Improving U-values will tend to reduce heating, but may increase cooling demand in air-conditioned buildings. Increasing the solar performance of the glass by reducing the solar gains into the building can reduce cooling requirement, but may also increase heating demand.
To assess which option (U-value or solar performance) will have the greatest effect, perform a test run on the actual building and look for the dominant load. Buildings with high internal gains and large areas of glazing will almost certainly be cooling dominated.
Increasing the thermal mass of the building, for example using exposed ceiling slabs, will tend to reduce cooling demand and moderate summer temperatures.
