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In addition to having a tight building envelope, there's also no air leakage to the outside from our HVAC system. Since the indoor unit and all ducts are inside the building envelope, the small amount of air that leaks from the system isn't lost from the house.If you're building a house, it's always best to put the HVAC inside the envelope. When you put it in an unconditioned attic or crawl space, leakage from the supply side of the system results in your attic or crawl space being more comfortable, which is of no benefit, and a negative pressure in the house. In addition to higher operating costs, this can cause indoor air quality problems or moisture problems in the house. The equipment, being in a harsher environment, won't last as long either.