Hygrothermal Simulation

Hygrothermal simulation calculates the coupled heat and moisture transport in building components transiently over time, based on real climate boundary conditions and the material-specific storage and transport functions. It is standardised in DIN EN 15026 and required wherever the steady-state Glaser method reaches its limits: for interior insulation, driving-rain exposure, timber components or the assessment of mould and condensation risks. For one- and two-dimensional component simulation, DELPHIN by Bauklimatik Dresden is available. It was developed at TU Dresden and is validated per DIN EN 15026.

What hygrothermal simulation calculates

In real operation, building components are exposed to heat and moisture flows simultaneously. Temperature gradients drive heat conduction, vapour-pressure gradients drive diffusion, and capillary forces transport liquid water. These processes are tightly coupled: moisture changes the thermal conductivity, temperature changes the sorption moisture, and phase changes (evaporation, condensation) release or bind latent heat.

The hygrothermal simulation solves the coupled balance equations for energy and moisture numerically across a spatial grid and over time. As a result it provides, for every point in the component, the time history of temperature and heat flux, relative humidity and water content, condensation and evaporation amounts, and, derived from these, the mould, frost and corrosion risks.

Unlike a pure thermal-bridge calculation, which considers only the temperature field, the hygrothermal simulation represents the full moisture balance, including storage, drying and the seasonal reversal of the transport direction.

Limits of the Glaser method

The Glaser method per DIN 4108-3 is the classic, steady-state verification procedure for climate-related moisture protection. It balances condensation and evaporation in a winter and a summer period under strongly simplified assumptions: purely steady-state treatment with constant boundary conditions per period, vapour diffusion only and no liquid water transport, no moisture storage in the materials, and lump-sum block climates instead of real weather data.

For simple, diffusion-open constructions the method gives usable results. For demanding assemblies, however, it significantly under- or overestimates the risk. DIN 4108-3 itself therefore refers, for such constructions, to the transient hygrothermal simulation per DIN EN 15026.

Typical use cases

Interior insulation

Retrofit interior insulation is the classic case in which the Glaser method fails. Because the insulation cools the load-bearing shell, the dew point shifts into the construction. Only the transient simulation can assess whether the accumulating moisture dries out again in the summer period and whether capillary-active systems safely redistribute it.

Timber components and wood moisture

For timber and wood-based constructions, the time history of moisture determines durability and load capacity. The simulation evaluates the wood moisture against critical limits and checks the drying behaviour of built-in construction moisture.

Driving rain and façades

Absorbent external walls and exposed masonry take up considerable amounts of water during driving rain. The hygrothermal simulation represents the driving-rain load as a boundary condition and evaluates wetting, frost risk and drying.

Mould and condensation risk

From the simulated temperature and moisture histories at surfaces and interfaces, the mould risk can be assessed, for instance via isopleth models that describe spore germination as a function of moisture, temperature and duration.

Software and normative basis

Hygrothermal component simulation is governed by DIN EN 15026; the WTA guideline 6-2 describes the practical procedure and validation. Tools for these calculations must represent the coupled heat and moisture transport transiently, and for junctions and thermal bridges two-dimensionally.

For exactly this task, DELPHIN by Bauklimatik Dresden is available. The program is developed at the Institute of Building Climatology at TU Dresden, calculates the coupled heat, moisture, air and salt transport in porous building materials one- and two-dimensionally, and is validated per DIN EN 15026. For VICUS users this yields a continuous workflow: VICUS Climate exports climate data in the C6B format, which can be read directly as a boundary condition into DELPHIN.

Distinction from thermal building simulation

Hygrothermal and thermal simulation answer different questions and work at different scales. Thermal building simulation, such as in VICUS Buildings, assesses the energy behaviour of whole buildings across the year: heating and cooling demand, room temperatures, summer thermal protection and comfort. It calculates thermally, not in terms of moisture. Hygrothermal simulation examines the moisture balance of individual components in detail and falls within the scope of DELPHIN.

The two methods complement each other: building simulation delivers energy demand and comfort, hygrothermal simulation the climate-related moisture protection. VICUS Buildings and DELPHIN share the same scientific background from TU Dresden.

When to reach for it

Hygrothermal simulation is the right method when the moisture balance of a component must be assessed realistically: for interior insulation, timber construction, driving rain or the evaluation of mould and condensation risks. Where the steady-state Glaser method reaches its limits, the transient calculation per DIN EN 15026 represents liquid water transport, storage and real climate data physically correctly. DELPHIN by Bauklimatik Dresden provides a validated tool developed at TU Dresden for this, and it fits into the planning workflow via the C6B climate format from VICUS Climate.

Further reading: Dynamic Building Simulation explains the time-resolved thermal calculation of whole buildings, Summer Thermal Protection (DIN 4108-2) shows the verification via overheating degree hours, and Steady-state vs. Dynamic Calculation places steady-state and transient methods in context.

References and Standards

• DIN EN 15026 — Hygrothermal performance of building components and building elements — Assessment of moisture transfer by numerical simulation
• DIN 4108-3 — Thermal protection and energy economy in buildings — Climate-related moisture protection
• WTA Guideline 6-2 — Simulation of heat and moisture transfer processes
• DIN EN 15026 / DELPHIN — Validation benchmarks, Institute of Building Climatology, TU Dresden / Bauklimatik Dresden