Mould Growth Risk

What are the three conditions mould needs to grow?

Mould colonises surfaces where three conditions align: surface moisture (relative humidity consistently above 80%), available organic material (dust, skin cells, dander, textiles, paper, paint, present in every inhabited space), and temperature above freezing (optimal range 15–25°C). The critical insight for building designers is that only one of these three is controllable by the building envelope: moisture. Nutrients and warmth are inevitable in an occupied home, so design must focus entirely on keeping interior surfaces dry.

How does surface relative humidity differ from room humidity?

Mould risk depends on the relative humidity at the surface of the wall or window frame, not the humidity of the air in the middle of the room. A cold surface can have 95% relative humidity even when the room air measures 50%. This is why thermal bridges, uninsulated corners, window sills, balcony junctions, become mould hotspots in winter: their surface temperature falls far below the dew point, and moisture condenses on them regardless of whether the room feels damp. The governing equation relates surface temperature, room air temperature, humidity, and the heat-transfer characteristics of the thermal bridge itself.

Location	Reason for Cold Surface	Typical Mould Risk	Primary Design Fix
Window reveal in uninsulated wall	Thin masonry; no exterior insulation; direct thermal connection to outdoors	Very high (surface RH often exceeds 90%)	External insulation (ETICS); thermal-break reveal lining
External corner of masonry flat	Geometric thermal bridge; interior area smaller than exterior; two walls meet	High (surface RH 85–95%)	External insulation; corner insulation units
Bathroom wall with no extract	Moisture generation exceeds ventilation; no thermal bridge needed if RH saturates	High (poor ventilation, high RH everywhere)	Local extract fan; controllable background ventilation
Bedroom wall blocked by furniture	Furniture traps humid air; prevents surface drying; if wall is cold, RH exceeds 80%	Medium to high (localised; often unnoticed)	Allow air circulation; move furniture 5–10 cm from walls; insulate wall

What is the temperature factor (fRsi) and why does it define mould risk?

The temperature factor fRsi quantifies how cold an interior surface becomes relative to the temperature difference between indoors and outdoors. It is defined as: fRsi = (interior surface temperature minus outdoor temperature) divided by (indoor air temperature minus outdoor temperature). An fRsi of 0.75 means the interior surface is three-quarters of the way to room temperature; an fRsi of 0.5 means it is halfway to outdoor temperature and therefore much colder. For mould prevention in Slovakia's climate, standards require fRsi ≥ 0.6 to 0.65 at thermal bridges, depending on design indoor humidity (usually modelled at 50–60%). Higher values (fRsi 0.7–0.8) are safer and eliminate most mould risk even if ventilation is imperfect.

The fRsi value is calculated during the thermal design phase using standardised procedures (e.g., finite-element analysis of the detail, or nomographs in STN 73 0540). It becomes the criterion against which the thermal bridge detail is assessed. If a window sill detail achieves fRsi = 0.63 but the climate requires fRsi ≥ 0.65, the detail fails the mould-risk check and must be improved, usually by adding insulation or repositioning the window frame.

Why did replacing windows without ventilation cause a mould epidemic in Slovakia?

Before the 1990s, old Slovak apartment buildings had leaky windows and uncontrolled infiltration. This unplanned air leakage served a dual purpose: it supplied the combustion air needed by gas heaters and stoves, and it incidentally dried damp materials in walls and around window frames. When occupants replaced single-pane wooden windows with new airtight PVC windows, two things happened simultaneously:

1. The building tightened dramatically. Infiltration dropped, removing the drying effect. Interior humidity rose because cooking, bathing, and occupancy moisture had no escape path.
2. The new windows, though thermally better than the old ones, were often installed into the same uninsulated masonry walls and frames. Window sills and reveals remained at their original cold surface temperatures.

The result: a perfect storm. The surface relative humidity at window sills and corners climbed from 75% (with infiltration-driven drying) to 90–95% (with airtight windows and elevated indoor humidity). Within months, black mould bloomed on sills, reveals, and exterior corners. This was not a failure of the windows themselves, but a failure to address thermal bridges and to provide controllable ventilation to replace the uncontrolled infiltration that had masked the problem for decades.

What are the most common mould locations in Slovak homes and why?

Four scenarios dominate mould complaints in Slovak residential practice:

1. Window sill and reveal in a flat after windows are replaced. The old wooden frame was larger and recessed deeper into the masonry; the new PVC frame fits into a tighter opening. The reveal surfaces (the sides of the masonry between frame and exterior) are now exposed, cold, and in direct contact with outdoor air. Surface moisture reaches 90% within days of cold weather.
2. External corner of an apartment block. The corner is a thermal bridge where two exterior walls meet at a right angle. Geometrically, the interior corner area is smaller than the exterior, so heat flows preferentially outward. The interior surface temperature at an uninsulated corner can be 8–12°C lower than the surrounding wall, turning the corner into a moisture sink even in dry climates.
3. Bathroom or kitchen with no extract ventilation. Cooking and showering release large quantities of water vapour. Without a fan or ducted extract, this moisture cannot leave the building. It disperses to cooler rooms, condensing on any surface below the dew point. Cold bathroom walls, uninsulated pipes, and window frames become saturated.
4. Bedroom wall blocked by a bed or wardrobe. Furniture pushed against an external wall creates a dead-air zone with no circulation. Humidity traps behind the furniture; if the wall is cold or uninsulated, the air stagnates at 85–95% RH. The occupant may be unaware because the open room air feels normal, but mould germinates in the dark, unventilated cavity.

What is the correct order of fixes for mould problems?

Fix	Action	Expected Outcome	Common Mistake
1. Remove thermal bridges	Insulate exterior; install thermal-break reveals at windows; use thermal-bridge kits at balconies; insulate corners with rigid foam.	Interior surface RH drops 10–20 percentage points. fRsi rises from 0.4–0.5 to 0.7+.	Skipped because it is expensive or disruptive. Ventilation and cleaning then become permanent triage, not cure.
2. Install controlled ventilation	Add local extract (bathroom fan with timer; kitchen range hood) and ensure background fresh-air supply. Consider heat-recovery ventilation in deep retrofits.	Indoor RH stabilises at 45–55% year-round. Surface RH at thermal bridges remains below 80% despite cold weather.	Extract installed but no background air supplied, creating negative pressure and pulling moisture into walls. Or extract vented into attic instead of outdoors.
3. Clean and treat residual deposits	Wash surfaces with dilute detergent or commercial mould cleaner; allow to dry thoroughly. Do not use bleach (it damages paint and leaves no residue). Consider a fungicide only on materials like untreated timber where mould can penetrate.	Visible mould removed; health risk eliminated if steps 1 and 2 are complete.	Treating mould without removing moisture. Mould returns within 2–6 weeks. Occupants blame the cleaner and repeat endlessly, wasting money and damaging materials with repeated cleaning.

Never treat mould alone. It is a symptom, not a disease. The disease is moisture; the mould is the consequence. Remove the moisture, and mould will not return. Leave the moisture, and no amount of bleach will help.

How does mould risk change during the heating season?

In Slovakia, mould risk peaks during the heating season, typically November through March. Winter brings two accelerators: outdoor temperatures drop (making cold surfaces even colder), and indoor heating raises the humidity set-point by 10–15 percentage points if ventilation is poor. A room that hovers at 45% RH in summer can climb to 60–70% RH in winter, pushing surface humidity from 75% (marginal) to 95% (critical) at cold details. Spring brings drying; outdoor temperatures rise and days lengthen, allowing surfaces to dry faster than moisture accumulates. Summer is usually safe because cold surfaces disappear. Autumn is a transition: as heating turns on and windows close, humidity rises sharply, but surfaces are still warm from summer, creating a brief grace period before October and November cold snaps arrive.

How does surface condensation differ from interstitial condensation in terms of mould risk?

Surface condensation occurs on visible surfaces (window sills, walls, pipes). It is obvious and tangible; occupants see wet patches and act. Mould can grow directly on the wet surface within days, making surface condensation an immediate mould risk. Interstitial condensation, by contrast, forms inside the construction layers where it cannot be seen. The immediate mould risk is lower because the air gap inside the wall is usually nutrient-poor and oxygen-limited. However, if interstitial condensation wets insulation or timber for extended periods, rot and mould can develop deep in the wall, emerging only when the damage is severe. Both types demand different design strategies: surface condensation is prevented by raising fRsi; interstitial condensation is managed by specifying the right vapour resistance and ensuring the annual moisture balance closes (condenses in winter, dries in summer).