Image by New Copper Photography
Building a home in Manitoba means dealing with some of the most dramatic temperature swings in the world. From deep cold winters to hot, humid summers, Manitoba’s climate puts pressure on a home’s exterior walls and creates unique challenges when building in our province.
The first thing that comes to my mind when thinking about a big temperature swing is insulation and insulation upgrades to make your home run more efficiently. Though the reality is that air, vapour, and weather barriers play a massive role not only in a home’s efficiency but also in its longevity and comfort.
Homeowners may often hear these terms, but the differences between barriers aren’t always apparent. Each one plays a unique role in keeping a home dry, efficient, and comfortable.
When barriers are installed correctly, they work together to prevent drafts, control moisture, and protect the structure for generations. When they’re done poorly, even a brand-new home can experience heat loss, condensation, mould, and premature wear.
The Barriers: What They Are and How They Work
Air Barrier: Controlling Air Movement
The air barrier is a continuous layer that must remain completely uninterrupted and prevent the movement of air through the building envelope. A building envelope is the outer shell of your home. Includes the walls, roof, windows, and doors, the parts that separate the inside from the outside. It’s often confused with a vapour barrier or weather barrier because each can also serve as an air barrier when installed correctly. I mention this in detail later in this article.
An example of an air barrier would be a 6-mil polyethylene sheet on the interior of the wall, or in some cases, a peel-and-stick membrane on the exterior.
The reason the air barrier is so significant is that air leakage is the biggest source of heat loss in cold climates. If done incorrectly, it can let humid air leak into wall cavities, which in the winter will show up as frost, and in summer as mildew and possibly mould.
A common occurrence in Manitoba in older homes or homes with an ineffective air barrier is “roof leaks.” People automatically assume the roof is leaking when water drips in, but fail to realize it hasn’t rained recently. What’s happening more often than not is that frost has been building up throughout the winter due to a failure in the air barrier, and as the weather warms, the frost melts, and water starts dripping inside.
Weather Barrier: Water, Snow and Ice Protection
Think of the weather barrier as wearing a rain jacket. It keeps you dry. Just like rain jackets, some are breathable, like Gore-Tex, and some are non-breathable, like good old-fashioned rubber.
In either option, the main failure points are anywhere there’s an opening, like head and arm holes, and anywhere there’s a zipper. On a home, think of the head and arm holes as your windows and doors, and think of the zippers as the seams in a weather barrier.
The critical points in the effective installation of a weather barrier are achieving a perfect seal around windows and doors, and ensuring joints are overlapped correctly and sealed.
When would you choose a non-breathable membrane over a breathable one? If your vapour barrier also serves as your air barrier, which I’ll cover in the vapour section, you would opt for a breathable weather barrier, such as Tyvek. If your weather barrier is also your air barrier, it needs to be an air-tight membrane, such as a peel-and-stick membrane.
It’s much more common to have the vapour barrier acting as the air barrier because it’s easier to get a complete seal around the entire home. Remember, the roof needs to be sealed with no penetrations with whichever air barrier is chosen.
Vapour Barrier: Keeping Moisture Out of the Wall Cavities
The vapour barrier’s job is to keep moisture from transferring into the wall and ceiling cavities. In Manitoba, we have warm, humid indoor air that is only separated by 6” from cold, dry frigid air. If those two meet, it can cause condensation or frost, which can eventually lead to mildew or mould.
The vapour barrier is installed on the warm side of the insulation and is typically a 6-mil polyethylene sheet. In most cases, it also acts as the air barrier. If the vapour barrier also serves as the air barrier, it must be perfectly sealed, with no penetrations through which air can leak.
If the weather barrier serves as the air barrier, then the vapour barrier just needs to overlap to keep moisture out of the wall cavities. You’ll see this in almost all older homes, as air barriers and air tightness weren’t standard in the building code back then.
Overview: How They Work Together
Air Barrier—
• Stops air from moving.
• Prevents moisture carried by air from entering wall cavities.
Weather Barrier—
• Stops water from entering from the outside.
Vapour Barrier—
• Keeps warm indoor moisture from reaching the cold parts of the wall.
Together They Provide—
• A dry, durable wall assembly.
• Higher energy efficiency.
• Better indoor comfort and air quality.
• Longer lifespan for structural components.
