Difference Between Air Barrier vs. Vapour Barrier
One of the most confusing things to homeowners (and even some builders!) is the difference between an air barrier and vapour barrier. Knowing where and when to use them and how to install and maintain an air barrier and vapour barrier in your home for the best energy efficient results, material longevity, and performance is what we’re talking about today on the blog.
As we covered last month in our article titled How the Right Control Layers Make an Efficient Home, an air barrier and vapour barrier are part of your home’s control layer system that helps regulate the movement of elements like air and moisture through your building. This week we’re going in depth with these two specific components to help you better understand what they do and why it’s important not to skip this step when building or renovating your home.
How Does It Work?
While each layer shares similar tasks and functions, and may even perform like one another, the air barrier and vapour barrier in your home do distinct and individualized tasks to help keep your home functioning at 100%.
To recap last month’s article, your air barrier is a system that:
- helps stop interior air from escaping your home
- is absolutely impermeable to air flow
- is continuous
- strong enough to change with air pressure changes
Meanwhile, a vapour barrier is a layer that:
- helps reduce and prevent moisture from entering your home
- is impermeable to vapour diffusion
- covers as much area as possible
- allows air flow
The primary differences between the two systems is that an air barrier MUST be continuous to work efficiently, and can be installed anywhere - meanwhile, a vapour barrier must be installed on the interior side of a structure and can cover as much surface as possible.
So how do these two layers work together? Depending on individual products and materials:
- your weather barrier can also perform as the air barrier
- you vapour barrier can also perform as the air barrier
- you thermal barrier can also perform as the air barrier, vapour barrier, and weather barrier
Working Together
Your air barrier and vapour barrier work together to provide security from the elements. Three common barrier systems are a rigid barrier, a membrane system, and a house wrap. These layers are either on the interior or exterior of your building.
Products like an interior rigid barrier offer advantages in material flexibility, but may or may not be adequate when it comes to moisture in the home. On the other hand, a membrane system may be able to act as both an air and vapour retarder. The last type, a house wrap, reduces the need for additional air sealing around electrical and plumbing outlets, is easily repaired, and can repel wind and weather quite effectively.
Different System, Different Installation
Where you install your air barrier and vapour barrier is just as important as choosing the right material for your home. Generally, a specific material will be rated for interior or exterior use.
An interior rigid or membrane barrier uses sealants, tapes, and gaskets to ensure complete continual coverage. Rigid air barrier materials aren’t always great vapour barriers though, so they need to be paired with a moisture control system to ensure you’re covered. A interior membrane system, on the hand, can serve as both the air and vapour retarder, so it needs to be continuous and without leaks.
Exterior air barrier systems also have rigid and membrane options, and often need to be paired with a vapour layer. But the exterior house wrap acts as both - it completely eliminates the need for an interior air or moisture barrier, and is more easily repairable and flexible to work with.
It’s important to remember that the material you choose will determine where you install it. Air barriers are generally more flexible, but moisture barriers are not - they need to be on the warm side of the building envelope
How Do These Layers Interact with my Insulation?
When it comes to air barrier and vapour barrier systems, there’s a lot of confusion in part because air can often hold a great deal of moisture in vapour form, and it brings that moisture with it as it moves throughout a building. If you improperly install one part of your barrier system or use the wrong materials in the wrong place, what usually results in an increase in moisture related problems in your home.
An common result of these kind of mistakes is that instead of preventing parts of your home from getting wet, incorrectly installed vapour barriers actually trap moisture in and prevent wet or damp areas from drying. This can be a huge problem in spaces like the basement or attic where insulation is often one of the first victims of too much moisture. Wet insulation doesn’t do its job in helping to regulate your indoor temperature, and may also be especially vulnerable to mold growth.
Air and Vapour Barrier Systems and Energy Efficiency
Your air barrier and vapour barrier are important tools when it comes to making your home more energy efficient.
An air barrier is there to help prevent unwanted air from leaking out of or into your home, and can usually be found in the walls, attic or roof of your home. Their immediate energy savings is obvious - by preventing your conditioned air from leaking out, you need to run your furnace or air conditioner less frequently to keep your thermostat level. But an air barrier’s benefits are often subtler as well - for example, when your furnace is constantly running there are increased maintenance costs and wear and tear on your unit. Increased air leaks and movement through your building may result in mold, moisture, or condensation trouble that can cause damage throughout your building.
Your vapour barrier provides similar energy efficiency benefits by cutting down on the need to run dehumidifiers and reducing the risk of moisture related problems in your building structure. Rising energy costs across the country are causing many more homeowners to pay closer to attention to their monthly bills and look for savings where it’s possible. Installing an air barrier and vapour barrier when building or renovating your home is an excellent option for making a more comfortable and energy efficient place to live and work.
Air Testing and Tightness
How do you know if you need an air barrier in your home?
One of the best ways to find out is to schedule an air test. This test will measure how quickly the air inside your home is being replaced by outdoor air. A tighter home brings you:
- increased energy efficiency
- lowered heating and cooling costs
- fewer drafts
- more regular indoor temperature
In an air test, a blower door is usually temporarily installed in lieu of your front door and the rate of air exchange is measured. Air can be blown out of your home (depressurization) or pushed in (pressurization). How much air the fan has to move to maintain an even indoor pressure will tell the technician how much or how little air is leaking from your home.
When you’re renovating or building from new, this test can be performed after you finish installing your insulation, taking care to prevent gaps or compression and thoroughly sealing all seams and joins before the test is run. Even a small hole can have a huge effect on the amount of air your home leaks, and a lot of care is needed to make sure that the air barrier is installed correctly and sealed tightly.
Good Building Science
Ensuring that you’re using the correct air barrier and vapour barrier when renovating or building a new home is important. It can have a huge effect on the longevity of your building materials, the air tightness of your home, and reduce the danger of moisture and condensation, all the while offering energy efficiency savings and rewards. When you’re adding a new component such as an air barrier and vapour barrier to your home, don’t be shy about asking your contractor questions to find the right solutions for your home’s needs.
