By David Braddy LEED GA
We have heard for years that a home can be built to tight and that a home has to breathe. Well a home does not have to breathe; you do and there are ways that are much safer than leaky walls. Before air conditioning, this was never really a problem. As a matter of fact many homes were built without much, if any insulation.
How many homes today do not have air conditioning?
This changed building science tremendously and created a new set of problems that we are just now starting to address and understand. This problem is called Vapor Drive
Here are a few simple facts to help you understand this:
- All siding will eventually leak and is not intended to be the air or moisture barrier
- Water Vapor always moves from warm and humid to the cold and dry side
- Water Vapor is driven through even the smallest crack or opening, and many building materials themselves.
- This water vapor is the primary cause of mold & rot, not necessarily an actual water leak.
- If air can move through a wall so does water vapor.
This is a problem that has caused mold issues in many parts of the country even though builders were following proper building procedures and local building codes. Up until 2007 the International Building Code classified the entire country as a cold climate with only one solution for vapor drive and this caused many serious problems. Unfortunately those outdated codes and methods are still in use throughout many parts of the country.
The standard solution was to put a vapor barrier or poly on the inside of the wall assembly, this is fine in a cold climate with warm humid air on the inside most of the time, as this keeps the moisture from entering the wall cavity from inside, but in our area for instance we do not have that kind of climate. We have hot humid summers and air conditioned homes. Even in the winters we have very little humidity inside of our homes to create vapor drive from the inside out, yet that is what we have been told is proper building method for our area.
In our area Vapor Drive forces moisture from outside to the cool dry inside. This is opposite of the old one size fits all code and the reason for its change.
When you fill a wall with a highly vapor-permeable insulation (fiberglass batts) and cover the wrong side with a non-permeable vapor retarder you can have moisture problems that are unhealthy for you and your home, as this can become a wood rotting, mold feeding liquid, created by Vapor Drive.
Make sure before you build or remodel you are using the proper method for your area, as many local codes have not been updated. A simple discussion with your builder and local building inspector will usually suffice, as there is a plethora of information on this subject.
One last thing; for an air or vapor barrier to be effective it must be continuous, vapor drive can be quite strong and find even the smallest openings in a protective barrier. Pay special attention around door and window openings and use high quality flashing around all openings to ensure a good seal in the building envelope.