Uncertainty-aware determination of thermal transmittance and systematic evaluation of insulation retrofit scenarios for energy-efficient building envelope

Thermal transmittance (U-value) assessment is essential for improving energy efficiency in buildings, particularly under real operating conditions where transient heat transfer and measurement uncertainties affect reliability. This study proposes a novel framework that combines non-intrusive infrared thermography, uncertainty quantification based on the Guide to the Expression of Uncertainty in Measurement (GUM), and dynamic heat transfer analysis for in-situ U-value estimation. The objectives are to evaluate and compare instantaneous, smoothing, and Cumulative Time-Integrated approaches and support the systematic assessment of insulation retrofit strategies. The proposed methods are applied to the external wall of a single-family house in Brazil, characterized by a warm and humid climate classification. In-situ measurements of indoor and outdoor air temperatures and surface temperature are used to estimate heat flux through convective heat-transfer modeling. Uncertainty propagation is incorporated using sensitivity coefficients, enabling a rigorous evaluation of the influence of measurement errors and environmental variability on U-value estimation. The framework is extended to analyze retrofit scenarios using insulation materials, including expanded polystyrene (EPS) and extruded polystyrene (XPS), combined with internal and external covering layers such as gypsum board and acrylic render. The results show that instantaneous U-values are highly sensitive to transient conditions and low temperature gradients, while smoothing improves stability but may introduce bias. The Cumulative U-value provides the most robust and physically representative estimate. Retrofit analysis indicates that insulation layers significantly reduce thermal transmittance, with external systems performing better under solar exposure and internal configurations enhancing indoor thermal stability. The study supports effective energy-efficiency interventions in building envelopes.