Why Indoor Humidity Drops in Winter in Ireland (Explained Simply)

Even though Irish weather apps show 90% relative humidity, most homes experience much lower indoor humidity in winter — often between 45% and 55%. For many homeowners, this feels confusing. After all, if the outdoor air looks “so humid,” why does your home become noticeably dry?

The answer is straightforward once you understand how cold air behaves, how moisture capacity changes with temperature, and how ventilation systems move air through your home.

Cold Winter Outdoor Air Holds Very Little Moisture

This is the key principle behind falling indoor humidity.

During winter in Ireland:

• Outdoor temperatures drop to 0–10°C
• Air becomes colder and denser
• Cold air can hold very little moisture, even when RH reads 90%

Think of air as a bucket:

• Cold air = small bucket
• Warm air = big bucket

At 5°C, outdoor air might be 90% full, but that bucket is tiny. It contains only a small amount of real water vapour in absolute terms.

When Cold Outdoor Air Enters Your Home, It Warms Up

Even with modest heating, Irish homes stay around 18–21°C in winter. When cold outdoor air enters through MVHR, trickle vents, or natural leakage:

• It warms up immediately
• The bucket gets bigger
• The amount of water stays the same

So the relative humidity drops sharply.

Example:

• Outdoor air at 5°C and 90% RH contains ~6 g/m³ of moisture
• When warmed to 20°C, the same air could hold ~17 g/m³
• Its RH falls to around 30–40%

This is the main reason indoor humidity falls so dramatically — even when the weather app says the air outside is “very humid”.

MVHR Makes the Drying Effect Even Stronger

Mechanical Ventilation with Heat Recovery (MVHR) increases this natural drying effect.

MVHR:

1. Extracts humid indoor air from bathrooms, kitchens, and living areas
2. Brings in cold, dry outdoor air
3. Recovers heat from the outgoing air
4. Warms the fresh incoming air without adding moisture

Once warmed, the incoming air’s RH collapses.

So MVHR doesn’t remove moisture directly. It simply:

• Supplies already-dry winter air
• Warms it up
• Lowers RH even further

Most homes stabilise at 45–55% RH, which is ideal for comfort and health.

Heating and Winter Sun Reduce Humidity Even Further

Two more factors contribute to lower indoor humidity:

Home heating

Radiators, heat pumps, and stoves raise indoor temperature. Warmer air has a much bigger moisture-holding capacity, so RH falls.

Solar gain

Even weak winter sun warms internal surfaces, slightly increasing indoor temperature and reducing RH again.

These effects are small individually, but they add up.

Why Lower Winter Humidity Is Good for Irish Homes

Dryer indoor conditions offer several benefits:

• Less condensation on windows
• Lower mould and mildew risk
• Healthier indoor air
• Faster drying of surfaces and building materials
• Better comfort and reduced mustiness
• Optimal performance for MVHR systems

Irish homes gain these advantages naturally due to the cool winter climate.

Why High Outdoor RH Misleads Homeowners

Weather apps display relative humidity, which is a percentage.
Ventilation systems (including MVHR) respond to absolute humidity — the actual grams of water in each cubic metre of air.

In winter:

• Outdoor RH is high
• Outdoor temperature is low
• Outdoor absolute humidity is very low

So the air entering your home is cold and dry, not damp.

This is why your indoor humidity drops, even when the outside reading looks extreme (90–99%).

Summary

Indoor humidity drops in winter because cold Irish outdoor air contains very little moisture, and when that air is brought into your warm home, its relative humidity falls dramatically.

This is normal, healthy, and expected — especially in homes with MVHR.

Related winter reminders

• Humidity guides
• Temperature and humidity in Ireland
• RH vs AH explained
• Servicing and maintenance
• MVHR systems

External Links

• SEAI – Indoor Air Quality Resources
• Department of Housing – TGD Part F (Ventilation 2019)

If your winter humidity feels unusually low or you want a professional check of your Mechanical Ventilation settings, Eco Vent can assess, calibrate, and optimise your system for healthier indoor air.