Retrofitting can be defined into two major categories

Building Envelope Sealing

The effectiveness of a reasonable air-tight building should not be overlooked. The air barrier for the building envelope can help control unwanted infiltration as well as reduce energy consumption while improving comfort.

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Fort the case shown there is a 50% building envelope air-leakage improvement. 

Before

Air-leakage source acknowledged

After

Air-leakage source fixed