what is the best counterargument for CCS that substantial performance did not occur?

Read the case below and answer the questions that follow.
Cold Case Storage (CCS) provides refrigerated storage facilities for food purveyors. CCS
contracts with Igloo Builders Inc. (Igloo) to build five refrigerated warehouses. The contract
called for type “A1A” brand insulation so that the warehouse would stay between 35 and 40
degrees with minimal energy. Igloo built the first four warehouses using the A1A insulation,
but then ran short of supply. Igloo’s construction manager did not want to miss the completion
deadline because her bonus depended on timely completion. Therefore, she ordered her
workers to substitute B1B insulation for the A1A insulation in the last warehouse. Upon
inspection, CCS refused to pay for the final warehouse, citing expert evidence that, because
the B1B insulation was not as efficient, the extra costs would total $1,000 (more during peak
hot months) per month in cooling costs.
If Igloo argues that it substantially performed by using the B1B pipe, what is the best
counterargument for CCS that substantial performance did not occur?