Interior mechanism of a simple hygrometer. As humidity rises, the coil expands and moves a needle. Source: Wikimedia Commons.
What Relative Humidity Means
Relative humidity (RH) expresses how much water vapour the air currently holds as a proportion of the maximum it could hold at that temperature. At 50% RH and 20 °C, the air contains half the moisture it could carry before condensation begins on cold surfaces. The same mass of water vapour at 10 °C would push RH well above 80%, which is why an unheated room in a Polish winter feels damp even with no obvious moisture source.
The relationship between air temperature and moisture capacity follows a non-linear curve. Outdoor winter air in central Poland typically carries very little absolute moisture — cold air simply cannot hold much. When that air enters a heated interior and warms up, its RH drops sharply, often to 20–30% without any deliberate humidification. That dry condition affects respiratory mucosa and wooden furniture alike.
The 40–60% RH Window
Several bodies, including the World Health Organization and ASHRAE Standard 55, describe a comfort and health range broadly centred on 40–60% RH for occupied residential spaces. That window reflects a balance: below 30% RH, mucous membranes dry out and some viruses transmit more efficiently through the air; above 65–70% RH, mould colonies begin to establish on permeable surfaces within days.
ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy) identifies 30–60% as the acceptable humidity range for winter conditions in mechanically heated spaces. The WHO's housing guidelines reference the 40–70% band as the target for general residential health.
In Polish apartments heated with district heating (ciepłownictwo miejskie), where occupants have limited control over supply temperature, RH in winter regularly falls to 25–35% unless a humidifier is running. Conversely, in poorly ventilated bathrooms or kitchens without exhaust fans, localised RH can exceed 80% for extended periods after showering or cooking.
How Hygrometers Work
Consumer-grade humidity sensors fall into two main categories: capacitive and resistive. Capacitive sensors — the type found in most digital weather stations and standalone hygrometers sold in Polish electronics shops — use a polymer film whose dielectric permittivity changes with absorbed moisture. They typically offer ±2–3% RH accuracy over the 20–80% range when new and properly calibrated.
Resistive sensors use a hygroscopic material between two electrodes. Electrical resistance through that material changes with moisture absorption. They are cheaper to produce but drift more over time, particularly after exposure to high humidity, and may require periodic recalibration against a salt solution standard.
Mechanical (Hair) Hygrometers
Older mechanical instruments use a bundle of human or synthetic hair that lengthens as it absorbs moisture. The interior of the hygrometer shown at the top of this article uses exactly that principle — a coil expands and contracts with humidity changes, moving a needle across a dial. These instruments are accurate to roughly ±5% in the 40–70% range under stable conditions, but they respond slowly (several minutes per significant step) and must be recalibrated seasonally in climates with large seasonal swings like Poland.
Placement Considerations
Sensor placement significantly affects readings. A hygrometer placed near an exterior wall receives colder surface radiation, which can make localised RH appear higher than room-average conditions. Central room placement at roughly 1.2–1.5 m height — away from windows, exterior walls and directly above heat sources — gives the most representative reading of occupied-zone conditions.
Seasonal Patterns in Poland
Polish residential humidity follows a recognisable annual pattern linked to heating season:
| Period | Typical indoor RH (heated apartment) | Main risk |
|---|---|---|
| November – February | 20–40% | Excessive dryness, respiratory discomfort |
| March – May | 40–55% | Usually comfortable range |
| June – August | 50–70% | Elevated humidity in poorly ventilated spaces |
| September – October | 45–60% | Transition; watch for condensation on cold surfaces |
Condensation as a Field Indicator
Visible condensation on window glass is one of the most immediate signs that surface temperature has dropped below the dew point of indoor air. In a typical Polish apartment with double-glazed windows, condensation appears on the inside of the frame (not the glass pane itself) when the frame's thermal performance is lower than the glass — a common situation with older PVC or aluminium frames installed without thermal breaks.
The dew point corresponding to common indoor conditions:
When window frame surface temperature falls below the dew point — possible during cold Polish nights even with adequate heating — moisture deposits on those surfaces. Regular wiping removes the water but does not address the cause. Improving frame insulation, increasing ventilation rate or reducing indoor moisture sources (drying laundry indoors is a significant contributor) are the interventions that actually shift conditions.
Mould Risk in Polish Housing Stock
The Polish housing stock includes a substantial proportion of large-panel concrete buildings (wielka płyta) constructed between the 1960s and 1980s. These buildings have known thermal bridging at panel joints, at balcony connections and at window installations. Surface temperatures at these bridges are measurably lower than mid-panel surfaces — sometimes 4–8 °C cooler — making them nucleation points for condensation and mould even when room-average humidity appears acceptable.
Mould colonies on interior surfaces in these locations are not evidence that overall humidity is too high; they indicate that localised surface temperatures are too low relative to current indoor air moisture content. Thermal imaging during heating season is the accurate diagnostic tool. Hygrometer readings alone cannot identify bridge locations.