They perform best when installed closest to the warm side of a structural assembly -- toward the interior of the building in cold climates and toward the exterior in hot/wet climates. Vapor retarder installation should be continuous and as close to perfect as possible.
Vapor barriers are installed along, in, or around walls, ceilings, and floors. Of course this is done to prevent moisture from spreading and potentially causing water damage.
Where They should be Used? A vapour barrier can be very useful in many different places within a home, and those are the places with high moisture or humidity. The best place for the barrier is the place under the foundation of the house, however, that cannot be done if your house had already been built.
Vapor barriers resist moisture penetration, keeping humid interior air from reaching cold surfaces where it could condense. In cold climates, vapor barriers go on the warm side of insulation. In hot, humid climates, they go on the exterior side.
Add a vapour barrier on the warm side of the wall within the first third of the thermal resistance value of the overall wall insulation.
A continuous geomembrane or plastic liner behind the drywall protects the interior walls from any water damage. Because bathrooms and kitchens produce the most water vapor of any house or building, it has become standard practice to also use semigloss paint on the walls to act as a secondary vapor barrier as well.
Install a New Vapor Barrier
If your vapor barrier was damaged or not installed correctly, it's important to replace it with a new one. This will help prevent moisture from seeping into your crawl space and creating the perfect environment for mold growth.
The common taboo against a double vapor barrier arises from a very real concern: If you have a Class I vapor retarder (less than 0.1 perm) on both sides of a wall, that wall has virtually no drying potential in either direction.
It's the first line of defence against high vapour pressure air entering the roof structure and causing interstitial condensation. Because of this, the VCL always sits at the warm side of the insulation.
Wiring – When wiring exists in the cavities that are being insulated, simply split the insulation so that it is placed around the wire on both sides, allowing the wire to be comfortably nested inside the insulation.
Don't: Put Vapor Barriers on the Bottom of the Floor Joist in the Crawl Space. Because vapor barriers are good at retaining moisture, putting plastic sheeting on the joists can cause moisture to build up under the floor.
Is DuPont™ Tyvek® a vapor barrier? No, DuPont™ Tyvek® is not a vapor barrier. It is made with unique material science to keep air and bulk water out while allowing moisture vapor inside walls to escape.
For crawl space projecs that require a vapor barrier installed within walls, builders commonly use a thinner plastic sheeting option such as 6 mil, but for the best durability in the crawl space, our vapor barrier experts instead recommend choosing 8 mil plastic sheeting.
Depending on where you live, the vapour barrier will either go on the inner surface of the insulation (if you live in a cold climate) or the outer surface of the insulation (if you live in a hot climate). It's safe to say that a vapour barrier will be needed for almost all projects.
In general water vapor moves from the warm side of building assemblies to the cold side of building assemblies. This is simple to understand, except we have trouble deciding what side of a wall is the cold or warm side. Logically, this means we need different strategies for different climates.
Provided you're not claustrophobic, installing a vapor barrier is a project most homeowners can DIY. You will need to calculate the square footage of your crawl space, so you can buy the appropriate amount of liner.
Vapour control layer building regulations
12 require that all new builds have an approved VCL fixed to the warm side of all insulation and cover the external framed wall area. It must be fitted at 250mm centres to the top and bottom of frames, around openings, at laps and wherever there has been any damage.
They perform best when installed closest to the warm side of a structural assembly -- toward the interior of the building in cold climates and toward the exterior in hot/wet climates. Vapor retarder installation should be continuous and as close to perfect as possible.
On most projects where insulation meets the under-slab vapor barrier, we have found that the project teams elect to install their insulation first and to lay the vapor barrier over top. Various industry leaders have commented on the advantage of placing the vapor barrier directly beneath the slab.
If moisture is a known problem, vapor barriers can be integrated into wall, ceiling, and floor construction to combat moisture affecting the home.
Should you leave a gap between the insulation and the roof? Yes, you should leave an air gap between the insulation and the roof to allow proper ventilation and prevent moisture buildup.
Once insulation gets wet, it's hard to dry it out. “It will sit there like a sponge, leading to mold problems and rot,” says Tom. Vapor barriers—sheets of plastic or kraft paper—keep water vapor out of the wall cavity, so the insulation stays dry.
The Cons of Vapor Barrier Installation
Although vapor barriers aim to prevent moisture from entering the structure, they can also trap moisture inside. If not properly installed or upheld, this could result in the proliferation of mold and other issues related to moisture.
Your home's vapor barrier, unlike an air barrier which prevents air from permeating through a structure, is generally a thick plastic sheeting that is placed in your home's crawlspace. In Oregon and Washington, in order for this plastic sheeting to be code, it should be black, and no less than 6mm thick.
A high-quality crawl space vapor barrier can be an effective solution for many years, as long as it is not damaged or punctured along the way. Generally, a crawl space vapor barrier that has been correctly installed by a professional and made of high-quality materials can have a lifespan of 20 years or more.