A probabilistic model of thermal transmittance (U-factor) for a building with lightweight construction is presented. Steadystate conditions of airflow and heat flow through the structure are assumed. Three cases, dependent on the direction of the airflow through the envelope, are considered: tight envelope, infiltration, and exfiltration. Thermal transmittance is treated in the model as a random variable varying with the changes of microclimatic conditions at the site of a building.

As an example, the test house situated in two different climatic zones is considered. The probability density functions of Ufactor are approximated on the basis of probability density functions of climatic parameters and the air change rate. First-order reliability method (FORM) techniques are used as a tool for the probabilistic approximation.