What would happen on Earth with an atmosphere of 50% CO2 an 50% water vapour?

Please consider what would happen on Earth starting with an atmosphere
of Carbon dioxide mixed 50:50 with water vapour. My estimate is that
the sun would visually dim very little but the whole IR band of
radiation would fall by some 38%. Therefore there would be 38% less
heat
reaching the ground.

That atmosphere would warm and convection currents would start to
operate. Due to expansion of gases the rising vapour would cool at the
saturated lapse rate some 5 °C/km). Soon a distillation process would
begin and Earth would regain something like the current temperatures
but actually cooler due to a decrease in thermal energy reaching the
ground.

At night the heat retention of the carbon dioxide and water vapour
atmosphere would save the planet from excessive temperature
fluctuations as observed on the Moon of 123°C to -238 °C.