Contrary to popular belief, MDF is water-resistant to a certain degree.
MDF handles water, vapor, and moisture rather well.
The problem is that it can be weathered, and the more weathered it becomes, then the more vulnerable to moisture it becomes.
Sustained and/or repeated exposure to water will damage MDF, but not as quickly as many people assume.
Why Do People Love MDF?
It is a cheap-to-produce wood, is durable enough, strong enough, and flame retarded enough to work well in many forms of household furniture.
It is easy to manipulate, and it is fairly light when compared to most other types of wood.
It means you can create a chest of drawers that are not too heavy to lift on your own or you can use it to make an outdoor sign that will last for decades.
These days, it is possible to buy water-resistant MDF that will work well in damp and highly humid areas.
It will not withstand flooding for very long but will maintain its shape and integrity while wet and/or damp for a long time.
Water-resistant MDF often contains resin, and sometimes has treated wood that makes it harder for water to penetrate.
Easy to Use and Versatile
The great thing about MDF is that you can treat it to make it far more water and moisture-resistant.
Even a coat of paint will make it more resistant to the ill effects of water.
The more elements you subject MDF to, then the faster it degrades.
The problem is that when MDF allows water to soak in, then it expands and warps, and this can be made worse if the MDF freezes when wet or is exposed to lots of heat while wet.
The Properties of MDF
If you put MDF under pressure from water, then it will not allow water through.
However, water vapor will eventually soak through, and water will eventually penetrate if the MDF is exposed to water over an extended period.
It actually takes a long time for the water to soak into the various wood elements and chemical/glue elements.
You often see MDF wood with swollen edges because that is where the water was most easily able to enter.
Even doing something like beveling the edges can help stop water soaking into the MDF as quickly.
Thankfully MDF is fairly easy to waterproof.
Be aware MDF will not last for very long under flood conditions, and once it is water damaged, it cannot be fixed.
Either you have to replace the piece of MDF by removing it from the furniture or replace the piece of furniture completely.
If MDF is moist over a long period of time, then it is also likely to become moldy.