U value in HVAC is the amount of heat transferred per meter per degree by the material More U more heat transfer. The reciprocal of U is R (resistance) so more R less heat transfer There are certain U values for certain material which you will find in ASHRAE fundamentals as property of materials. The overall U value is taken when the wall has many components like there will be a plaster or cladding so all the values are sum up as R not U and then overall U value is calculated.