Hygrothermal Convergence Errors

A convergence error in the context of hygrothermal HAMT simulations, refers to the discrepancy that occurs when iterative numerical methods fail to reach a stable solution within a predefined tolerance level. These error indicate that the simulation's equations (governing heat and moisture transfer) are not balanced properly, leading to unstable or non-physical results.

What Causes a Convergence Error?

One primary cause of convergence errors is improper boundary conditions. When environmental inputs, such as temperature, humidity, and wind speed, are unrealistic or inaccurately defined, they can disrupt the balance of energy and mass equations within the model.

Another significant factor is the accuracy of material properties or there inappropriate placement within a wall system. Using incorrect or oversimplified characteristics for materials can lead to erroneous moisture and temperature gradients, which further destabilise the simulation.

Additionally, inappropriate time step sizes can cause convergence issues; too large a time step may miss important transient phenomena, while too small a time step can result in excessive computational load and instability. HAMT defaults to 60 steps per hour.

How to Mitigate a Convergence Error

To mitigate convergence errors, it is crucial to refine input data, ensuring that all boundary conditions and material properties are as accurate and detailed as possible. We recommend the following steps.

Step 1 - Run Period

A Run Period can be set for up to 10 years, with convergence errors occuring in some wall systems after year 5. Reducing your Run Period to 3 - 5 years can resolve errors and a useful first step.

Step 2 - Cavity Ventilation

The Cavity Ventilation Rate is set to nominate an air change rate behind the outermost wall material, typically a cladding or masonry finish. If the air change rate is too low, convergence errors can occur due to the build-up of moisture within the cavity.

Step 3 - Internal Conditions

Internal Conditions impacts the amount of moisture generated by the model that is available for diffusion into the wall. If the moisture load is too high, a function of both the Air Exchange Rate and the Moisture Generation Mode, convergence errors can occur.

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