE VTR 150 (PF)
Filter dimensions	302x137x130 307x131x130
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	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Maximum protection – ePM1 70% removes up to 40% more smog, pollen and fine dust than standard filters (50–60%).
Manufacturer product number	145777 211121 212120
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.

Manufacturer of MVHR	Brofer
Manufacturer	CleanFilter
Air handling unit	RDCD25SK RDCD25SKE RDCD25SKC RDCD25SKHE RDCD25SKHCE RDCD25SKHC RDCD25SKH
Filter dimensions	216x198x22
Class EN779	G4+G4
Class ISO16890	Coarse 75%
Protection level	Basic: larger solid particles, dust, fluff.
Quantity of filters	2
Advantages	Made in the EU – reliable quality and strict safety standards German filtration material – hygienic (VDI 6022) and PFAS-free, for cleaner and safer air ISO 16890 tested – performance verified by an accredited laboratory in Germany Lower running costs – lower filter price, lower energy use, and usually longer time between replacements than higher-efficiency filters.
Manufacturer product number	KFTR060A
Remarks	ℹ️ This MVHR unit has 3 filter slots: Supply air: G4 (mandatory) + optional F7/F8 (for pollen, smog, fine particles). Extract air: G4 (mandatory) 👉 The two G4 filters are always required. The F7/F8 filter is optional for extra protection. ℹ️ Additional F7/F8 supply air filter is sold separately and available in our shop.