Table of Contents
Why Temperature and Humidity Matter in Every Irish Home
Temperature and humidity are closely connected, and this relationship explains almost all indoor humidity behaviour in Irish homes. Moreover, temperature influences how much moisture the air can hold, what relative humidity (RH) will be, when condensation forms, and how mechanical ventilation systems behave across the seasons.
Because many Irish households experience humid summers and dry winters, understanding this link helps diagnose comfort issues without blaming the ventilation system unnecessarily. As a result, homeowners and installers can make better decisions about airflow, heating, and moisture control.
For background reading, see:
Warm Air Holds More Moisture (Absolute Humidity & Temperature)
Warm air can hold far more moisture than cold air. This rule is universal and drives nearly all humidity behaviour in Ireland. Additionally, even a small rise in temperature increases moisture-holding capacity dramatically.
Approximate maximum moisture capacity
| Temperature | Max Moisture Capacity |
|---|---|
| 5°C | ~6 g/m³ |
| 10°C | ~9 g/m³ |
| 15°C | ~12 g/m³ |
| 20°C | ~17 g/m³ |
| 25°C | ~23 g/m³ |
Because of this, Irish winter air is cold and dry in absolute terms, even when outdoor RH looks high. Conversely, Irish summer air is warm and moisture-rich, even when RH seems moderate.
This is why:
- winter AH is ~4–6 g/m³
- summer AH is ~11–16 g/m³
- the home feels dry in winter and humid in summer
For more details, see:
Moisture sources inside a home → (A5)
Relative Humidity Depends Entirely on Temperature (RH & Temperature Relationship)
Relative Humidity (RH) does not measure how much moisture is in the air. Instead, it shows how “full” the air is relative to its maximum capacity at that temperature.
Therefore:
- Increase temperature → RH decreases immediately
- Decrease temperature → RH increases immediately
Example
- 10°C air at 80% RH
- Heat it to 20°C → RH drops to ~40%
- No moisture removed — only temperature changed
This is exactly what happens when cold Irish winter air is heated indoors: RH collapses, even though the AH stays the same.
This temperature effect is why relative humidity jumps in cool bedrooms, north-facing rooms, and underheated spaces.
Absolute Humidity Is the True Moisture Value (Moisture Content in Irish Climate)
Absolute Humidity (AH) measures the actual moisture content in grams per cubic metre. It does not depend on temperature and is the meaningful value for:
- mould risk
- condensation potential
- ventilation moisture removal
- indoor comfort
Typical Irish values:
- winter AH ≈ 4–6 g/m³
- summer AH ≈ 11–16 g/m³
This explains seasonal comfort differences more accurately than RH ever could.
How Temperature Controls Humidity in Real Irish Homes (Season-by-Season)
Let’s examine how temperature shapes indoor climate throughout the year.
Winter: Cold Air Heated Indoors → Relative Humidity Drops
- Outdoor: ~5°C at 90% RH → AH ~6 g/m³
- Indoors heated to 20°C → AH stays ~6 g/m³ → RH ~35%
Consequently:
- homes feel comfortably dry
- windows stay clear
- condensation disappears
- mould dries out
- clothes dry faster indoors
Because temperature increases moisture-holding capacity, RH falls dramatically.
For MVHR airflow performance:
Commissioning & airflow testing →
Summer: Warm Air Indoors → RH Remains High
- Outdoor: 16–20°C at 65–85% RH → AH ~12–16 g/m³
- Indoor: 19–22°C → AH stays the same → RH remains high (often 60–70%)
Therefore:
- homes feel clammy
- towels dry slowly
- bedrooms feel “heavy”
- MV cannot reduce humidity
- windows may mist overnight
This is explained in detail in Article about summer humidity in Ireland.
Why Cooler Rooms Always Have Higher RH (Condensation Risk in Cold Rooms)
Cooler rooms naturally have higher RH even when moisture is unchanged.
Example
- Air AH = 10 g/m³
- At 20°C → RH ~55%
- At 15°C → RH ~80%
This is why:
- unused rooms feel damp
- north-facing rooms develop mould sooner
- corners and external walls are most at risk
- slightly heating a room can drop RH by 5–10%
If your extractor vents or ductwork need maintenance:
Ventilation servicing →
Why MVHR Behaves Differently in Winter vs Summer (Humidity Removal Explained)
MVHR can remove moisture only when outdoor AH < indoor AH.
Winter
- cold air = very low AH
- heating reduces RH
- MVHR extracts moisture effectively
- indoor RH decreases naturally
Summer
- outdoor AH = same or higher than indoor
- MVHR brings moisture in
- RH stays high
This is normal and expected.
Learn more about MVHR airflow:
Heat recovery ventilation explained →
Why Dehumidifiers Work in Summer but MVHR Cannot Reduce Humidity
A dehumidifier:
- actively condenses water
- reduces absolute humidity
MVHR:
- cannot condense water
- cannot remove moisture in summer
- only exchanges air
Therefore:
To reduce humidity in summer → you must reduce AH → only a dehumidifier can do that.
If your home uses MEV or DCV:
MEV/DCV overview →
Practical Tips for Homeowners (Managing Temperature and Humidity Indoors)
✔ Warm cool rooms slightly
Even a 1–2°C increase can drop RH by up to 10%.
✔ Reduce MVHR boost during summer
Boosting brings in more humid air.
✔ Use a dehumidifier during humid weeks
Especially June–August.
✔ Avoid drying clothes indoors
Each laundry cycle releases 3–5 litres of moisture.
✔ Understand temperature–RH physics
Your home isn’t faulty — it’s reacting to Irish climate conditions.
For full summer moisture management:
Reduce humidity in summer → (Section C1 — upcoming)
Temperature and Humidity – Summary
Understanding the link between temperature and humidity helps homeowners manage indoor comfort all year. Warm air holds more moisture, so RH depends entirely on temperature. Absolute humidity explains mould risk, condensation behaviour, and MVHR performance. Because Irish winters are dry and summers are moisture-rich, indoor humidity changes naturally with the seasons.