My personal opinion you may as well go to the Farm Bureau or the local visitors and convention bureau as go to the Better Business Bureau.  They have no authority and very little effect ... other than making you feel good that you did something.

I wasn't sure what he was trying to prove with a psychrometer until I pondered on it a bit.  Relative Humidity measurement is difficult under ideal conditions.  The psychrometer is one of the more accurate instruments, but even so I didn't understand what he was trying to prove. And I am not sure the small differences shown are statistically valid.  It's kind of like a seed corn plot showing one variety is 2 bushels better than another.  It's interesting but may not prove a thing. Here is what a manufacturer of RH sensors found (from http://www.veriteq.com/humidity/calibration.htm) "Standing and breathing in close proximity to a RH sensor can cause it to jump several percent".

There was a discussion on here down below about insulating a concrete floor even if you were not putting radiant heat in it. One suggestion was insulate anyway because the concrete will conduct heat/cold and youwill benefit from the insulation. 

That is when it hit me.

Aluminum window frames will conduct heat or cold as long as there is a thermal path to the outside.  Squirting foam in the frames may stop air flow but the thermal transfer is going to happen just because the aluminum will transfer heat/cold to wherever you don't want it to be. This is why you were getting the temperature variation.

It finally hit me.  What you are measuring thermally and what he is measuring by checking relative humidity are two different things.  He is trying to show your windows do not allow air movement.  He's probably correct.  You are seeing a thermal leak due to the windows and you are correct as well.

You can probably prove your point with an infrared thermometer

Point it at the wall and the frame and other objects and show the thermal leak through the frame.  But even though you prove it, you won't accomplish anything. His frame is sealed.  It just has a design flaw that can't be fixed without replacing the whole thing with something else.

Both of you are correct. He is correct his window is sealed.  You are correct his window is allowing heat transfer.  But it is still sealed.

My suspicion is unless you want to pursue a lengthy, costly, and probably unfruitful lawsuit your best move is to go "Aw ####" and move on.