Hi camila,
i am not sure about this fully may be any senior person here can guide you better. i just want to share my experience with u. TRNSYS actually calculate heat transfer coefficient during simulation after every time interval on the basis of the material's properties, does not use constant value during whole simulation period. its value is dependent on outside and inside of building weather conditions. external and internal convective heat transfer coefficient( that are input values). my suggestion is to ignore and try to validate your computed and experimental results by comparing. in my case i confirm overall heat transfer coefficient manually by using following equation..
U-value (overall heat transfer coefficient) = Q/ A(Ti - To)
Q is heat losses through the wall you want to check( select form layer output in TRNBuild), A area of the wall, Ti and To inside and outside temperatures. so u can calculate U-value for each time period. hope this will work.
Please let me know if you get some other information about this.
Regards
adnan