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November 22, 2019
This series of articles looks at the fundamentals of building science and how you apply them in diagnosing the conditions that are affecting the performance of a building.
When we talk about building science, we often parcel heat, air, and moisture flows into separate categories, but not one of them is a sole operator when it comes to building performance.
When you are analysing or diagnosing a building, you need to think in 3-dimensions (at least) to figure out what’s going on. You need to consider several physical conditions (heat flow, air flow, moisture flow in the house and through the envelope, interior/exterior temperature differentials, wind conditions, occupant usage patterns) and how they are impacting the performance of the house. When you consider how to improve performance, you have to understand how a change to any one of those physical conditions will impact all of the others.
Building science is complex. Temperature and pressure differentials are dynamic.
Also, you can’t rewrite the rules of physics. There is only one playbook, and if you don’t know it, you’re going to do damage to houses, people, and definitely your reputation. So let’s get on to it:
First up is the stack effect. It’s driven by temperature differences (interior to exterior, bottom to top), and pressure differences (outside to inside, bottom to top.).
Stacking Things Up
Every building experiences the stack effect, some more than others. Taller buildings have a stronger stack effect than short ones. Leaky buildings have stronger stack effect than tight ones.
Heat, air, and moisture flows are all impacted by the stack effect. Air carries heat and moisture, so if it’s entering and leaving a building, so are heat and moisture. If you have no control over air, you have no control over heat or moisture, and the three of them are going to be trouble if left to their own devices in a house.
Stack effect drives heat loss through air leakage, and contributes to occupant discomfort through temperature stratification (and air leakage). Stack effect can also drive humidity problems (too much or too little). Controlling stack effect is the key way to make a house more comfortable and energy efficient. So the first step in any retrofit project is to determine how you are going to control the stack effect before you consider increasing the insulation levels.
All buildings experience the stack effect, and all buildings have a neutral pressure plane.
The neutral pressure plane is the invisible ‘line’ in a building where the pressure inside the house is equal to the pressure outside the house - it’s neutral. That doesn’t mean it’s static, but you really want it to be stable. The neutral pressure plane is driven by the stack effect, so the stronger the stack effect, the more active the neutral pressure plane is.
The neutral pressure plane is a challenge to think about. It’s not something you can test for, see, feel, or hear. It’s both a concept and a real phenomenon in a building.
Four Things That Drive the Neutral Pressure Plane
Here’s the physics behind the neutral pressure plane, and it’s driver, the stack effect:
Stack effect is driven by temperature differences, whether it’s cooler at the bottom, warmer at the top, or cooler outside and warmer inside.
Stack effect is driven by infiltration and exfiltration rates, air gets pulled in at the bottom and pulled out at the top.
Wind pressure increases infiltration/exfiltration rates and changes the ‘line’ of the neutral pressure plane.
The neutral pressure plane has a negative pressure zone below and a positive pressure plane above.
A house with high air leakage rates will have a neutral pressure plane that moves a lot when the wind blows or when the outside temperatures are lower than inside in the heating season. Or both, when it’s windy and cold. The pressure plane will tilt according to the orientation to the prevailing wind.
The results can be drafts, cold spots in specific areas of the house but when the wind blows from one direction, moisture problems in upper stories in the heating season, mold growth in attics, and occupant discomfort.
Minimizing infiltration/exfiltration minimizes stack effect and stabilizes the neutral pressure plane. That’s the goal of air sealing work, always. We want to gain control over air movement so that we can minimize heat loss and improve occupant comfort.
Here’s a short video from our House as a System module describing the neutral pressure plane.
The complete House As A System module is featured in three of Blue House Energy’s key building science courses:
Learn more about analysing the stack effect and the neutral pressure plane in Part 2 of this article.