Why Understanding Air Pressure Differences is Key to Designing an Efficient Home

Most people understand the concept of insulation, and why it is important. Heat wants to move from your warm comfortable home to the cold outside, always seeking a balance of energy. Insulation slows that process down, so more is better. 

However, insulation is only part of the solution of a comfortable and efficient home. Indeed, there is a point at which adding more insulation is no longer cost effective, because the relationship between insulation and heat loss is logarithmic. The more insulation you add, the less effective each additional inch becomes. The first inch of insulation is extremely effective at reducing heat loss, the second inch is still effective, but less so, etc…

Another often overlooked way that heat escapes your home is through what is called air infiltration. Warm air finds its way out of your home through the many small, or not so small, gaps and cracks in the walls and ceilings. As this air leaves your home, cold air is drawn in through other gaps, creating cold drafts and robbing your home of its efficiency. Unlike the relationship between heat loss and insulation, the relationship between air infiltration reduction and heat loss is linear. Every square inch of gap that is closed off is as effective as the last. Certainly, there is a point where the methods needed to close those gaps becomes more costly, but a lot can be done with effective gaskets, expanding foam, and caulking. There is a saying in building science circles, “Gaskets are better than foam, is better than caulk, is better than nothing.” This is due to the longevity, effectiveness, and expense of each of these methods. 

In order for air to move from one place to another, two things are needed. A pressure difference, and an opening. Thinking in terms of meteorology, wind is caused by air rushing from areas of high pressure to those of low pressure. Without a difference in pressure, the conditions are what sailors would call “dead calm.” No air movement at all, even though there are no physical boundaries to be seen. Now consider a balloon. Using some amount of effort from our lungs, we force air into a well sealed enclosure and pinch it closed with our fingers. Even though there is a significant pressure difference between the air inside the balloon and the air outside, there is no opening, no path for pressure equalization, and therefore, no movement between the two areas. How do we know there is a pressure difference between the balloon and the air outside? Let the balloon go. Air rushes out of the opening, propelling the balloon like a rocket in the other direction.

Air pressure within and around a home is constantly fluctuating. Wind may be blowing on the home, exerting a positive pressure on the windward side of the home while at the same time, the aerodynamic drag exerts a negative pressure on the leeward side. Warmer air is more buoyant than cooler air, so inside the home, warmer air rises to the ceiling of the second floor. This exerts a greater pressure on the ceiling and walls in the higher areas of the home, while simultaneously creating areas of low pressure in the basement. The low pressure in the basement means that air wants to be drawn into the home, possibly bringing in soil gases. This is particularly troublesome in areas with high radon content. Since it is not practical to constantly equalize the air pressure between the inside of your home and the great outdoors, sealing leaks is typically the more effective way to control air infiltration.

This is why at evoDOMUS, we painstakingly detail connections and possible weak points in the building envelope with an eye toward air sealing. Major possible weak points, such as the union of two modules, are gasketed to make sure that the seal between them is tight and effective. Areas that are less likely to experience movement over time, such as around windows and doors, are foamed, and finally, small gaps that are too small to receive foam or a gasket are caulked. The net result is a cost effective yet sound strategy for preventing heat loss due to air infiltration.

Each evoDOMUS home is tested at least twice for air infiltration by a certified HERS Rater using specialized and calibrated blower door equipment — once after the modules are set, in order to find any remaining weak points so that they can be addressed, and once near completion to determine the final result. This equipment measures the amount of air flowing through the fan when the specific target pressure is reached. Air pressure is measured in a unit called “Pascals” (Pa.) The target pressure difference for this test is 50 Pa. With a little math including the air volume contained within the home, the Rater is able to calculate the number of times that the air inside the home is completely exchanged at the target pressure over a given time, yielding a result such as “three air changes per hour at 50 Pascals pressure difference,” abbreviated as “3 ACH (50).” There have been a lot of these tests performed throughout the country, and it is generally accepted that a typical new home built to code will test in at about 6 or 7 ACH (50). A home that is certified under the Department of Energy’s ENERGY STAR® Homes program must test below 5 ACH (50). 

An evoDOMUS home is typically below 2 ACH (50) because we want the air that you have paid to heat and cool to stay in your home as long as possible. Another benefit of this is that the air within the home has time to equalize itself in terms of temperature. A more constant, lower powered HVAC System provides smaller amounts of conditioned air into the home, which naturally wants to find thermodynamic equilibrium. The tight envelope gives this process more time to occur, letting the natural process quietly equalize energy throughout the home instead of simply rushing out through gaps and cracks. The result is a more evenly comfortable home, where all rooms are closer to the same temperature without having to move around large volumes of air with big furnace fans.