Air sealing is mathematically the most effective energy upgrade a homeowner can perform. However, aggressively foam-sealing an older home that contains atmospherically-vented gas appliances (water heaters and furnaces) can trigger a lethal phenomenon known as "Backdrafting."
The Physics of Backdrafting
Older gas furnaces burn fuel and exhaust the deadly Carbon Monoxide (CO) fumes up a metal flue pipe and out the roof. They rely on the natural "draft" of the house to pull the fumes upward.
If a DIY homeowner buys 30 cans of spray foam and perfectly seals every leak in the attic, the house becomes airtight.
The Fatal Reversal
When the house is suddenly airtight, turning on the kitchen exhaust hood or the bathroom vent fans creates a negative pressure vacuum. The fans violently suck air inward from the path of least resistance. The easiest path is straight down the gas furnace exhaust flue. The deadly Carbon Monoxide is sucked backward, out of the furnace, and dumped directly into the living room.
- ✔ Professional auditors conduct a Combustion Appliance Zone (CAZ) depressurization test.
- ✔ We use manometers to measure the exact pressure threshold where the flue pipe fails.
- ✔ We stop sealing the envelope before that pressure threshold is ever reached.