Pressures Driving Air Leakage

Morning all...the blog is still acting crazy and I hope to have it fixed soon. So if you can deal with not having bold copy, color or photos - just reading...I'll proceed on. No grammar or spelling chex either...

Pressures in a house contribute greatly to air leakage. Again, the more air leakage, the higher the energy bill and less comfort (both on your body and wallet).

The first type of pressure is called the Stack Effect. Cooler air is denser than warmer air, and this density difference creates a pressure that causes air to move. Hot air rises to the top and cooler air falls to the bottom. If the home has leaks, warm air leaves through higher openings and cool air enters through lower openings. This pattern of air leakage is called the stack effect because it resembles airflow in a chimney. The pressures created by the stack effect are greatest at the highest and lowest points in the building. Therefore, a hole in a basement or attic will allow more air infiltration than an equal-sized hole near the neutral pressure plane. This is why we focus on the basement and attic so much on our retro-fit jobs.

The second type of pressure is called Wind Pressure. The wind's speed is greater the higher from the ground you measure. As building height increases, wind's force against the building increases. Wind speed is affected by trees, fences, neighboring buildings and hills that block or divert it. Wind pressures push and pull air through holes in the shell - again causing air leakage.

Next pressure, Chimney & Exhaust Pressures. Chimneys, exhaust fans and clothes dryers create a slight vacuum indoors because they exhaust air out of the building. Replacement air, for air exiting exhaust devices, is called make-up air for exhaust fans or combustion air for combustion appliances. Make-up air and combustion air enter through air leaks, intentional openings, or ducts. Make-up air or combustion air may even come down a chimeny if negative pressures become too great - a dangerouse situation called backdrafting.

The last pressure is Duct Pressure. The furnace blower circulates air through the furnace and its supply and return ducts. Supply registers blow air into a room, pressurizing nearby areas of the room. Return registers suck air out of rooms, depressurizing areas near these registers. If the ducts are leaking, or return air is resticted, rooms may have high positive or negative pressures. These pressures are often large enough to double or triple the building shell's air leakage, compared to air leakage when the furnace blower is off.

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