# How does opening a window change the air pressure of a bedroom?

I open my window and now whenever I open or shut my bedroom door i slam it hard accidentally.

There is less air resistance to my bedroom door.

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Opening your window is like opening a coke can. Shake the can when its closed, it will increase pressure by exciting the carbonated fluid which makes the can feel harder when you squeeze it. But when you open it and shake it, the coke spills out the top.

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What?

A room is not airtight like a coke can.

The more likely answer is that by opening a window you are allowing air currents to move through the room, and they are pushing slightly on the door.

DrGreg
Gold Member
I open my window and now whenever I open or shut my bedroom door i slam it hard accidentally.

There is less air resistance to my bedroom door.
When you close the door quickly, you are pushing air out of the room. When the window is shut, this causes a temporary slight decrease in air pressure, and the difference in pressure applies a force to door to slow it down slightly.

If the window is open, as the door pushes air out of the room, more air comes in through the window, the air pressure doesn't decrease and nothing slows down the door.

If the window is open, as the door pushes air out of the room, more air comes in through the window, the air pressure doesn't decrease and nothing slows down the door.
Why would air come in through the window when you close the door? Shouldn't air be pushed out of the window?

DrGreg
Gold Member
Why would air come in through the window when you close the door? Shouldn't air be pushed out of the window?
I am thinking of a door that opens inwards, as most room doors do in the country I live in. Maybe it's the other way round in some other countries.

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This all depends so much on the setup. Across an area slight temperature differences (humidity differences and whatnot) cause a slight force that constantly accelerates air until you have wind. Wind blowing over your building will cause a pressure difference between the front and the back. Adding to this is a pressure difference because one side of the house is usually warmer. Since a house is not air sealed opening a window will often cause a draft of air to gush through the window, because you break the pressure flow equilibrium that you had with closed windows.
Then we have the turbulent flow around houses that causes the pressure in front of the window to oscillate, and this will lead to wind in and out of the house even if the rest of it was perfectly sealed.

So if I understand your question correctly your door tends to slam when the window is open. This is probably caused by wind blowing back and forth faster then your hand can adjust to it. So you try to close at a certain speed, but suddenly the wind pushes in the same direction and you slam. If the wind goes in the opposite direction you don't slam but you push harder, and then suddenly the wind swings again and you slam.

turbo
Gold Member
The same thing happens in a well-built car. If you are used to riding alone, you get a feeling for just how much force you need to use to close and latch the door. If someone else is with you and has a door ajar, you will slam the door if you use the same amount of force that you normally do.

Welcome to PF foamlover!

Possibly due to a "draft" which might have a couple of causes.

Open the door a "crack" and light a match near the opening. Blow out the match and observe the movement of the smoke... with the window open and again with the window closed. Any difference would be a sign that a draft is occuring.

The draft might result from ambient (outside) wind direction or from the effects of a forced air ventilating system inside the building. If the latter, consider the location of outlet registers and return-air ducts and try the match experiment with the ventilating system fan turned off.

Let us know how the experiment works out...

There is usually a wind outside. At your window, the air is blowing some particles away and some others take their place. But if you think about it, air isnt very dense.

Now, when you open your window and some of the particles have been blown away, the air particles in the room, right at the window, can take their place. The same thing happens with the free space in the room which was created by the particles moving away to the outside of the window. And this happens again and again.
This can happen in many ways though, for example in reverse when the pressure inside the room is smaller than outside the window.

When you open the door, it just creates a free path for the air, equalizing the pressures.

This is entirely how I understand it though. Feel free to correct me.

The bedroom door is inward. I walk towards the bedroom and the door swings inward towards the bed.

The window and the door are both on the same sides of the room as well. It is hard to explain but the window is probably as close as possible to the door as it can be if you were to chose a random spot for the window.

In the event of a tornado, the strong winds that blow by houses can be catastrophic not due to the direct wind force, but due to the Bernoulli effects. Doing research on the topic for one of my papers for my fluid mechanics class, I found that pressure differences of nearly 20kPa (~3psi) are apparent. This means that extremely large forces can be caused by winds blowing parallel to the side of the house with your window.

Not sure if this is what was causing what you experienced, but it's interesting nonetheless!

DrGreg
Gold Member
The way this has been described, I doubt it has much to do with the wind. I think this would happen even on a windless day. The explanation I gave in post #6 needs no wind.

You might like to try this experiment. With the window closed, hold your door open by just a few inches or cm, put your hand close to* the gap between the door and the frame, and close the door quickly. You should feel on your hand a puff of wind, as air has to get out of the way of door and some of it comes through the gap. This "cushion of air" acts as brake to slow the door down.

Now repeat the experiment with the window open. You should find that the puff of wind is noticeably less this time. If there is also a window in the corridor or room that is outside your room, the puff of wind should be greatest when both windows are shut, and least when both windows are open.

You might also like to try the experiment on windy days and windless days and see if that makes a difference. If you can actually feel the wind blowing through your room, then it might make a difference, but otherwise not.
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*Health and safety: but not too close to the gap to risk trapping your fingers in the door, please!

Entropee
Gold Member
Simply put, when the window is closed, the air you are forcing into your room by closing your door has nowhere to go but back out, so the door requires more of a force to close. When the window is open the forced air goes right out the window and the door closes with ease and can easily be slammed. My room does the same thing. Sorry that I gave a non-physics answer, wasn't sure what you were looking for.