USI Blog

ACHnat and Air Flow: The Secret to Saving Money in a New Home

1024px-Ranch_style_home_in_Salinas,_CaliforniaArchitects often consider the aesthetic flow of a home when they design, but the structure and ACHnat also affect the flow of heat and air. Smart choices with building envelope design can lead to lower energy bills, a more comfortable home and improved air quality.

The design of a comfortable home has long depended on air flow. In cold climates where heat came from fireplaces, homes were designed with many rooms that could be closed off so that the heat could stay in the main living areas. In warm climates like Hawaii, homes have multiple doors that open on all sides to allow a cross-breeze to naturally keep things cool. As upgrades have been made in insulation and central heating/cooling, open floor plans have become more feasible without compromising air flow. It’s important to consider what region you’re in.

Mike Carpenter, USI National Sales & Technical Trainer, adds that “of the 9 out of 10 homes that have a mold issue, the reason is due to lack of air flow. I see it all the time, especially in the southeast, where you have high-volume ceilings at 12 feet and passageways (doors, arched openings, etc.) at 8 feet. Hot air gets trapped in these upside-down pools with zero air flow. That raises the humidity to above 70 percent.” In southwest Florida (USI West Coast Insulation), for example, the considerations are slightly different in the fact that insulation must be perforated to allow the system to properly remove moisture from the home. This is also why cycle times for an AC air exchange rate are also important. An AC that runs for the ideal amount of time and cycles off and on at consistent intervals is important to prevent mold in moist climates.

Optimizing Flow

When constructing a home, there are a number of things that you can accomplish. If you want to naturally warm a home, consider a row of windows and/or skylights on the south side of the home. This can create a greenhouse effect and help to lower energy costs. If you are building a home in a place with very hot summers and very cold winters, then you can dissipate that same heat if the windows open to create cross-breezes. Fans can help to even temperatures within a home if hot air is trapped at the top of the home. Multi-story homes will have a harder time controlling heat on each level, though fans along stairwells and in high-ceiling sections can help.

A home that is not well-sealed will have a number of cold breezes that come from unwanted places. Poorly sealed windows, gaps in construction and uninsulated foundations are all causes for cold (or hot) air to seep in, as well as moisture. Good caulking and insulation are essential to avoid this.

A well-sealed home has a more difficult time getting rid of moisture. This is why vents and windows are essential. Cooking and bathing are a constant source of moisture in the home, and they need a place to escape. In addition to vents, a kitchen hood can also help to vent water outside. Otherwise, keeping a home warm enough and with enough air flow to avoid condensation is important. You do not want water to settle as it can lead to mildew.

ACHnat – Air Changes per Hour

What we are normally concerned about is ACHnat, which stands for Air Changes per Hour that “naturally” happen in a house. A lot of the numbers you will see are based upon the house under a set pressure. Those can be used to come up with the ACHnat number. Pat Dwyer, Branch Manager at USI Champion Insulation in Houston, TX, says that “for a normal batt and blow house, ACHnat is usually around .5. That means that every hour, half the volume of the air in the house leaks out to be replaced by air from the outside, like from the garage and attic. This air will have a lot of humidity and particles in it – whether pollen, dirt, heat (attic) or CO from the garage. The air handler now needs to try to dehumidify, cool and clean that air as it pulls from the returns. So .5 ACHnat means that 12 times the volume of your house in leakage has to be treated each day on top of the heat that is making its way into your house from radiant and conduction heating.”

  • BIBS or cellulose has an ACHnat of .2 (so 4.8 times the volume of your house has to be treated for the effects of leakage.)
  • Spray foam houses are around ACHnat .1 (so only 2.4 times the volume of your house to treat.)

Dwyer adds: “think of the difference in the volume of moisture and contaminates between a batt house at 12 x the volume vs. 2.4 with foam each day. The air conditioner in the batt house has to work at least 4 times as hard. Some people always ask about the fresh air vents we put in houses in which we install spray foam insulation, and how that is different than air leakage.”

The big differences are:

  1. We pick where the air comes from – soffit vs. attic or garage
  2. That air is introduced into the return before the coils and is filtered and conditioned before it enters the house
  3. Having a fresh air intake helps to put a positive pressure on the house, which keeps pollen, dirt, and hot air from leaking past doors and windows

Learn More

To learn more or for more information on insulation, contact USI online to find a branch near you and get a free quote. USI believes in excellence in every step of the process and ensures timely completion and quality of service, time after time.