Why Is The Gap So Wide Under The Doors In My Manufactured Home?

Have you ever walked into a mobile home and immediately noticed the 2 to 3-inch gap underneath the doors? If you have, I bet you’ve wondered why?

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The Short Answer

These gaps under the doors act as the air return system for the HVAC system. The air comes into the rooms via the HVAC registers in the room and then exits the rooms through the gap underneath the door if the door is shut.

Why Such A Large Gap?

When I was about 11 years old, wayyyyy back in the day, my cousins lived in a brand-new double-wide 4 bedroom mobile home. The first time we went for a visit, even at that age I noticed what appeared to be enormous gaps under the doors. I also noticed that the doors seem very thin and hollow. I can remember wondering “why even have doors?”

Large Gap Under Mobile Home Interior Door

My grandparents retired into a mobile home about a year later and I remember seeing the same thing in their home. My grandfather actually worked in his later years as a field repairman for a mobile home manufacturer.

Being the inquisitive kid that I was I remember asking my grandpa about the huge gaps under the doors. With humor and patience he took the opportunity to explain to me what they were,

Forced-air HVAC systems “exhale” the air via the HVAC registers located in every room in the house. They are engineered to try and distribute the air evenly for a steady temperature in the home. These same systems “breathe” the air back in generally through a central vent located in one part of the home. Usually a hallway where the HVAC air handler resides.

The gap under the door exists so that if the door to any particular room is shut, the air can escape the room and return to the central air-return duct back into the system?

So Why Don’t Site-Built Homes Have These Gaps?

Well, in some areas they do. Just not quite as big. Way back before most homes had central HVAC systems, doors to homes often went down close to the floor. We live in Arizona, where most homes have had central air-conditioning since the 1960s. Homes in our area have always had gaps under the doors; usually about 1 inch. They serve the same purpose; to let the air circulate out of the room and back through the HVAC system.

Normal Gap Under Site-Built Home Interior Door

Many older homes had multiple air-return registers throughout the home, but as contractors began to cut costs, many homes built in the 1980s and 1990s had only one air-return register, just like mobile homes.

In the 2000s, the HVAC systems became more energy efficient and sophisticated. Contractors began again to have multiple air-return registers throughout the home. These are where you change your HVAC filter each month. My new home has six of them located throughout the home.

Newer Designed HVAC Systems

Not only were HVAC systems changing for site-site-built homes, but they were changing in the same ways for manufactured homes as well.

HVAC contractors went even further with the efficient air-flow designs for the systems. In some homes vents through the walls were installed above the doors in each room, allowing the air at the top of the room as well as under the door to circulate out of the room.

Many higher-end HVAC systems also have air ducts running through the ceiling of the room connecting rooms together to equalize the air-presser and air-flow. These changes have been made in many newer manufactured homes as well as site-built homes.

Ok, But Why Is The Gap Under The Door In Mobile Homes So Much Bigger?

That, my friend is a good question. Yes, it is for air circulation, but so are the 1-inch gaps in site-built homes. The larger the gap, the more sound that seems to transfer between rooms. Because smaller gaps will transfer the air as efficiently as larger gaps, there is no real reason the gaps need to be 3 inches.

When we remodel manufactured home, we almost always replace the interior doors with new and better ones. When doing this, we install pre-hung doors purchased from our favorite big box home stores. These doors come pre-cut and pre-hung and have only the smaller 1-inch gap under the door.

These work and look great in the homes we renovate and still provide the needed space underneath the door for the air returns. We also install vents above the doors, so that the room gets better air circulation overall, improving the efficiency of the HVAC system.