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The door to a bedroom is closed 24/7. Someone told me, "Be careful. It sounds like you are pressurizing the room."

The room is fairly pressurized, I suppose; I can feel air rushing out from underneath the bottom of the door if I stand outside the room and in front of the closed door.

But what is bad about that? I don't understand the practical significance of air pressure.

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    We're all assuming you have forced hot air in your house. If you don't then you've got real problems!
    – Carl Witthoft
    Commented Jan 5, 2017 at 16:50
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    You might want to explain just what this someone is thinking is the harm if the room gets 'pressurized'.
    – whatsisname
    Commented Jan 5, 2017 at 19:32
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    I was asked to design a pressurized safe room at an Ammonia loading facility a few months ago. Scrolling though my jaw dropped at the title of this. While I learned a ton about pressurized rooms, none of it is applicable to you other than there are basically zero health effects to the type of pressure that you are experiencing.
    – Myles
    Commented Jan 5, 2017 at 19:52
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    My house has one central return duct for the bedrooms and the master bedroom door seals relatively tightly. One effect is that when the door is mostly closed, the air pressure forces the door the rest of the way closed rather abruptly. If I am trying to sneak out of the room to play video games after my wife has fallen asleep, the slamming of the door wakes her up and I have to go back to bed.
    – stannius
    Commented Jan 5, 2017 at 20:04
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    @stannius Kudos for being the first person yet to provide one real downside to a pressurized room. I would agree that a door that energetically closes itself is a nuisance (and, if you have young ones in the home, a hazard).
    – Fil
    Commented Jan 5, 2017 at 20:31

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In a properly configured home, the return air ducting is the exit path for airflow. There's no significant pressure buildup in any room regardless of door position.

In older homes without return ducting, or vents solely in common areas, the return air path is indeed through or around the door. Still, pressure buildup is extremely small unless the door is extraordinarily well sealed.

You should probably ask "someone" what his or her concern actually is. The pressure differential in your situation is miniscule--probably not even enough to pop your ears. The only downside I can come up with is the accumulation of dust bunnies outside your door.

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    My home has forced air heat with hot air registers in each room, but no return air ducts in any room, the return ducts are in the downstairs and upstairs hallways. So closing any room door does make the room "pressurized" in that the only escape is around and under the door. I don't think there's any real harm aside from a little higher backpressure for the furnace blower and perhaps a little less heat in the room since there's less airflow.
    – Johnny
    Commented Jan 5, 2017 at 16:30
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    When a room doesn't have a return duct, it's common to install a vent in the door to relieve the back pressure, especially when the door is expected to be closed all of the time, such as a bathroom door with an automatic closer.
    – Dave Tweed
    Commented Jan 5, 2017 at 16:33
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    Or the wall, I've seen that.
    – Harper - Reinstate Monica
    Commented Jan 5, 2017 at 18:15
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    David Tweed, I am not trying to be contentious, but I have not seen vents in bedroom doors in tract houses which do not have return vents. They would be unsightly and would diminish privacy. Of course, in the days before a/c in the south we had very high ceilings (12 and 14 ft) and there were large swing opening transoms over the doors to allow airflow. This is really unrelated to the question of what happens in closed rooms with forced air central HVAC.
    – Jim Stewart
    Commented Jan 5, 2017 at 20:42
  • Unless it causes problems for the furnace.
    – Joshua
    Commented Jan 5, 2017 at 23:33
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A typical interior doorway has a gap of about 3mm along each side and the top, and often 5-10mm underneath. This gives an area of around 20,000 mm^2, or about the same area as 150mm (6") duct. Unless you take action to prevent airflow through these gaps (e.g. installing draft excluders or an automatic door closer that pushes the door tightly against the frame), you will not see a noticeable build-up of pressure. Now the gap around a door does not exactly provide an unobstructed path for air flow, but even assuming a high friction coefficient for air leaking through the doorway, a 0.05kPa pressure difference (which is very small) would cause about 0.5 to 1 cubic metre (about 15-30 cubic feet) of air flow per minute. Your room will still get adequate ventilation, and you won't find a noticeable pressure change between the two sides of the door.

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As far as "practical effects" goes, the amount of heated (cooled) air going through this room is less than it would otherwise be. This could affect appliance life since it would run longer/more to get the room to temperature, depending on the magnitude of the change of course!

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  • "This could affect appliance life since it would run longer/more to get the room to temperature". As I wrote in my comment to Eric Johnson, my experience has been quite the opposite. Eliminating points of escape for the air in the room (e.g., a doorway) increases the effect of the (heated or cooled) air from the vents, thereby moderately lessening the frequency and/or intensity at which these central appliances need to be run.
    – Fil
    Commented Jan 5, 2017 at 20:46
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If your forced air HVAC system does not have return ducts or if the return in this room is too restrictive, then the return air must flow under the door to a hallway and down to the hallway HVAC intake. If you don't use this bedroom much, you might want to restrict the incoming air at the duct in this room and/or open wider the vents on other rooms to decrease the pressure in the a/c vents and balance the heating and cooling.

If you use this room a lot and need this amount of airflow in, you might consider cutting 1/4 inch off the bottom of the door to allow more flow out of the room. I installed new oak doors on the bedrooms in our house and I erred on the side of leaving the doors too close to the tile floor. This means I have to carefully balance the restriction louvers on the air ducts to avoid pressurizing a particular room.

If there is insufficient space under the door the room may become pressurized and if there is a bathroom off the bedroom you may lose conditioned air through the exhaust vent in the bathroom.

Edit: I now think from the comments and answers that a whole bedroom could not be pressurized enough to force any significant air out the bathroom vent.

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    Based on the way in which this answer is written, it is evident that a pressurized room is to be avoided. I still don't understand the downside, though. What negative results occur when a room doesn't have a large opening for air to escape out of the room?
    – Fil
    Commented Jan 5, 2017 at 18:36
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    I pray that your answer to my above question is not "The room will be pressurized."
    – Fil
    Commented Jan 5, 2017 at 18:41
  • a 1/4 inch gap on the bottom of a 36" wide door gives 8 sq inch of ventilation space, which is actually more than you get from a typical 4" return duct inlet. You don't need to go anywhere near as far as that.
    – Periata Breatta
    Commented Jan 5, 2017 at 19:30
  • I certainly was not thinking of any physiological consequences of having a very slightly pressurized room, and I didn't originally interpret the OP's statement as anyone being concerned about this, but I now realize that some people imagine a problem where there is not one. My only thought (expressed above) is that air could be forced out of the conditioned space into the attic or outside through say the bathroom vent. The effect of this would be that this would result in an overall lower pressure in the house and unconditioned air would flow in through leaks and vents.
    – Jim Stewart
    Commented Jan 5, 2017 at 20:58
  • Leaks in the (pressurized) HVAC ducts into the attic are said to cause a slight under-pressure in the conditioned space which draws in unconditioned air from outside the conditioned space. But on the question as to whether a whole room could be pressurized so much that it could cause this effect the analysis in the comments and answers indicates that there is enough clear space around doors that this effect would be insignificant.
    – Jim Stewart
    Commented Jan 5, 2017 at 21:05
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You could freeze your AC coil or trip your furnace out on over heat, due to lack of air flow. As far as humans are concerned, there is no concern.

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The air will want to move through the path of least resistance. If you block the exits of a room off, it would have the same effect as blocking off the entrance of the air into the room. This will result in the positive pressure, but also no additional airflow into the room as the air will flow into other rooms that have return.

This could result in the room being cold because no additional air would be flowing into it.

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  • "If you block the exits of a room off, it would have the same effect as blocking off the entrance of the air into the room." Huh? Can you explain this in detail? Maybe this is true in a theoretical environment. But, at least in my case, the reason that the door is always kept shut is to intensify the influence of the (heated or cooled) air that is coming out of the room's three floor vents. In other words, when the heat is running and the door is closed, the temperature of the room is a few degrees higher than if the door had been kept open.
    – Fil
    Commented Jan 5, 2017 at 20:04
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    You two are describing two different scenarios. If the return air path is blocked completely, before long enough pressure builds that the furnace cannot force heated air into the room, eventually causing a cooler temperature. In reality, this can't happen--there are always some leaks. However, if the return is only partially blocked, the room tends to accumulate warm air quicker than in other rooms, resulting in a higher temperature.
    – isherwood
    Commented Jan 5, 2017 at 20:15
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I think people are overlooking the more significant issue of any kind of pressure imbalance in a heated or cooled house - condensation.

If a room is hot and forcing air into non-desirable spaces (cracks in the wall or ceiling for example) the drastic change in temperature along surfaces can cause condensation in areas that should be dry. This is a problem in humid environments especially.

Health wise - no issue. A house isn't sealed well enough to build up significant pressure, nor would an air pump keep working in the pressure was too high.

On the plus side - you shouldn't be getting many flies or mosquitos in the room.

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  • Except that it is not forcing the air into the walls or ceiling, it's blowing out under the bathroom door. If there's hot, humid air in the bathroom, it's being blown out into the hall or wherever, cooling and yes, possibly condensing there.
    – FreeMan
    Commented Feb 13 at 13:35

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