Take a grand old city library with a domed exterior and palatial marble rotunda inside. In the mid-1950’s, expand the landmark with an addition behind the 1898 building. Then, in 2010, top off the addition’s sturdy roof deck with the icing on the cake: a 33,000-square-foot green roof plus an array of 132 solar panels. Result: a big bang for your energy-savings buck. The Central Library is one of Milwaukee’s showcase buildings for green infrastructure. Its green roof is an inv