Improving ground heat transfer simulations in cold climates through field-validated boundary conditions

Highly insulated slab-on-ground construction with sub-slab insulation is widely used in cold climates. Accurate assessment of heat loss, hygrothermal behaviour, and frost heave risk requires understanding ground temperature dynamics around and beneath buildings. This study combines long-term field measurements with hygrothermal simulations to evaluate ground heat transfer modelling accuracy.

Temperature data were collected at three sites in Estonia using measurement piles on undisturbed ground and self-installed sensors near a building. One-dimensional simulations were conducted over nine years using a verified finite element software for transient heat, air and moisture transfer in building materials, while three-dimensional building-coupled simulations were performed over three years using a verified finite element software for transient 3D heat transfer. Boundary condition configurations were varied to assess their impact on accuracy.

Simulations using ground-temperature boundary condition below the surface buffer layer on undisturbed ground achieved the best accuracy, root mean square error (RMSE) ranging from 0.2 to 0.5 K across all depths and sites. Daily deviations up to ±1.4 K occurred at shallow depths during seasonal extremes. In contrast, using air temperature, snow cover, and its constant thermal conductivity increased RMSE up to threefold and caused deviations up to ±3 K. Heavy rainfall events produced transient temperature differences, especially in upper soil layers.

Near the slab perimeter, model–measurement discrepancies were larger, likely due to simplified treatment of moisture and thermal inertia. Results indicate that accounting for dynamic soil moisture and snow properties, along with improved representation of thermal capacity near building edges, could enhance simulation performance.