To estimate the loss of tendon force for a post-tensioned timber connection a series of tests are being conducted at the ETH in Zurich. Several post-tensioned specimens are being observed in different climate conditions. One set of specimens is in a climate chamber, where the relative humidity and temperature are kept constant. The second set of test specimens is positioned in an uncontrolled environment, where temperature and relative humidity change daily. The two environments allow estimating the influence of changes in relative humidity and temperature on the loss rate of tendon force. First results show that the relative humidity influences this rate, making it a key variable to estimate the total loss in post-tensioning force during the lifetime of a building.