The more I study the International Residential Code (IRC), the more I understand it. However, one of the things that do not make sense to me is why they split the venting requirements of dryers from the makeup air requirements.
So, if you are renovating your laundry room and the last thing you want to do is read through paragraphs of regulations to find the requirements for makeup air, then you’ve come to the right place. Here’s what the building code says about makeup air and laundry rooms.
Laundry rooms only require makeup air if there are gas dryers present, and these are rated 200+ CFM. Dryers encased in closets need makeup air. Sometimes, makeup air is required when the exhaust ratings are lower than 200 CFM because this exhaust interferes with the function of gas-fuelled appliances.
Dryers May Necessitate Makeup Air in Laundry Rooms
The requirements for makeup air in laundry rooms depend on the appliances present.
With only a washing machine, you are unlikely to need makeup air.
However, if you have a dryer in your laundry room, it may be necessary to supply makeup air. This is because dryers pull air out of the room (I’m referring to vented dryers here).
The drawn-in air passes through the dryer, serves its purpose, and is then exhausted through ductwork to the outside.
Usually, when air is removed from a room in this way, it can be replaced with air from other parts of the house or from the outside through open windows and doors and even through the cracks or gaps present in many houses.
There are three scenarios that make makeup air necessary in laundry rooms containing vented dryers:
- When rooms are tightly sealed. Weathertightness is increasingly achievable, and with all the benefits associated with this, there are few reasons to live without it. However, one drawback is that there are fewer ways that air can enter your house to replace the air exhausted by the dryer.
- The amount of air entering the house through open windows and doors and/or cracks or gaps is not enough to counter the air lost from a dryer with a high CFM (exhaust rating). Even non-weathertight houses can struggle to replace all the air removed by larger machines.
- Sometimes, enough air can be brought into the house to make up for the air lost through the dryer,. However, this inflow of air is so extreme that your laundry room and nearby areas of the house are afflicted by strong drafts that make the house uncomfortable and expensive to heat or cool.
What Happens if There Is No Makeup Air?
The third scenario presented above necessitates the supply of makeup air to counteract excessive drafts and energy losses. The first two necessitate makeup air to prevent the formation of a negative air pressure system in the laundry.
Negative pressure in the laundry has many consequences, some of which can also influence the rest of the house.
If there is not enough makeup air to replace the air that is removed through dryer exhaust systems, the functionality of the dryer can be impaired.
This movement of air is important to dry the clothes (think of how much more quickly clothes dry on the clothesline on a warm, windy day compared to a cold, still day). So, if there is no air to pull in, the dryer can’t work as well.
For gas dryers, depressurization and lack of makeup air can mean that there is also not enough air for complete combustion to take place.
Gas dryers use a burner assembly to burn a pilot light to heat the air that is used to dry the clothes.
Without sufficient air for combustion, the burner assembly cannot function. This air is supplied as combustion air, not makeup air. However, if there is insufficient makeup air, then the supply of combustion air can be interfered with.
Section G2407.4 of the IRC says that exhaust systems cannot interfere with the functioning of gas-fuelled appliances.
In addition to these issues, a lack of makeup air can result in backdrafting. Backdrafting affects drains like those in the washer and laundry sink. It can even affect drains in other rooms if the deficiency is severe enough.
This is smelly, horrible, and even harmful to health.
Backdrafting also affects vented appliances, like gas dryers, fireplaces, heaters, etc. This results in combustion gases being pulled back into the house instead of pushed outside where they are less harmful to the health of the inhabitants.
Laundry Rooms Losing 200 CFM Need Makeup Air
According to Section G2439.5 of the IRC, if more than 200 CFM (cubic feet per minute) is exhausted from a room, makeup air should be provided.
The CFM for each dryer depends on factors such as the radius of the exhaust vent and the power of the blower fan.
For most domestic dryers, air is exhausted at 100 to 250 CFM. Many newer fans may favor 200 CFM.
However, if you have two dryers that exhaust air at 150 CFM, for example, you will still need to provide makeup air as the CFM from each dryer is compounded.
With regards to how much makeup air must be provided, it needs to match the amount of air that has been exhausted.
If your house is not very airtight, then some of this can be countered by what is known as infiltration and the makeup air that has to be deliberately provided will be lowered.
Dryers in Closets Must Be Provided With Makeup Air
In the IRC, Section G2439.5.1 states that closets designed to contain clothes dryers must be supplied with makeup air.
This can be achieved through an opening with an area of at least 100 square inches, or by other approved means, such as through vents/ducts or makeup air systems.
That means that if you have a dryer that is placed in a closet within a laundry room, it is not necessary to supply makeup air to the laundry room itself, but air must be supplied to the closet.
These requirements are to ensure that enough makeup air is available for the proper functioning of the dryer, as tightly sealed spaces can rapidly become depressurized.
Local Requirements Can Differ
The IRC is a generalized code of regulations adopted by most states (Wisconsin is the exception).
However, in some cases there may be local changes to the IRC, and in these cases, the local adaptations take precedence.
For example, in Nevada there are slightly different requirements for dryer makeup air.
In this case, makeup air allowances are divided for Type 1 clothes dryers (dryers used primarily for domestic use) and Type 2 clothes dryers (for business or commercial purposes).
Type 1 clothes dryers are required to have makeup air provided according to manufacturer instructions.
Type 2 dryers are required to be provided with makeup air via a free area of at least 1 square inch for every 1000 Btu/h of the total input rating of the dryer.
The requirements for closet dryers are the same as stated in the IRC.
Local requirements likely differ by state because of differences in climate, economy, and resources, as well as differing state government policies.