The Simple Explanation of Vapor Barriers

OK, admit it.  You’ve heard the term vapor barrier more than once but you don’t really know exactly what one is.  Worse, it seems like the more often you hear the term, the more confusing it gets.  So here it comes, the simplified explanation of the vapor barrier.

A vapor barrier is anything that completely stops the movement of water vapor.

It’s that simple.  So why does it sometimes get so confusing?  One reason for the confusion is we have so many similar-sounding terms.  In addition to vapor barriers, we also have water barriers, air barriers and vapor retarders to name just a few.

Another reason is that sometimes when people speak about a vapor barrier, they are thinking of a material and other times they are thinking of an entire home.  A roll of plastic sheet or a piece of glass will stop the movement of water vapor.  They are examples of vapor barriers.

When we ask whether a house has a vapor barrier, what do we mean?  If the house has glass windows, then it contains vapor barriers.  But does it have a continuous barrier, forming a complete boundary all around the home?  Maybe, maybe not.

Understanding Water Vapor

It helps to know a little bit about water vapor and how it moves.  Of course water can exist in three states:  solid, liquid and gas.  Solid water is ice.  Liquid water is, well… water.  Water vapor is a gas.

Water vapor mixes with air.  When air moves, the water vapor moves with it.  This is the major way that water vapor moves in and out of our homes.  This means that something which stops the movement of air (an air barrier) will also stop most of the movement of water vapor in and out of our homes.

In addition to moving with air, water vapor can also move by diffusion through building materials.  Some materials do not allow diffusion.  We refer to those materials as vapor barriers. 

Then There was Tyvek

This brings us to one of the more confusing parts of the vapor barrier story:  Tyvek.  Tyvek is a brand name for a type of house wrap.  House wraps come in different types and are primarily used as water barriers.  This means they stop the movement of liquid water.  When it rains, the rain water cannot get through the house wrap.

If the house wrap is properly installed continuously around the boundary of the home then, in conjunction with the roofing materials, the home has a water barrier.

Water vapor can travel right through the house wrap.  On purpose.  The benefit of this is the ability to separate the need for a water barrier from the decision of whether to use a vapor barrier.  The Tyvek is a water barrier but not a vapor barrier. 

Do You Need a Vapor Barrier?

So now we know that a vapor barrier is anything that completely stops the movement of water vapor.  Do our homes need them?  In the climate of the St. Louis area, the short answer is, it depends… and that’s a story for another day.