Vapor Barriers in walls, why polyethylene can be problematic

It would probably surprise many home builders to hear what really causes moisture accumulation in walls, and what to do to prevent it. An understanding of how water vapor moves through walls is important, so a good place to start would be with our page explaining moisture movement in homes (see related articles below).

The traditional approach to preventing water vapor from penetrating walls in homes is a 6 mil polyethylene vapour barrier, or 'vapor barrier' for our southern neighbours. This is an ideal building practice in the extreme northern communities of Canada, less so as you come further south. Despite it being used extensively in residential construction, it can be overkill in most Canadian homes, and can cause problems of its own.

"One of the problems in the building industry is that we have a spreading 'cult-like' mentality that worships at the 'church of polyethylene'. This cult views the answer to all moisture problems as the installation of a polyethylene vapor barrier on the inside of buildings. This cult is responsible for many more building failures than building successes. It's time that the cult deprogramming started."

- Joe Lstiburek, Principal of Building Science Corporation


The USA & Canada has many climatic zones, so there is not one building envelope that can possibly serve them all. The automatic installation of a polyethylene vapour barrier in every home from the Hudson Bay to the vineyards of Southern Ontario to the deserts of Arizona meets the state & provincial building codes, but completely ignores the reality of how different those climates are.

Many parts of the country can range from extreme cold to extreme heat and humidity, with temperatures that vary as much as 60 degrees Celsius or more. In areas like that, the vapour barrier that works great in February isn't doing you any favours in July. During those 30+° Celsius days with relative humidity levels upwards of 80% and an indoor air-conditioned environment some 10 degrees cooler, that vapour barrier is on the wrong side.

Is the solution then to not install a vapour barrier? No, but since there isn't a perfect solution that meets the needs of both climatic extremes, we should find a solution that at least takes them both into account.

The vast majority of Americans & Canadians live in a temperate climate, so for most of us a vapour barrier (or more accurately semi-permeable vapor retarder) that allows a certain amount of water vapour to pass through a wall could actually serve us better over the course of the year.

As warm, humid air cools, air molecules shrink and squeeze out the moisture. This can be a problem if it happens inside your walls, so vapour barriers are there to mitigate that.

In order to prevent condensation from forming, a vapour barrier should be placed on the warm side of your insulation to stop warm, moist air from condensing on a cold surface inside your wall.

In cold climates like Canada, for most of the year the vapour barrier should be on the inside of the insulation. In hot climates like the southern U.S. for example, it should be installed on the outside of the insulation. 

In both cases, the vapour barrier is tasked with preventing warm, humid air from shedding its moisture as it meets a cool surface, no matter which direction it is travelling.

The most important thing to realize is that there is no fixed rule regarding vapor barriers. Building practices should always be determined by the climate zone in which you are building.

Understanding vapor barriers:

The National Building Code of Canada stipulates that for residential buildings, a vapour barrier must have a water vapour permeance of less than 60 ng/Pa*s*m2 or 1.0 Perm. That means that no more than 60 nanograms of water vapour can pass through a square meter of the material in one second. Nanograms are pretty small by the way, that's one billionth of a gram.

Traditionally, a polyethylene vapour barrier (with a vapour permeance rating of 3.4 ng) is installed behind the drywall in new Canadian homes. In fact, you would be hard pressed to find a home being built in Canada right now that does not have it, or something equally impermeable to moisture. This doesn't mean that there aren't other options out there, they just aren't being applied.

In the US any material that has a perm rating of 1 or less is considered to be an adequate vapor retarder for residential construction. As requirements vary between states we suggest giving your local permit office a call to establish their recommendations. The perm rating is a measure of the diffusion of water vapor through a material & the table below shows the perm rating of some common building materials that are consistent with the ASHRAE Handbook of Fundamentals and other industry sources.

US perm rates for materials
US perm rates for common materials ASHRAE Handbook

The problem is largely because the 6 mil poly that gets installed as a vapour barrier is mistaken for, and almost entirely relied upon to act as the air barrier. The purpose of the two barriers should not be confused - the job of the vapour barrier is to control vapour diffusion, the job of the air barrier is to control air leakage.

6 mil poly can work effectively as an air barrier if it is carefully sealed, but so can other materials. Well-sealed drywall in itself makes a great air barrier. But unless you install polyethylene with the express purpose of it being an air barrier, it likely isn't doing the job. And in fact, the term 'air barrier' is rarely if ever used in mainstream residential construction, and it really should be.

Vapour retarder latex primers:

Firstly, the classification of a material as either an impermeable ‘vapour barrier’ or a semi-permeable ‘vapour retarder’ is determined by how much water vapour passes through the material under specific conditions.

There are vapour retarder primers on the market that exceed the requirements of the National Building Code of Canada & local US building code regarding water vapour diffusion, with a vapour permeance in the area of 30 to 36 ng, which is about half of the 60 ng often allowed by code.

Vapor barrier primer meets building code
Vapor barrier primer meets building code © Ecohome

So concerns that primers are insufficient to control vapour diffusion are unfounded, they just aren't widely used. But keep in mind that the construction industry can be slow to adopt new practices, regardless of the merits. So don't be intimidated if you want to break the norm.

Air leakage:

Now that we have looked at some options regarding vapour barriers time to understand the difference to air barriers, and firstly it should be pointed out that the water vapour permeating through building materials - the reason for installing a vapour barrier - is not the monster it has been made out to be. 100 times more water vapour is carried though a wall assembly by air leakage, than is carried by vapour diffusion. So the air barrier is 100 times more important than the vapour barrier.

Therefore, we really don't need to go to the extremes that we do regarding vapour barriers, as it actually takes the focus away from what we should be thinking about, which is creating an effective air barrier.

So here is the summed-up case for the “poly-free” house, and a bit of perspective:

  • Water vapour diffusion through building materials accounts for only about 2% of moisture penetration through walls, and a vapour retarder primer can be twice as effective as it needs to be.
  • Polyethylene is some 15 times more resistant to water vapour diffusion than it needs to be; it's expensive to buy and install; is environmentally questionable; and it can actually cause problems in the summer months.

In much of the country, you could take the time and money you would have spent on installing polyethylene on the entire exterior wall of your home, and instead put those resources into a latex vapour retarder paint on primer and a properly sealed air barrier. There are hard cost savings to be had doing this, and an improvement in both performance and durability.

The one glitch in the system, is that building inspectors can also be subject to the same conditioning that a lot of builders are, and do not realize that in many cases there are better options available than polyethylene for controlling water vapor in homes. When you take your plans in to get a permit, make sure it is clear what material you plan to use for water vapor control, so that you can go into battle then, and not during a home inspection after construction is complete.


Lstiburek (2004):

U.S. Building Code requirements for vapor retarders are proposed based on climate and properties of other materials in the wall assembly. Identified hygrothermal regions include those applicable to Canada. Most assemblies do not use polyethylene and incorporate latex paint or vapor semi-permeable interior finishes.

The following main principles are recommended:

  • Avoid vapor barriers where vapor retarders will work, avoid vapor retarders where vapor permeable materials will work.
  • Avoid the installation of a vapor barrier on both sides of the wall assembly.
  • Avoid using poly, foil faced batts, reflective barrier foils, and vinyl wall coverings on the interior of air-conditioned assemblies.
  • Ventilate enclosures

To read why you shouldn't install air conditioning in a home with a polyethylene vapor barrier see here, from the EcoHome Green Building guides