Why put plastic over insulation?

Author: Jeff Boyer Jr.  |  Last update: Sunday, August 31, 2025

Yes, covering your insulation with a vapor barrier plastic before installing drywall is crucial. It prevents moisture buildup that can lead to mold, rot, and even structural damage. A polyethylene vapor barrier also adds a layer of fire protection, making your home safer.

Should I put plastic over insulation?

Big no-no! Plastic traps moisture, which can cause mold in your attic. Insulation needs to breathe. Think of it like a sweaty sweater under plastic - not comfy!

Why is insulation wrapped in plastic?

The main point of vinyl plastic is to stop air movement, within the wall structure, which in turn stops moisture migration. A breathable moisture barrier as in Tyvex is only efficient if there is a minimal air change within the wall envelope.

What is the plastic called that goes over insulation?

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.

When should you not use a vapor barrier?

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.

How Vapor Barriers Work

What are the disadvantages of a vapor barrier?

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.

Do I need a vapor barrier over fiberglass insulation?

Being an inert material, it is a non-viable food source for mold and will not support mold growth. Fiberglass insulation cannot fight the battle against moisture intrusion alone, however. Vapor barriers applied over the face of the insulation provide additional benefits in the fight against moisture.

How do I know if I need a vapor barrier?

A vapor barrier is likely required if the building is cladded with absorptive material and if the structure is in United States climate zones 4C, 5 through 8.

Can any plastic be used as a vapor barrier?

As you can see, thicker polyethylene sheets have very low perm ratings under 0.1, making them viable options as vapor barriers if local codes allow it. Thinner sheets or other plastic types may not meet the requirements.

How much does a vapor barrier cost?

Vapor Barrier Installation Cost

Per square foot, expect the crawl space vapor barrier cost to land between $1.35 and $2 per square foot, or a total of between $1,500 and $4,000 for labor and materials.

What is the best plastic to cover insulation?

What are the Best Vapor Barriers to Use with Insulation? The best insulation vapor barriers are made of polyethylene plastic and are reinforced to stand up against wear and tear. For extra protection, choose an insulation cover with mold and fire protection.

Can you put plastic on top of insulation in an attic?

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.

Is there a difference between vapor barrier and plastic sheeting?

Whereas the most common under-slab vapor retarder is a 6-mil sheet of generic polyethylene—with a permeance closer to 0.1 perms— vapor barriers are typically composed of high-quality plastic sheeting and/or metal film, like aluminum, that greatly impedes the diffusion of water vapor.

Where should the vapor barrier be installed?

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.

What mil plastic over insulation?

The thicker the vapor barrier, the more durable it tends to be. For instance, a 6 mil plastic sheeting may be suitable for a basic level of moisture protection, but a 12 mil or 20 mil vapor barrier would offer superior resistance to punctures and tears.

Where should you not put insulation?

Never place insulation close to water heaters, oil burners or anything hot. The backing is especially flammable and will be a major fire hazard for your home.

Can I use painters plastic as a vapor barrier?

Encapsulation Vapor Barrier Thickness Matters. A typical painters plastic is 6-mil or less, and is usually only 0.5 mil. This type of plastic is used to create barriers between rooms. Water restoration companies will use 0.5 mil but many times use a 6-mil plastic to create containment areas.

Is black or clear plastic better for vapor barrier?

Do Use A Black Vapor Barrier vs a Clear Barrier. Black or white vapor barriers are better than clear ones since they absorb heat, which creates a buffer zone between the warm air in your home and the cold air outside. This reduces condensation on surfaces, helping to protect against moisture damage.

Can I use plastic instead of Tyvek?

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.

What happens if I don't use a vapor barrier?

Even if the concrete foundation is enough to keep water away from the building, water vapor can easily rise to the surface and damage a building's foundation, subfloor, and main floor. A vapor barrier is crucial to protect a building from the many negative effects of water vapor on building materials.

What insulation does not need a vapor barrier?

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.

Is Tyvek a vapor barrier?

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.

Can I use kraft paper as a vapor barrier?

Kraft-faced insulation includes a paper vapor retarder, which helps prevent mold and mildew. Kraft-faced insulation should be installed in exterior walls, exterior basement walls, and attic ceilings by pressing the product into the wall cavity with the paper side facing outward, towards the installer.

Do you need a air gap behind insulation?

It provides an additional layer of insulation by trapping air and helps to prevent moisture buildup by allowing for ventilation. So, it's always recommended to leave an air gap when installing insulation, whether it's in your walls, roof, or floors.

What do you put over fiberglass insulation?

PIB (Polyisobutylene) can also be used over fiberglass insulation. PIB is a type of vapor barrier that is designed to prevent moisture from penetrating through walls, ceilings, and floors.

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