Temperature Factor (fRsi)
A dimensionless metric (fRsi) that predicts internal surface temperature at thermal bridges to assess condensation and mould growth risk.
What is the temperature factor fRsi?
The temperature factor, denoted fRsi, is a dimensionless metric ranging from 0 to 1 that quantifies the minimum internal surface temperature at a thermal bridge or junction detail. It serves as the primary tool for assessing whether cold spots in a building envelope will trigger condensation and mould growth under given indoor climate conditions. Unlike the U-value, which measures heat flux, fRsi directly predicts surface temperature behaviour, making it essential for comfort and durability assessment in residential design.
The metric is named for its use in compliance checking against regulations such as the Slovak national standard STN 73 0540 and the European calculation standard ISO 13788. Architects and engineers use fRsi to evaluate junction details, window frames, balcony penetrations, and other weak points where insulation is compromised or material changes occur.
How is the temperature factor calculated?
The fRsi formula relates the minimum surface temperature to the temperature difference between indoors and outdoors:
fRsi = (Tsi,min - Te) / (Ti - Te)
Where:
- Tsi,min = minimum internal surface temperature at the thermal bridge (degrees Celsius)
- Ti = internal air temperature (typically 20°C in residential settings)
- Te = external air temperature (reference: -5°C or lower depending on climate zone)
The result is always between 0 and 1. An fRsi of 0.75 means the surface is 75% of the way between the outdoor and indoor temperatures. For illustration: in conditions of -5°C outside and 20°C inside, an fRsi of 0.75 yields a surface temperature of 13.75°C.
Practical calculation requires finite-element thermal modelling per ISO 13788 because fRsi depends on junction geometry, material properties, and surface resistances inside and out. This cannot be determined from U-value alone. Many European countries maintain thermal bridge catalogues (e.g., DIN 4108 supplementary sheets) with pre-verified construction details and their fRsi values, allowing designers to check standard junctions without running simulations each time.
Why is the 0.75 threshold critical for mould and condensation risk?
The threshold fRsi = 0.75 represents the boundary below which mould is likely to develop, given typical indoor humidity levels of 60-80% and normal moisture sources (cooking, bathing, human occupancy). This threshold comes from ISO 13788, which correlates fRsi to the psychrometric conditions under which fungal spores germinate.
| fRsi Value | Surface Temperature at -5°C Outside, 20°C Inside | Risk Assessment | Typical Application |
|---|---|---|---|
| fRsi >= 0.75 | Tsi >= 13.75°C | Low mould risk; complies with residential standards | Well-insulated windows, external wall junctions |
| fRsi 0.65-0.74 | Tsi = 11.25-13.74°C | Moderate risk; acceptable in commercial buildings; requires controlled humidity indoors | Single-glazed frames, lightly bridged corners |
| fRsi < 0.65 | Tsi < 11.25°C | High mould risk; surface condensation likely; unsuitable for residential without major renovation | Thermal bridges, poorly insulated corners, cold balcony slabs |
The distinction between temperature factor and surface condensation is important. Surface condensation describes the physical phenomenon (visible water droplets) that occurs when air humidity saturates at a cold surface. Temperature factor predicts whether that surface will be cold enough for condensation to form. Low fRsi creates the preconditions; high indoor humidity triggers the event. Together, they form the complete picture of condensation and mould risk.
How do thermal bridges affect the temperature factor?
A thermal bridge is a pathway of high thermal conductivity through the insulation layer, typically at junctions where materials change (e.g., concrete balcony slab meeting an exterior wall) or where geometry forces insulation to taper (e.g., window frame corners). At these locations, heat flows out more readily, and the internal surface temperature drops significantly compared to the rest of the envelope.
The fRsi metric quantifies exactly how much colder a thermal bridge surface becomes. A well-designed junction might have fRsi = 0.78 (safe), while a poorly detailed one at the same location might be fRsi = 0.55 (risky). The difference is typically resolved through thermal breaks (non-conductive spacers), additional insulation, or geometry changes that increase the internal surface area or reduce the thermal path length.
| Construction Detail | Typical fRsi Value | Mitigation Strategy |
|---|---|---|
| Triple-glazed window frame with thermal break | 0.80-0.85 | Highly effective; standard in passive-house design |
| Single-glazed frame, aluminium without break | 0.40-0.55 | Install thermal break spacer; consider triple glazing |
| Balcony slab penetration (unbroken concrete) | 0.50-0.65 | Separate insulation layer inside; concrete-to-concrete junction with insulation recess |
| External corner, 15cm mineral wool insulation | 0.72-0.75 | Marginally compliant; increase to 20cm or upgrade material conductivity |
| External corner, 30cm mineral wool insulation | 0.82-0.88 | Excellent performance; well above regulatory minimum |
What is the relationship between fRsi, dew point, and humidity?
The dew point is the air temperature at which moisture begins to condense if no further cooling occurs. In a room at 20°C with 70% relative humidity, the dew point is approximately 14°C. If a surface temperature drops below this dew point, condensation will form immediately, regardless of the fRsi value.
fRsi allows precise calculation: ISO 13788 determines whether the coldest surface stays above the dew point. If fRsi is too low, condensation occurs during winter. Critical insight: good U-value does not guarantee condensation avoidance. A wall with U=0.20 can have poor fRsi at corners (geometry effects) and produce mould. Conversely, an older facade with modest U-value passes fRsi checks if corner insulation is generous.
How is temperature factor assessed in practice?
Assessment follows ISO 13788, specifying junction geometry, material properties, and indoor/outdoor boundary conditions per the relevant standard. Thermal simulation tools (THERM, Flixo, AnTherm) compute the temperature field and extract fRsi. The result is compared against the regulatory threshold fRsi,N defined in STN 73 0540-2 for Slovak residential buildings.
Many architects consult approved thermal bridge libraries instead of running simulations. These catalogues provide pre-calculated fRsi values for common junctions, speeding up design review and ensuring consistency.
What does fRsi tell you about mould growth versus visible condensation?
Mould and condensation are related but distinct. Condensation is the visible symptom (water droplets); mould is biological colonisation that follows when surface moisture is sustained above 80% relative humidity for several days. A high fRsi (0.75+) reduces but does not eliminate mould risk; if indoor humidity remains very high, mould can grow on marginal surfaces (fRsi 0.70-0.75). A low fRsi (below 0.65) makes mould almost inevitable in residential settings.
Slovak building standards reference both fRsi and humidity criteria because neither alone is sufficient. The normative requirement is fRsi >= fRsi,N as defined in STN 73 0540-2; for residential buildings this commonly corresponds to fRsi >= 0.75. The robust solution integrates thermal bridge control, moisture management (ventilation and humidity discipline), and sometimes active dehumidification in high-moisture spaces (kitchens, bathrooms).
Frequently asked questions
- What does an fRsi value of 0.75 mean in practical terms?
- An fRsi of 0.75 means the internal surface at a thermal bridge is 75% of the way between the outdoor and indoor air temperature. If outdoors is 0°C and indoors 20°C, the surface would be at 15°C. This threshold protects against mould formation when indoor humidity is moderate (60-70%).
- How is temperature factor different from U-value?
- U-value measures total heat loss through a building element; fRsi measures the minimum surface temperature at that same element. U-value alone does not tell you whether mould will grow. A window with U=1.0 might have fRsi=0.80 (safe) if well-designed, or fRsi=0.50 (risky) if the frame detail is poor.
- Can I calculate fRsi from building materials alone?
- No. fRsi depends on geometry, material conductivity, and boundary conditions (indoor/outdoor temperatures, surface resistances). A corner junction and a window frame with identical U-values can have different fRsi values. Calculation requires finite-element analysis per ISO 13788 or reference to approved thermal bridge catalogues.
- What distinguishes temperature factor from surface condensation?
- The surface-condensation article describes when water droplets actually form on a surface. Temperature factor is the predictive metric that lets you calculate whether condensation will form for any given indoor climate. fRsi assesses the surface's thermal performance; condensation is the resulting phenomenon if fRsi and humidity align dangerously.
- Does a high fRsi eliminate mould risk completely?
- fRsi ≥ 0.75 greatly reduces mould risk but does not eliminate it entirely. Mould requires both a cold surface and sustained high humidity (above 80% on the surface). If indoor humidity is poorly controlled, even fRsi=0.85 can fail. Thermal bridge design works best paired with ventilation and moisture management.
- Are there different fRsi thresholds for different building types?
- Yes. Slovak norm STN 73 0540 and most EU standards require fRsi ≥ 0.75 for residential buildings. Offices and retail spaces sometimes accept fRsi ≥ 0.5 as adequate, depending on occupancy density and moisture generation. Certain spaces (swimming pools, industrial kitchens) may need even higher thresholds.