One question I get asked a lot is how to deal with vapor, what is the best vapor control product, etc… Before I get into vapor control, lets look at the four control layers and their order of importance:

  1. Water Control Layer
  2. Air Control Layer
  3. Vapor Control Layer
  4. Thermal Control Layer

The first three all deal with moisture. (There is always some moisture in air.)  The order of importance is based on how much moisture the control layer sees.  Of course rain and snow will produce the most moisture, and present the highest risk to a structure.  This is the reason the water control layer will always be on the exterior of the building, we want to keep as much water as possible out of the buildings assemblies.

The air and vapor layers can be either inside or outside, or maybe both.  In the case of the vapor control layer, the location will be determined based on your climate.  In a heating dominated region such as mine, the vapor control layer will always be on the warm in winter side of an assembly.  The air control can be either or both.  Air transportation of moisture is almost always more important to control.  This is the reason you want to build a “tight” structure.

So, what is the best way to control issues with interior vapor, either moving in air or through building materials?  These are in no particular order and the importance will most likely be based on the design of the structure. First, control the amount of moisture in the structure.  Second, control the amount of air moving through the building assemblies.  Third, use a product that will limit vapor movement through building materials, I recommend a smart vapor control product.  But probably the best way to control vapor is to not give it a surface to condense on.  In other words, keep the first condensing surface above the dew point temperature.  It’s funny how the least important control layer, thermal, can be used to solve for vapor.  Use exterior insulation.  If enough is used, there is no longer the need for a smart vapor control product.  We can switch to a simple and inexpensive vapor control, painted drywall.

Part of the reason for this blog post, besides the fact that I’m asked about vapor often, is I’ve been involved in several recent building diagnostic investigations where water and ice had formed on the exterior cladding during the heating season.  There are several reasons for this accumulation of wintertime moisture, all could have been prevented by the use of exterior insulation.

In my opinion, these are the best ways to control interior vapor concerns, at least in my climate.  That being said, I would like to hear your thoughts on the subject.  Please leave me a comment!

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