Vapour Diffusion Retarders are Better than Vapour Barriers

An air barrier has to be a continuous (sealed) barrier to stop air flow through the envelope. A vapour barrier stops moisture diffusion through building materials. They have two distinct functions, but it’s confusing because in many cases, one material has been the go-to material for BOTH of those control layers.

The material in question is 6 mil polyethylene sheathing. Many builders are now turning to other materials as air barriers on the exterior of the envelope, meaning that the poly is now a single-purpose material: a vapour barrier.

What’s a vapour barrier?

In order to combat vapour diffusion, US and Canadian building codes require a vapour barrier: a membrane that has a perm rating less than 1.0 perm. A material with less than 1 perm is not a perfect barrier, so we often switch out the term ‘barrier’ with ‘retarder.’ Sometimes it’s a more complete description: vapour diffusion retarder, and often that’s shortened to VDR. 

In cold climates, the vapour retarder is installed on the warm side – typically directly behind the drywall. This location prevents the warm and relatively humid air from traveling into the wall where it may condense.

But here’s the thing:

Building scientists agree that the use of vapour impermeable retarders or barriers are not always necessary and in fact can be a poor strategy in all except the coldest climates such as northern Canada and Alaska. This is because the use of vapour impermeable barriers has two significant flaws:

1. The vapour impermeable barrier is on the wrong side during the summer months in air-conditioned buildings. That used to be more regionally specific in Canadian housing, and more focussed on larger buildings. With the increased uptake of heat pumps, supplying both heat and cooling, we’re seeing a lot more houses with air conditioning than we did previously.
2. Construction is never perfect. That’s ok, for the vapour barrier, as it’s there to control diffusion through materials and that’s a slow transfer of moisture driven by the difference in water vapour pressure and the material's resistance to this pressure. When the poly is also the air barrier, it’s a big problem, as the air barrier is supposed to stop the flow of moisture-laden air into the building assemblies - which can account for 100x more moisture movement than diffusion. 

So while it’s not a big deal if the vapour barrier is not continuous and sealed, it’s a REALLY big deal if the poly-as-air-barrier is not continuous. 

The challenge is this: with an impermeable vapour barrier installed on the inside, the envelope is limited to drying only to the exterior. 

According to many experts, including Joe Lstirbrek at Building Science Corporation, vapour impermeable barriers can be avoided completely in all building types except those with very high humidity levels, such as indoor pools and spas, and those in extreme cold regions - oh - that’s a lot of Canada, eh?!

So, we’re kinda stuck with 6 mil poly. Right?

Not really.

The building code requires the installation of a vapour barrier. Part 9.25.4.2 states that “Vapour barriers shall have a permeance not greater than 60 ng/(Pa s m2)” (approximately 1 US perm). Here’s where the confusion and reliance on poly comes in. The code also states that any material other than polyethylene shall conform to “Vapour Barrier, Sheet, Excluding Polyethylene, for Use in Building Construction.” It’s easy to see how reading the building code could lead you to believe that polyethylene is the required standard. Poly is the default material, but it’s not the required standard. 

One of the challenges here is the interpretation of the building code by the building official, the authority having jurisdiction (AHJ), ie, your building official. Whether you can forego the poly depends on the individual AHJ.

The assumed requirement for using an impermeable barrier, and specifically 6 mil poly, is the result of limited research back in the day as well as limited technology in terms of materials. It certainly has made a long-lasting mark on the industry, as even today, there are many builders, AHJs and engineers who will say that poly must be in the assembly.
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3 Guidelines for Drying Assemblies in Canadian Houses

Some notes on drying assemblies for Canadian houses, and US houses in heating dominated climates:

1. Install building materials so that the least permeable material is installed on the interior side, with the permeability increasing as you move through the building assembly towards the exterior. This allows the building to dry to the outside.
2. Avoid the dreaded vapour sandwich: don’t install low vapour permeable building materials like rigid insulation, or spray foam on the outside if you are using poly on the inside. 
3. When in doubt, use a smart vapour retarder. These materials change at the molecular level as humidity changes, helping the wall dry. The typical range is from one perm or less when the cavity is dry, to greater than 10 perms when the cavity is moist. This promotes drying within the wall if it becomes wet.  These are good alternative options, especially for buildings with exterior insulation, in basement upgrades, and interior retrofits.