high humidity indoors and why humidity is high in Ireland - humidity Ireland

Why Irish Homes Feel High Humidity Indoors During Summer

Most Irish homeowners notice a familiar pattern every year. During winter, indoor air feels dry. During summer, however, the same home often feels high humidity indoors, damp or clammy. Additionally, many people report that indoor Relative Humidity (RH) seems stuck between 60% and 75% from June to August. Although this feels worrying, it is almost always normal for the Irish climate.

As a result, homeowners often wonder whether something is wrong with their MVHR system, their ventilation settings, or even the house itself. However, once you understand how Irish summer air behaves, the cause becomes clear. This guide explains why Irish homes feel high humidity in summer, how humidity affects comfort, and what you can realistically do to manage it.

For background reading, you may want to review Relative vs Absolute Humidity here:
Understanding moisture terms →

What causes high indoor humidity in summer ?

Although winter air often shows a very high RH outdoors, it actually contains very little real moisture. Conversely, summer air looks only slightly humid, yet it carries far more water vapour. This contrast explains almost all summer dampness issues in modern homes.

Typical Irish winter conditions

Temperature: 2–8°C
Outdoor RH: 75–95%
Absolute Humidity (AH): ~4–6 g/m³

Typical Irish summer conditions

Temperature: 15–20°C
Outdoor RH: 65–85%
Absolute Humidity (AH): ~11–16 g/m³

Although RH appears lower in summer, the actual moisture load is more than double. Consequently, when this warm moist air enters your home, the absolute humidity stays high. Additionally, because indoor temperatures are mild, the RH rises further.

For a deeper explanation of winter moisture behaviour, see:
Why humidity drops in winter →

Why MVHR/MEV Cannot Remove High Humidity Indoors in Summer

Many people expect MVHR to “dry the air,” yet that is a misconception. An MVHR system is not a dehumidifier. It only exchanges indoor air with outdoor air through a heat exchanger. Because of that, the system can remove moisture effectively only when the air coming from outside is drier than the air indoors.

In winter

Outdoor air is extremely dry once heated. Therefore, every air change reduces indoor humidity. This process also aligns with the moisture-removal principles described in advanced humidity articles such as:
Why MVHR removes moisture in winter → (Section D1 — upcoming)

In summer

Outdoor air is usually wetter than indoor air. Therefore:

Every MVHR/MEV air change brings in more moisture.
Extracting stale air simply replaces it with equally humid air.
Boost mode increases humidity even faster.

Consequently, a healthy MVHR system will often make indoor humidity slightly higher in summer, not lower. This is expected and normal under Irish weather conditions.

If you suspect your system is not functioning correctly, you may check airflow performance here:
Ventilation commissioning & testing →

Why Homes Feel High Humidity Indoors in Summer Even at Mild Temperatures

Indoor comfort depends on both temperature and humidity. When moisture levels rise, evaporation from your skin slows down, making the air feel heavier and stickier.

Comfort comparison

21°C at 40% RH → feels fresh
21°C at 70% RH → feels muggy

Therefore, even mild Irish summer temperatures can feel uncomfortable. Bedrooms often feel worst because they stay cool and maintain high moisture absorption. Furthermore, bedding, fabrics, and mattresses tend to hold moisture, which increases the clammy sensation.

To understand why temperature changes humidity readings, see:
How temperature affects moisture →

Why Opening Windows Often Makes Things Worse

Opening windows feels like the right decision when a room feels “stuffy.” However, during Irish summers, this usually increases indoor RH because outdoor air commonly sits between 65% and 85% RH.

When you open windows:

Warm, humid outdoor air rushes inside.
Indoor RH rises quickly.
Cool surfaces absorb moisture.
Bedrooms can feel even more uncomfortable afterwards.

Therefore, window ventilation improves freshness but rarely helps with drying during summer months.

Why Winter Feels Dry Even When Outdoor Relative Humidity Ireland Is High

Winter air at 5°C and 90% RH contains very little actual moisture. Once this air enters your home and warms up to 18–21°C, its RH drops dramatically to around 30–45%. Consequently, the MVHR system brings in moisture-poor air all day, creating a clear drying effect throughout the home.

This natural winter drying:

Reduces condensation
Lowers mould risk
Keeps surfaces dry

Why Bedrooms Often Feel the Dampest in Summer

Bedrooms are often the most uncomfortable rooms during humid periods. Several factors combine to increase indoor RH overnight:

People breathe for 6–9 hours continuously in a closed space.
CO₂ rises, so people instinctively open windows.
Outdoor humidity flows indoors during cooler nighttime hours.
Bedrooms are generally cooler, which pushes RH upward.

North-facing rooms and older rooms with insufficient insulation often experience even higher humidity, especially during long periods of humid Irish weather.

If your MVHR system is underperforming, servicing may be required:
Professional ventilation servicing →

Why Dehumidifiers Work in Summer (and Mechanical Ventilation Doesn’t)

A dehumidifier actively removes moisture by condensing water out of the air. MVHR, by contrast, cannot remove moisture unless the outdoor absolute humidity is lower than indoor absolute humidity.

In most Irish summers:

Outdoor AH ≥ indoor AH
Mechanical Ventilation cannot reduce humidity
A dehumidifier is the only reliable tool

Because of this, many Irish households use a small portable unit in bedrooms or living rooms during humid weeks. MVHR continues to provide fresh air; the dehumidifier handles moisture removal.

For houses with continuous extract systems, you may compare MEV/DCV here:
MEV & DCV systems overview →

How To Reduce high indoor humidity in summer? – Practical Ways:

Below are realistic steps that align with Irish climate behaviour and modern ventilation guidance.

1. Use a dehumidifier during humid spells

Especially in bedrooms or living spaces. Even small units make a noticeable difference.

2. Avoid unnecessary MV boosting

Boost mode increases outdoor moisture intake. Use it only for showers and cooking.

3. Keep MV on normal or low speed

Lower airflow equals lower moisture intake from outside.

4. Use night ventilation only when suitable

Check outdoor conditions:

Temperature should be cooler than indoors
Outdoor RH should be lower

5. Reduce indoor moisture sources

Avoid:

Drying clothes indoors
Long unventilated showers
Leaving bathroom doors open afterwards

6. Keep indoor temperatures stable

Cooler rooms show higher RH. Increasing temperature by only 1–2°C can reduce RH by 5–10%.

To understand ventilation settings seasonally:
Seasonal ventilation guide → (Section C2 — upcoming)

Below are internal links using varied anchor texts, based on your existing sitemap URLs.

Summary. What Causes High Indoor Humidity In Ireland During Summer

Irish homes feel damp in summer because outdoor air carries far more moisture than winter air. Consequently, mechanical ventilation systems cannot dry incoming air, and opening windows often increases humidity. Bedrooms frequently feel the worst due to nighttime breathing and cool temperatures. Although this can feel uncomfortable, it is entirely normal for the Irish climate.

The best approach is simple:

Understand how summer humidity behaves
Avoid over-ventilating
Use a dehumidifier when needed
Manage internal moisture sources
Adjust MVHR settings seasonally

By following these steps, your home will feel fresher, drier, and more comfortable throughout the humid Irish summer.

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