This is part 2 of a series. Find Part 1 here.
The stack effect runs the house.
Taller buildings have a stronger stack effect than short ones.
Leaky buildings have a stronger stack effect than tight ones.
The goal of air sealing work is to minimize infiltration/exfiltration, which minimizes the stack effect.
By gaining control over air movement, we minimize heat loss and improve occupant comfort.
The neutral pressure plane (NPP) is directly related to stack effect. It occurs where there is no pressure difference between the inside and the outside. It’s location is determined by both the amount of air leakage in the house and the location where the air leakage occurs. The house is a dynamic system, so the location of the NPP changes depending on wind and outside temperatures (the drivers of the stack effect).
PUSH ME, PULL YOU
To simplify things, let’s talk about air being pulled by negative pressure and pushed by positive pressure:
Outside the house, positive pressure (above grade or below grade) pushes air into the house and negative pressure outside the house pulls air out.
Inside the house, negative pressure causes air to be pulled into the house (infiltration), and positive pressure causes air to be pushed out of the house (exfiltration). This means the air pressure below the NPP is negative, and the air pressure above the NPP is positive.
The location of the NPP is related to where the largest amount of air leakage happens. In most cases, infiltration happens at the bottom of the house and exfiltration happens at the top of the house.
Infiltration is strong: Low NPP
If the rate of air leakage at the junction between the foundation and the main floor (rim joists, exposed floors, sill plates) is higher than that in the attic, then air gets into the lower part of the house more easily than it can leave the upper part. Infiltration ‘pulls’ air into the house faster than exfiltration can can ‘push’ it out. The house gets ‘pumped up’ so there is more air pressure in the top of the house, and the NPP gets pushed down.
Exfiltration is strong: High NPP
If there is more air leakage at the ceiling (attic hatch, top plates, electrical and plumbing penetrations) than at the foundation and lower floors, the air gets out more easily from the upper part of the house than it can get into the lower part. The house ‘sucks’ as it is under a slight vacuum, pulling air in from the foundation to replace what’s pushed out at the top of the house. The NPP gets pushed up.