Now to finish with a third letter, U, this ones is the representation of the sum of all the thermal Resistance material we previously found. This is the measure of the complete assembly ability to transmit heat to cold. Everything around us is energy, and that must be in equilibrium, if not it will seek to find it. From hot (high energy) to cold (low energy), it’s the hot molecule slowing down to the lower energy state, until equilibrium has been reached.
The lower the U, the better your assembly will resist this transfer, keeping heat in your house and the cold out, and vice versa in a tropical climate.
Minimum R value for a wall from the national building code is R=17 (U=0.34)
A passive house wall has to be a minimum R=38 (U=.15)
Lets turn this to a wall with dimensional limestone, from the outside in:
Stone 200mm/air 50mm/insulation 150mm/stone 300mm we get a R= 34 (U=0.166)
That is more effective than a standard wall with minimal materials and substantial benefits. No maintenance required on the exterior, many different trades have been eliminated (less$$). Reducing the trades on sites reduces errors of installation. This is just the entrance of a rabbit hole if you decide to dive in.