I bought a duplex in 1991 and live in the lower. My outside walls were so cold in the winter that you could not touch them. I had insulation blown in, and they cut hole in the house under the siding. I did not see it. The walls were so much warmer that they were the same temp as the inside walls, to say nothing about the lower heat bill Just be sure the contractor is licensed and insured and get more than one bid.