Generally speaking, vapor barriers are considered an essential moisture-blocking tool for climates that experience significant levels of humidity. For that reason, building codes in more humid locations will often require them, in which case you might not have much say about whether to install one or not.
Most climates require a vapor barrier to cover ceiling insulation, but not all insulation requires a vapor barrier. It's always important to refer to your state's building codes.
Definitely do not add a plastic barrier between insulation layers. And as suggested above, make every effort to seal any openings where warm moist house air can leak into the attic. Vents, chimneys, wiring holes, etc. And do not cut off attic ventilation during the winter.
Attic Ventilation
area shall be no less than 1/150 of the attic floor area if no vapor retarders are used, or 1/300 of the attic floor area when a vapor retarder having a permeance of one perm or less is used.
That said, radiant-barrier roof sheathing is effective at lowering attic temperatures. Since it doesn't cost much more than ordinary roof sheathing, it makes sense to install it on new hot-climate homes. New homes in a cold climate, on the other hand, shouldn't use radiant-barrier roof sheathing.
Moisture problems, air leaks, inadequate attic ventilation, dust accumulation, compatibility issues with existing insulation, and condensation are some of the common challenges associated with radiant barriers.
In Most Cases, Attic Insulation Does Not Need To Provide A Vapor Barrier. The good news is that blown-in insulation should not need a vapor barrier. Blown-in is primarily used on the attic floor, and vapor barriers are not a concern for attics in our climate zone.
If you are installing insulation with a vapor barrier, make sure it faces the interior of the house. Another option for a vapor barrier is to take sheets of plastic and lay them between the ceiling joists. Then, using a staple gun, tack them to the sides of the joists.
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.
When calculating how much attic ventilation you need, your roofer will either use the 1/150 or 1/300 rule. The one they use determines how much Net Free Ventilation Area (open area for air to pass through) you'll need for every 150 or 300 square feet of attic space.
You can use plastic if you put polyethylene between your drywall and the wall studs. This way, you should get a tighter wall without trapping moisture inside the exterior walls. If you are framing out a below-grade basement, however, do not use plastic as a vapor barrier.
Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.
Also, when laying down additional insulation, work from the perimeter toward the attic opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all insulation at least 3 inches away from recessed "can" lights, unless they are rated IC (Insulated Ceiling).
Because a vapor barrier is a low permeable barrier, it keeps moisture from passing through. This is beneficial to keep moisture away from a dry surface but will trap moisture behind it if installed on a water-damaged concrete wall, causing the wall to fail further.
The ceiling air barrier usually consists of taped, mudded drywall, and should be continuous with all holes through it (such as for wiring, electrical boxes, light fixtures, flue and chimney pipes, duct chases, heating registers, soffits, etc.) thoroughly air-sealed before insulation is installed.
“For minor projects, the absence of a vapor barrier leads to water intrusion, leading to an unstable foundation or a cracked slab.” The American Society for Testing and Materials (ASTM) outlines national standards for underslab vapor barriers.
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.
After the insulation is in place you will want to add a vapor retarder, sometimes called a vapor barrier, if you need one. Not every wall does. A vapor retarder is a material used to prevent water vapor from diffusing into the wall, ceiling or floor during the cold winter.
Cold Climates: In regions with cold climates, the temperature difference between indoor and outdoor environments can lead to condensation within the ceiling. Vapor barriers become essential in preventing this condensation. High Humidity Areas: Buildings in high humidity areas are prone to moisture-related problems.
For hot-humid climates, attics should not be vented, and you should not install any vapor retarders in the assemblies interiors. In hot-dry climates, your attic can be vented, though, like, in hot-humid climates, you should not install any vapor retarder or barrier.
As a general rule, many people who are trying to reduce summer cooling expenses choose to staple the foil to the bottom of the roof rafters, while people looking for a winter heat retention benefit opt for laying the foil insulation over the existing insulation.
Even if the attic space is unheated, it's typically warmer than the outside air in winter. If you have chosen unfaced batting insulation for your rafters, you should staple sheets of plastic over the batting to accomplish a moisture barrier.
Unfaced insulation is great for new construction, remodels, walls, floors, ceilings, basements, attics and crawlspaces. It's best used for interior wall applications that do not face the outside and also in rooms that don't need moisture control, like living rooms, dining rooms and studies.