Whether you are one who believes that houses need to breathe or don’t, there is one item all can agree on – nature abhors a vacuum. When this vacuum is caused by mechanical equipment, well, things start to get interesting; bathrooms taking forever to clear up condensation, kitchen smells that linger, clothes taking to long to dry, equipment working harder (& failing faster) than it should, & the list goes on. In order for mechanical equipment to work properly it needs a fresh supply of air.
The Codes:
For years all the code books have dealt with was combustion air (Boilers, Furnaces, Water Heaters), makeup air for kitchen exhaust (very limited) and fireplaces. While they addressed bathroom exhaust and dryer venting requirements, they never addressed the need for make-up air for these items. As for kitchen exhaust they limit the requirements for makeup air to only units serving a gas stove (or that have naturally drafted appliance) that exceeds 400 cfm. For the most part the codes have been quiet, leaving it to the manufacturers to deal with makeup air requirements. With that said we wouldn’t need to worry about kitchen exhaust as the manuals state as you should crack a window… Oh, you didn’t read the manual…
 In the 2024 Mechanical Code things do start to change though they are still limited:
1502.6 while it would be great if it applied globally, it doesn’t as it only applies to dryer exhaust, more specifically based on the language – only heat pump / condensing style dryers. Regular gas and electric dryers exhaust sits right at 200 cfm while heat pump style ones require more CFM to work properly.
Kitchen exhaust also got updated but that only eliminates the option of using worst case depressurization test as a get of jail free card as they mandate an exterior air duct. |
Do I have an issue?
More than likely the answer is yes if you are researching this or you notice items like the ones above. Granted it could also be caused by improper exhaust duct installation but that has been covered in other pieces (see links above).
The most accurate way is to measure with the exhaust fans running or HVAC system running with the door closed and using a manometer. If the reading is 3 PA or less that is deemed acceptable. If it is higher than that it shows you have a pressure imbalance.
Don’t have a manometer or want to pay for someone to test, just look around the door jambs & flooring. If the carper is darker aka dirty in the area of the door or you see a bunch of dust on the jambs that is a sure giveaway. Another one is, are the doors harder to close or do they shut when equipment starts running?
Can Undercutting a door help?
In short, yes it can. Is it enough for what you need, well that is a better question (see below for chart). Are there other options available, yes there are. One good item is even if undercutting a door is not enough the effects are cumulative. For example, in a bathroom, you probably have a supply duct for your HVAC. Granted you probably have it minimized so you probably only get 10 – 20 CFM of air that way but adding in the door undercut just might be enough for your needs.
How much of an undercut do I need?
As a general rule of thumb (which is based on testing) you will get an average of 2 CFM per square inch of air space keeping you in that 3 PA or lower range. This chart was based off of that with items being rounded to the nearest 1/8 of an inch. Thus, instead of 1 1/24″ for a 24″ door it reads 1″. Yes, I also know that a 24″ door isn’t a true 24″ especially if doorstops are in the way but with leakage around jambs you should be good. If you want to add a 1/16 to an 1/8 of an inch or more to the measurement, you are always free to.
So, if you install a code minimum bathroom fan (50 CFM) assuming a standard 30″ door the undercut should be 7/8″. Please note that this 7/8 of an inch is the entire open space under the door to the top of the highest flooring (yes that includes shag carpet). You may actually have that by default from the door manufacturer as most jambs are about an inch longer though you would have to measure to be sure.
As a practical matter, exceeding 1 1/2″ to 2″ of undercut, things start to look off so if you are exceeding that mark you will probably need to add another item to get to where you need.
 Cutting doors – a sharp saw blade meant for finish work or crosscutting with no kerfs is optimal for cutting doors. I also will wrap the door with blue tape around the cut area & if you are not using a track saw where the plate will run. Be careful butting hollow core doors as most are limited to a 1/2″ maximum without requiring you to add a filler into the bottom after the cut. |
Dryers in closets or other small rooms:
Whether you tripped the 2024 code requirement or not, dryers need air to properly dry clothes. As mentioned most dryers need 200 CFM or 100 square inches while heat pump styles do need more. Unfortunately, many washers & dryers are shoved in a small closet and unless you want to leave the door open you won’t get enough make up air in there. Cutting 3″ off a door isn’t really an option. So how about louvered doors – while they can help most modern louvered doors are more for looks than actual air flow. You would actually need to look at the NFA (Net Free Area) to see how much air it actually lets in. In most cases you will have to add a transfer grill of some sort.
One word of note on transfer grills – if you are using the high low method the size of the grill doesn’t matter as much as the size of the stud cavity. For example, while a 20×20 grill might list at 300 cubic inches of NFA you are actually limited to the 50.75 (3.5 x 14.5) area inside the stud cavity – granted that is still 103 CFM’s but still less than the 200 CFM required. Now if you add the 103 CFM to a door undercut you should have the 200 CFM you require.
