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March 5, 2020
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. When more air leakage occurs high in the house, the neutral pressure plane is low. When more air leakage occurs low in the house, the NPP is high.
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.
How does this impact comfort, a healthy indoor environment, building durability and energy use?
When the NPP is low, it can cause condensation problems in the house, and exfiltration will push moisture into wall assemblies. Exfiltration is not your friend, especially in cold climates. The air that’s being pushed into wall assemblies via air leakage is warm and moist. When those wall assemblies are wood-based, they are mold-factories-in-waiting. Now you have rot and potential indoor air problems as well.
When the NPP is high, it can push warm moist air into the attic, contributing to ice damming, ‘attic rain’, and problems with rot. It can also cause a different set of indoor air problems. Depressurization can lead to soil gas infiltration and combustion spillage because of the negative pressure in the lower part of the house. Combustion appliances below the NPP have to work against negative pressure, while those above the NPP get an assist from positive pressure.
Exhaust fans and other devices can deepen the negative pressure and raise the NPP so that more of the living space is below the NPP. This is a problem encountered in newer houses with modest air leakage rates and no chimney. In an old house with a big old chimney bringing in gobs of dilution air, the NPP can actually be above the living space. The wall assemblies are dry, but so are the people inside.
How air sealing can help, or not...
Air sealing work in a basement will move the NPP up, while air sealing work in an attic will push it down. Either or both of these moves will reduce the stack effect. This is good, because air leakage locations that are further from the NPP cause more havoc with the performance of the house. According to Joe Lstribrek: The “effective area” of a hole is multiplied by its distance to the neutral pressure plane and this product is added to the next product of the “effective area” of a hole multiplied by its distance to the neutral pressure plane for all of the holes above [or below] the neutral pressure plane.
Lowering the NPP by air sealing from the top down reduces the negative pressure drive in the lower part of the house. However, if you only seal your attic, and don’t stop infiltration at the foundation, you lower the NPP and the exfiltration now occurs in your walls. Now you have a different problem. So you need to seal the top and bottom of the house to minimize the stack and wind effects, and then add make up air where needed if there are any combustion appliances in the house.
Air sealing work minimizes infiltration and exfiltration, stabilizes the NPP, allowing us to control comfort levels and optimize energy use. Never forget that comfort is the prime selling point for energy retrofits and high-performance new construction.
A zonal test with a blower door and an infrared camera will help determine what parts of the house are more connected to the exterior and where you are up against thermal bypasses. More on zonal testing and ‘connection to the exterior’ in an upcoming article in this series.
Hat tip to Jon Eakes and his 1999 article on the neutral pressure plane for getting me revved up on this in my building science infancy.
Coming up: Part 3 of this series, exercises to challenge your understanding of the neutral pressure plane.