Will just cutting a bigger gap under the door fix it?

It often helps meaningfully and is one of the lower-cost fixes, but the gap needs to be sized to the airflow the room's register actually delivers, not just made 'a bit bigger'.

Is this a sign my ducts are leaking?

Not usually. Duct leakage tends to reduce delivery regardless of door position, while this symptom is specifically tied to whether the door is open or closed, which points to return air instead.

Would a return-air grille in the door itself help?

A door-mounted transfer grille or a jump duct through the wall above the door are both standard ways to give supply air a path back to the central return without leaving the door open.

Does this affect the whole system, or just this room?

It's usually localised to the closed room, though in houses with limited overall return capacity, closing several doors at once can affect the whole system's balance.

Hot & Cold Rooms

Why Does My Room Get Hot When the Door Is Closed?

A room that heats up specifically when its door is closed usually has a return-air problem, not a supply problem. Closing the door without a return grille, transfer grille or sufficient undercut traps the supply air's pressure with nowhere to exit, so less cool air can actually enter the room even though the register keeps blowing.

Key takeaways

This pattern points specifically at return air, not duct leakage or sizing.
Supply air needs an exit path back to the return, or pressure builds and restricts new air entering.
The fix is usually a return path, not more supply airflow.
If the room is fine with the door open, the branch itself is very likely delivering enough air.

Most likely causes

No return-air path from the room

Most bedrooms rely on the gap under the door to let air travel back to a central return. Closing the door without a transfer grille or dedicated return traps supply air's exit route.

Undersized door undercut

Even where a gap exists, it may be too small to pass the volume of air the room's supply register delivers, throttling supply as pressure rises with the door shut.

Room pressurisation reducing supply

As pressure builds behind a closed door with no outlet, the pressure difference between the duct and the room narrows, and less air actually leaves the supply register than when the door is open.

Whole-house return imbalance

In houses with limited return capacity relative to supply, closing any door redistributes pressure and airflow across every room served by the same trunk, not only the closed one.

What you can safely check yourself

Feel register airflow with the door open, then again a minute after closing it.
Look under the door for a gap and estimate its height against the door width.
Check for a transfer grille, jump duct grille or dedicated return vent in the room.
Try propping the door open an inch and note whether the room recovers.
Check whether other rooms feel different when this door is closed, suggesting a shared-return effect.

Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.

What a professional should measure

Room pressure measurement with the door open and closed.
Static pressure and airflow measurement at the supply register in both conditions.
Assessment of total return grille capacity against total supply air being delivered.
Evaluation of options: transfer grille, jump duct, or dedicated return.

When duct leakage could be involved

This symptom is a pressure-balance issue between supply and return air, not typically a duct leakage issue. Duct leakage would generally show a similar shortfall whether the door is open or closed, since it happens inside the duct itself before air ever reaches the register.

Because the pattern flips cleanly with door position, it's a strong signal to look first at the return-air path rather than the duct run's condition.

When this probably isn't a duct-sealing problem

The room cools normally with the door open, ruling out a chronic delivery shortfall.
The change happens quickly and consistently as soon as the door closes.
Airflow at the register is strong immediately after the door closes and only tapers as pressure builds.
Other rooms sharing the same branch are unaffected.

When to call an HVAC contractor

Duct Daddy specialises in duct leakage testing and duct sealing. We do not perform mechanical HVAC repair, refrigerant work or equipment replacement. Contact a qualified HVAC contractor if any of the following apply:

The room stays hot even with the door left open.
You hear whistling or feel strong resistance closing several doors in the house, suggesting a broader return-air shortage.
Adding a transfer grille or undercutting the door doesn't meaningfully change the airflow.
You want a dedicated return added, which involves duct modification best assessed by a professional.

Frequently asked questions

Will just cutting a bigger gap under the door fix it?: It often helps meaningfully and is one of the lower-cost fixes, but the gap needs to be sized to the airflow the room's register actually delivers, not just made 'a bit bigger'.
Is this a sign my ducts are leaking?: Not usually. Duct leakage tends to reduce delivery regardless of door position, while this symptom is specifically tied to whether the door is open or closed, which points to return air instead.
Would a return-air grille in the door itself help?: A door-mounted transfer grille or a jump duct through the wall above the door are both standard ways to give supply air a path back to the central return without leaving the door open.
Does this affect the whole system, or just this room?: It's usually localised to the closed room, though in houses with limited overall return capacity, closing several doors at once can affect the whole system's balance.

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