Hygrothermal Analysis & WUFI Simulation | SerVilla

In ultra-low energy passive home construction, continuous insulation (U ≤ 0.12 W/m²K) and hermetic envelope airtightness make moisture management the defining discipline of building physics. The traditional steady-state Glaser calculation method (DIN 4108-3) cannot account for driving rain absorption, hygroscopic material buffering, or inward summer reverse vapor drive. Developed by the Fraunhofer Institute for Building Physics (IBP), transient WUFI (Wärme und Feuchte Instationär) modeling simulates the hourly coupled heat and moisture transport across building assemblies over multi-decade meteorological cycles. SerVilla validates all cold-formed steel envelope assemblies to guarantee absolute freedom from interstitial condensation and mold growth.

1. Transient Moisture Transport vs Static Glaser Limitations

The historical Glaser methodology relies on a static, one-dimensional assessment assuming constant boundary conditions (e.g., indoor +20°C, outdoor -10°C):

• Glaser Flaws: It neglects liquid capillary suction, latent heat effects, material moisture storage functions, and transient solar drying.
• WUFI Simulation (DIN EN 15026): Solves coupled differential transport equations hourly:

Where w is moisture content, \phi is relative humidity, D_\phi is liquid transport coefficient, and \delta_p is water vapor permeability. The modeling proves that relative humidity across all internal interfaces remains well below the critical 80% threshold for spore germination.

2. Steel Framing & Interstitial Condensation Mitigation

Cold-formed steel studs possess high thermal conductivity (λ ≈ 50 W/mK). Placing insulation exclusively between studs creates cold thermal bridges where interior air could condense:

• Exterior Continuous Insulation Wrap: SerVilla wraps the entire exterior frame with 100 to 160 mm of high-density rockwool (150 kg/m³), maintaining steel members within the warm thermal envelope.
• Temperature Factor (f_Rsi): Corner and jamb interfaces maintain f_Rsi ≥ 0.85 - 0.92, far above the European minimum threshold of 0.70.

3. Humidity-Variable Smart Vapor Retarders

Conventional low-density polyethylene (LDPE) vapor barriers have a fixed, impermeable diffusion resistance that traps moisture inside the wall assembly during warm summer weather:

• Smart Polyamide Membranes: Feature a molecular pore structure that adapts to ambient relative humidity:
• In winter (dry indoor climate): s_d ≥ 25 m (Vapor-tight, preventing indoor moisture from entering wall cavities).
• In summer (humid conditions): s_d ≤ 0.25 m (Vapor-permeable, allowing any accumulated moisture to dry inward into the conditioned space).
• This continuous drying capacity provides lifelong building envelope safety.

4. Mold Risk Assessment via Sedlbauer Isopleth Analysis

Fungal spore germination does not require liquid water; a sustained surface relative humidity above 80% combined with temperatures between 10°C and 30°C is sufficient:

• WUFI applies the Sedlbauer bio-hygrothermal isopleth model, evaluating transient microclimates against substrate-specific critical water activity curves.
• SerVilla wall assemblies maintain interface relative humidity below 65% even under sub-zero exterior conditions, providing total biological protection.

5. Hygrothermal Analysis Comparison Matrix

6. Dry Construction & Site Humidity Control

SerVilla utilizes hydrophobic mineral wool insulations with low moisture absorption. Because the lightweight steel structure requires zero wet concrete or masonry work, no residual construction water is sealed within the envelope. The mandatory post-construction Blower Door test (n₅₀ ≤ 0.60 h⁻¹) ensures convective vapor transport through unintended gaps is eliminated.