Make-up Air

Do you know what make-up air is? When we talk about make-up air in a home, we are describing the air that must enter the home when air is exhausted out of the home. Think about the different exhaust fans in your home; bathrooms, kitchens and even the dryer. When these fans are activated, air is removed from the home at a measured rate. The make-up air is the air that replaces what is exhausted from the home. This equation is very easy; for every 1 cfm (cubic feet per minute) of air that leaves, 1 cfm of air enters.

In the majority of homes there is usually plenty of air that takes the place of the exhausted air. The problem comes when we consider where the air comes from. If your home is on a crawlspace, there is a very good chance that the majority of the make-up air comes from here. That’s good news right? Probably not and there are several reasons why, the biggest being its not very healthy. In the greater Hampton Roads area we have summers with very high humidity. This means that there is a lot of moisture in the air. When a ventilated crawlspace is allowed to have the same moisture content as the outside air, several problems can arise. The biggest problem is that this scenario can lead to mold growth on the wood surfaces in the space. In turn, this mold growth can lead to moisture damage and dry rot to the wood components inside the crawlspace. Condensation is also an issue if there is ductwork in the crawl. When the warm, moist air comes in contact with cold ducts, the air condenses and water is formed. Naturally, when you turn on the 500cfm Viking hood above the range, air is potentially being brought in from the belly of the home.

I know you are asking yourself how the air gets from the crawlspace into the home. Think of Thanksgiving at your home and how everyone knows that the turkey is ready. Even if you are three rooms away, you know the turkey is done because you can smell it, the odor comes with the air. It gets in through voids and unsealed penetrations between the conditioned space and the unconditioned space. These areas can be as simple as a hole for the cable television or a penetration for a plumbing drain. In older homes there may also be opportunities for air to leak around hot water delivery pipes for radiator systems.

If air can be brought into the home through the crawlspace, it can also enter from the other end of the home, the attic. The good news is that attic air is not nearly as unhealthy as the air from the crawlspace. It is still air that is not conditioned however. If you have a set of pull-down stairs in your home, consider insulating and sealing this enclosure to reduce the amount of leakage. You may also insulate and seal the attic hatch found in most homes with simple weather stripping.

Make-up air can also come from the most natural place, which is from the exterior of the home. This is actually the air that we want to replace the air that is exhausted. This is not usually the case however; buildings of all types are more leaky than they need to be. A common myth is that homes need to breathe. A home does not need to breathe, the occupants in the home need to breathe.

A home that is built to Energy Star standards has an air change rate (ACH) of .33 or less. This means that there is a total air change approximately every three hours, which is a number that means the home is as tight as it needs to be. If a home is tighter, or less than .33 ACH, forced mechanical ventilation is recommended to provide proper make-up air for the occupants. Most existing homes have an air change rate much higher than that, usually between .5 and up. The higher the air change rate means the more penetrations and holes are present between the exterior and the interior. The key is to hire a professional with the knowledge, certification and tools to show you the optimal places to air seal and increase the energy efficiency in your home.

Exhaust fans play an important role in your home. Bathroom fans, when used, take the warm moist air created in a shower and exhaust it outdoors. The kitchen exhaust fan removes smoke and odors from the space and the dryer vent is necessary in order to dry the clothes properly. Your fans should be used as much as is necessary, just consider where the make-up air comes from in your home.