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Vapor Barrier: All You Need to Know

What is a Vapor Barrier?

A vapor barrier is designed to prevent the diffusion of vapor (water molecules in the air) through a home’s wall assembly. Different materials will have different degrees of vapor permeability, that is the material’s ability to allow water vapor to pass through it. Technically speaking, vapor barriers can be many different types of material, from gypsum board to rubber membranes to polyethelyene copolymer sheets! 

Why Would I Need a Vapor Barrier?

Simple physics dictates that moisture flows from an area of higher concentration towards an area of lower concentration of moisture, or from a warmer to a cooler space within a building material. Real-life example: over the course of one heating season a third of a quart of water will diffuse through an 8’ x4’ section of drywall. When that moisture gets in your walls bad things happen, like mold, mildew and structural damage.

How a Vapor Barrier Works

A Vapor Barrier isn’t an Air Barrier

If you want a comfortable and efficient living space you need to control for (in relative order of importance) :

  • Water – liquid water
  • Air – leakage
  • Vapor – water molecules moving through a surface
  • Temperature – heat transfer

So once you’ve figured out how to keep water from entering your house, you need to make it is airtight. Air tightness is not the purpose of a traditional vapor barrier. They solve for vapor diffusion, not air leakage. And air, as it turns out, carries a lot of moisture. The movement of water vapor through a 1-inch square hole is significantly greater than the movement of water vapor as a result of vapor diffusion through a 32 sq. ft. sheet of gypsum sheathing. So your moisture problem needs to be solved by addressing vapor and air. We’d further note that airtightness is the most cost-effective measure we can take to improve indoor air quality, comfort, durability, and energy efficiency of your home.

Another Issue with Vapor Barriers

Should moisture enter your wall assembly, you definitely don’t want to trap it, which is a risk with a vapor barrier. They are intended to prevent assemblies from getting wet. However, if moisture does find its way into your walls, vapor barriers often prevent assemblies from drying. For example vapor barriers installed on the interior of assemblies prevent inward drying in any air-conditioned enclosure. As we know, traditional insulation does not handle moisture well (mold, mildew) so your effective R-value of your wall can decrease dramatically as well. 

Vapor Variable Barriers aka Smart Air Barriers Solve for All This

The importance of air tightness is really not debatable. Further, building science consensus is now to allow vapor an escape path so assemblies can dry out either inward, outward, or both. Walls don’t need to breathe but they do need to dry. So we need an air barrier that also allows for moisture management (vapor variable). It should remain tight in winter when humidity in the cavity is low to prevent moisture from entering but also needs to increase permeability in summer to let moisture escape, helping to keep the wall dry. Hence a Vapor Variable Barrier or also referred to as Smart Air Barrier.

Fortunately, there are a number of products that can function as the primary AIR barrier (think air tightness) and as a vapor barrier too. An airtight smart vapor barrier (or retarder) will protect your assembly by keeping conditioned and moist air out of the insulated enclosure all while addressing air tightness. 

Products

Intello Plus is an air barrier, smart vapor control, and dense-pack reinforcement. It functions as a vapor retarder in winter (low permeability), protects against condensation, and maximum vapor openness in summer, high permeability. 

Pro Clima DB+ is a reinforced paper-based smart vapor retarder and air sealing membrane. It’s an ideal environmentally friendly air barrier option. 

Where Does This Smart Vapor Barrier Go? 

There are lots of opinions out there about where to place a barrier. Climate zones play a big role. 

Our view is that for mixed climates the best place for a smart vapor barrier is inboard of the insulation because it will prevent conditioned air from entering the insulation layer and really help minimize or eliminate condensation.  

Vapor Barrier Assembly Diagram

Assembly

Our friends at 475 High Performance Building Supply list out the proper assembly order in a mixed climate, from outside in : 

  • A protective back-vented rainscreen and vented roof
  • A vapor-open, windtight, water control layer
  • Fibrous insulation: Gutex wood fiber insulation or cellulose
  • A vapor variable airtight layer: INTELLO PLUS or DB+
  • A protective service cavity with interior finish