What is a healthy summer humidity level in an Irish house?

Indoor humidity is usually described as relative humidity (RH) – the amount of water vapour in the air compared to the maximum it could hold at that temperature.

As a simple rule of thumb:

• Most homes feel comfortable around 40–60% RH.
• Short peaks above 60% (for example during showers) are normal.
• Persistent readings above 65–70% RH, combined with mould or musty smells, are a warning sign.

In summer, Irish outdoor air often carries a lot of moisture. Even if the temperature is only 18–22°C, high RH can make rooms feel stuffy and can encourage mould growth in corners, wardrobes and behind furniture.

A small digital hygrometer is the easiest way to check whether your “muggy” feeling is real or just temperature-related.

Why summer humidity levels in Irish homes often feel too high

Several common Irish conditions combine to push summer humidity up:

1. Moist climate and warm, moist outdoor air

In many parts of Ireland, outdoor RH is high for much of the year. When warm, humid air enters the house and is not properly extracted, it adds to internal moisture from cooking, showers and laundry.

2. Airtight upgrades without updated ventilation

Modern windows, doors, insulation and airtight membranes are excellent for energy efficiency. However, if the building was not designed with continuous mechanical ventilation, moisture and pollutants can build up quickly in summer when people keep windows closed for security or noise.

3. Lifestyle moisture: showers, cooking, laundry

• Long, hot showers
• Boiling and simmering without lids
• Drying clothes indoors

All of these release large amounts of water vapour indoors. Without good extraction, bedrooms and living rooms end up feeling damp and stuffy in the evening.

4. Poorly performing existing fans

Older intermittent fans or badly installed ductwork may move far less air than homeowners expect. A “running” fan is not always a compliant or effective fan – especially if it is clogged with dust, poorly ducted, or never set to the correct boost rate.

How ventilation systems control summer humidity

Ventilation does not magically “dry the weather”, but it can control indoor humidity by constantly removing moist air and replacing it with outdoor air.

MVHR (Heat Recovery Ventilation)

MVHR (Mechanical Ventilation with Heat Recovery):

• continuously extracts moist air from wet rooms,
• supplies filtered fresh air to living rooms and bedrooms, and
• in winter, recovers heat from the extract air.

In summer:

• MVHR still removes moisture generated indoors (showers, cooking, laundry).
• Many units have a summer bypass, which avoids recovering unnecessary heat when outside air is cooler.

MVHR is especially effective in airtight NZEB-level homes, where opening windows is less desirable due to noise, security or external pollution.

MEV and DCV systems

MEV (Mechanical Extract Ventilation) and DCV (Demand Controlled Ventilation):

• provide continuous extract from wet rooms,
• draw replacement air through background inlets or leakage paths, and
• can increase extract rate automatically when humidity rises (DCV).

In an Irish climate, a good MEV/DCV system is often the most practical way to stabilise summer humidity levels in existing homes and apartments.

Intermittent extract fans

Traditional bathroom or kitchen fans:

• run only when switched on or when the light is on,
• may not achieve the flow rates required under Part F if under-sized or badly ducted,
• rely heavily on user behaviour (boost use, keeping doors closed, etc.).

They can help with summer humidity if used correctly, but they rarely offer the same stability as properly designed whole-house systems.

Practical steps – managing summer humidity with your ventilation system

1. Measure first, then adjust

Place a hygrometer in:

• a main bedroom,
• the living area, and
• one bathroom.

Track RH and temperature for at least a week. If regular evening readings are above 65–70% RH, or if you already see mould spots, your existing strategy is not enough.

2. Run your system continuously

For MVHR and MEV/DCV systems:

• Keep the system running 24/7 on its normal background setting.
• Never switch it off “because it’s summer” – this allows moisture and pollutants to build up and can damage building fabric over time.

3. Use boost in a targeted way

Boost is your main control for short, intense moisture spikes:

• Turn on boost during showers and leave it on for 10–20 minutes afterwards.
• Use boost during heavy cooking and for a short period after.
• When drying clothes indoors in a utility room, close the door and run boost for longer.

Avoid leaving boost on all day in very humid weather – that can just pull more moist air through the house without solving the root cause.

4. Control moisture at source

Ventilation works best when it does not have to fight unnecessary moisture loads:

• Prefer outdoor drying or a vented/heat-pump dryer.
• Use lids on pots and pans.
• Close bathroom and utility doors when generating steam.
• Keep extract valves and grilles clear of towels, furniture and dust.

5. Combine with smart window use

You can still open windows with MVHR or MEV:

• On cool, less humid nights, “night purge” (cross-ventilation) can lower temperature and help the ventilation system.
• On very humid nights, windows are useful more for cooling than for drying, so continue to rely on your mechanical system to manage moisture.

When ventilation alone is not enough

In some situations, even a good ventilation system may need help:

• Very wet outdoor periods with no break in the weather
• Homes with unusually high internal moisture loads
• Existing buildings with cold surfaces that attract condensation

In these cases, you may consider:

• A dehumidifier in the most affected rooms, working alongside MVHR/MEV.
• Air conditioning for both cooling and some dehumidification, particularly in well-sealed, south-facing homes.

If humidity remains stubbornly high despite these measures, it is worth booking a ventilation service or commissioning test to:

• verify airflow rates and balance,
• check ductwork and filters, and
• confirm that the system is set up in line with Irish standards and manufacturer guidance.

When should you speak to a ventilation specialist?

Contact a qualified ventilation contractor if you notice:

• RH levels above 65–70% for long periods,
• persistent musty smells or visible mould, especially in summer,
• condensation on internal surfaces, or
• uncertainty about how to use your MVHR or MEV/DCV controls.

A professional can assess whether your current system is:

• correctly sized and commissioned,
• compliant with Part F requirements, and
• suitable for how the home is now used (after retrofits or layout changes).

Internal linking

How to Use Your Ventilation System →
How to use MVHR boost mode →
Best winter vs summer settings →
MVHR servicing →
Ventilation vs air conditioning →