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Air Mixing Through Ducts in Dirt Crawl Spaces

If you have ducts in your crawl space, which most do, then you have a built in air highway to efficiently mix all the air in your home from crawl space to the upper floor. Ducts are made of segments of sheet metal, or combinations of sheet metal, duct board and flex duct. Between the sections and at the elbows there are joints which are simply bent, crimped or screwed together. These joined segments allow for duct leakage.

Supply ducts are under pressure. crawl space air ductsAs pressurized air passes by loose joints and seams in the duct, a little air leaks out. About 2% of the air in the duct leaks out at each joint. That's 2% per joint, which adds up. When you have a lot of supply duct leakage, a couple things happen. You depressurize the living spaces above and pressurize the crawl space. This pushes the damp, smelly, moldy air from the crawl space up into the living spaces faster than it would move in that direction otherwise. Its a no win situation. It costs you money and you and your family breathe diluted crawl space air all day.

Return ducts suck air from the rooms you want to heat and cool, to the heating/cooling unit. They run through the crawl space, and when they leak, they suck in the bad air from the crawl space directly into your duct system and blow it right back into the house. So that's a losing situation too.

Sealed duct systems do exist. A contractor would paint duct mastic on every joint in the duct system. It is an arduous process.

EPA Indoor Air Quality Reference Guide - Understanding Indoor Air Quality Problems