Damp in brand new porch

Hi The structure will take a while to dry out as it's very new,I think that you are probably correct in assuming it is most likely condensation. Your Builder should come back anyway to reassure you that this is the problem. Keep the porch heated during the very cold weather and regularly open the windows to let the moist air out. If it stays wet into Spring you would need to insist your builder returns and rectifies any problems.

It does sound like condensation has your windows been fitted with trickle vents. if you have fitted a radiator in your porch this could cause this if you have no ventilation. I would suggest getting your builder back for a visual inspection

if you see condensation on the window in the morning then it could be that dripping onto the screed or could also be from the window frame there are holes and chanels in the frame that let any water that gets into the frame out.. to the outside.. if these get blocked or the frame incorrectly fitted water can get in and give you a damp screed!!! ... its not correct get your builder to check it. Heating the porch will just encourage condensation on the cold window..

Once the porch was built ( brick/block work ,roof on and water tight ) the internal walls should dry out pretty quick and should be dry before plastering , I would think theres a problem with the d.p.c. at ground level or water is getting in from maybe over a window/door opening ,I would need to inspect, I would not be advising to wait until spring.