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SpecialistMichael
SpecialistMichael, MS, CSCS
Category: General
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Experience:  Senior Information Specialist
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Why does a door sometimes swell when it rains heavily? Can

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Why does a door sometimes swell when it rains heavily? Can you please explain how this phenomenom works? Thank you.
My name is XXXXX XXXXX you for using JustAnswer. This is a great question, its my pleasure to assist you one.

The reason a door swells during the rain is because of the material it is made of - wood. In the case of a metal door, there is still a high percentage chance the frame and threshold are made of wood too. Unsealed wood is an incredibly porous surface and when there is moisture in the air like during rain, or on hot muggy days, the water molecules that are in the area simply migrate into the porous surface. Due to the lack of space then within these pores in the wood, it has to expand as changes in heat and or space obviously create a barely measurable pressure difference.

You would see the same with an unsealed sundeck, any other non sealed woods - moisture migrates, causes a change in space between the material its inside and it just has to expand. Think of this as trying to stuff more people in a very tightly packed room...but the room walls are made of rubber. The room will probably expand in all directions due to the new people being added, but even a tiny bit more from the heat(like that a on a muggy day).

*To break this down even more, on hot rainy days the heat excites the "steam"(water vapor in the air) so much that even that can cause greater expansion. Any addition in heat causes excitation of materials at a molecular level at least to some degree. This excitation translates into a greater pressure exerted by the gas(water vapor in this case) and thus greater expansion of a material like wood that due to its porosity readily accepts migration of water vapor. So basically its not the rain itself, but the fact that the concentration of water in the air(humidity DURING rain and the fact unsealed porous wood and wood in general accepts migration of that water vapor.

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Laughing Mike
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Customer: replied 4 years ago.

How about when the same phenomenom happens on a very cold day? What is it that causes a door to swell on a very cold day instead of the water molecules and heat in a rainy day situation?

Great question.

Its actually a function of the wood and the moisture in the air, not the heat. The heat only excites that water vapor.

From a chemistry standpoint everything moves along a gradient, from high to low. So if wood is dried completely then built into a house or a door or whatever. When you end up in a situation of moist air, the vapor naturally want to move into a space where their is "less". Think of it as 2 rooms, 1 room is full of people, the other room is empty, people will naturally move into the other room until it equalizes. Now with wood there is a limit to equalization but thats why water vapor(moisture) wants to move into the wood - its a lower concentration.

If you think of natural trees, they have tons of vasculature for transporting water and nutrients and shuttling all different things.

When they manufacture wood, they always need actually dry the wood so that it warps less and is more stiff compared to only partially dried wood. As simple as it sounds, the same thing happens to potato chips and corn chips, oatmeal. Anything that is dried when its manufactured "wants" to reabsorb moisture. This is one of the reasons pressure treated wood is preferred over non pressure treated - it lasts considerably longer and wears much better. These are the reasons we stain wood decks, paint wood and wood shingles, it provides a barrier to slow that moisture absorption process.

If this has helped, please remember to press accept so I am credited with assisting you. Also we can continue our correspondence by simply pressing reply, even after accepting each answer.

Laughing Mike
SpecialistMichael and 49 other General Specialists are ready to help you