Air Pressure in Sealed Window

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  #1  
Old 02-19-10, 06:37 PM
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Air Pressure in Sealed Window

I sealed windows with the clear plastic film. The window in question is a sliding glass window about 4'x5'. I know it is sealed well, I can feel the difference. Some days it seems to have positive pressure in the space because it billows out a little into the room. Some days it has negative pressure in it because it does the opposite. What causes this?
 
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Old 02-19-10, 07:36 PM
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It's likely the direction the wind outside is blowing. Air exerts pressure on a house. Depending on the direction that force is coming from, air is either trying to come in that side of the house, or go out thru that side of the house... which reminds me of Boyle's law...

...look at the "bell jar model lung", something I remembered from science class in school. Amazing that popped into my head. And they thought I wasn't paying attention! lol

The windows in my truck whistle, depending on which way the wind is blowing. If I'm driving down the road and the wind is from the north, the window to the south whistles, and vis versa. Its kind of annoying. Be glad your door doesn't whistle. lol
 
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Old 02-20-10, 04:36 AM
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Hmmm, so when the film moves towards the window, its reacting to the wind blowing against the other side of the house as opposed to the air being sucked from the window side of the house? So I guess the film doesnt seal it completely.

The ucsb link is dead BTW.
 
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Old 02-20-10, 05:27 AM
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Could air temperature play a role? Hot air expands so I assume cold air contracts - just a guess, I don't know for sure
 
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Old 02-20-10, 05:31 AM
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Funny, the link loads for me. http://www.physics.ucsb.edu/~lecture...ng%20Model.htm

Actually, the film could be sealing completely, that's the thing. It's reacting to air pressure. The pressure on one side of the film is greater than the pressure on the other side. It's the difference in pressure when comparing the air between the film and the door, compared with the pressure between the film and the rest of the house.

The window film is reacting just like the sail on a sailboat. The film offers resistance to pressure. Whether or not air is leaking around it is beside the point. Air *is* leaking around the door, and *is* leaking in and out of your house in multiple places but is not necessarily leaking around the plastic film. That diaphragm has to react to positive and negative air pressure.

If you have forced air heat, you might detect a change in the plastic film when your heater turns on and off. Same thing... it's a change in air pressure.
 

Last edited by XSleeper; 02-20-10 at 06:28 AM. Reason: tried to fix link
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Old 02-20-10, 05:34 AM
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I couldn't get the link to work either
 
  #7  
Old 02-20-10, 05:42 AM
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You put the film on the door because the door is leaky. So when there is more air pressure against the door, it pushes through those leaks and pushes against the window film. If the film didn't completely seal, it probably wouldn't billow out like it does.

It may also not be the wind. I had placed plastic over doors to my basement. The colder it got, the more the plastic would bellow. The would be from the stack effect. As the hot air rushed out of my top floor, the cold air would rush into my basement.
 
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Old 02-21-10, 05:57 AM
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Equlibrium

The amount of positive pressure applied to the windward side of an area has an opposite yet equal amount of negative pressure on the leeward side of the same area. In aerodynamics the force on the leeward side in known as "LIFT". This is what causes planes to fly. Unlike a plane our homes and vehicles surfaces are not designed for LIFT but instead designed to avoid LIFT.

The window in this situation have the same force applied to it depending on which way the wind is blowing but the window is rigid enough not to change shape. However it is not air tight enough to avoid air infiltration. It is good to note this one of the reasons for air barriers.

On the other hand the plastic is far more air tight than the window but not nearly as rigid. The result is the plastic so called bellows in the direction by the pressure that's being applied, inward/positive; outward/negative. It is good to note that the air pressure switches we find on heating systems today work on this same principle.
 
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