Should You Install A Basement Vapor Barrier?

Jessica Stone
by Jessica Stone

Vapor barriers go between the insulation and drywall to prevent problems that come with moisture such as mold. However, vapor barriers can be just as harmful as they are helpful depending on where you install them. So, should you install a basement vapor barrier?

Never install a basement vapor barrier because it can cause mold to grow and spread. Water can become trapped between the insulation and drywall which will damage the sill plate. This can cause wood rot and which costs $600 to repair, and you may spend up to $5,000 on basement mold remediation.

In theory, it sounds like a great idea to install a basement vapor barrier, but it can cause expensive damage. Follow along as we explore why you should not install a basement vapor barrier.

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Should You Install a Basement Vapor Barrier?

You should not install a basement vapor barrier because it can cause mold to grow. Water can condense and form on a polyethylene vapor barrier and become trapped in a basement. This creates the perfect conditions for mold to grow, particularly because basements are already prone to mold.

Vapor barriers go between the drywall and installation on the warm side of a wall. There is nowhere for the water to go when condensation accumulates within a basement vapor barrier. Water can drop to the wood sill plate in your basement and erode it or allow mold to thrive.

Wood can rot due to prolonged water exposure in your basement because of a vapor barrier. It costs an average of $600 to repair dry rot wood, and mold remediation can cost over $5,000.

What is a Moisture Barrier?

Moisture barriers are located behind a framed wall, unlock vapor barriers that go behind the warm side of a wall. The purpose of a moisture barrier is to direct moisture that forms away from the walls to protect your insulation. Moisture barriers direct water to the floor where it is less likely to cause damage.

You don’t need a basement moisture barrier if you have spray foam insulation. However, it may be necessary to install a moisture barrier in your basement if you have batt insulation. That is because bat insulation may not be suitable to withstand moisture and is likely to cause mold to grow.

Quick Tip: Spray foam insulation is considered to be the best way to insulate a basement. It provides the greatest coverage, and R-value, and serves as a moisture barrier in and of itself.

Types of Moisture Barriers

If you have batt insulation installed in the basement of your home, there are essentially three types of moisture barriers you can implement to keep the space dry.

  • Rigid foam. Aside from spray foam, attaching rigid foam to your basement walls is arguably the best method to stop moisture. The rigid foam can then be sealed with spray foam at the corners and any gaps. Once installed, you’ll then be able to start framing the wall.
  • Poly sheeting. Installing 6-millimeter poly sheets is another option to prevent moisture from entering your basement walls. These cannot be affixed directly on the wall. You’ll first need to a strip of wood to the concrete wall that is six to twelve inches above the exterior grade. The poly sheets will be stapled to the wood strip and run the length of the wall to direct any moisture down to the floor.
  • Exterior house wrap (Tyvek). This method will also be hung using a piece of wood that is attached to the concrete wall. House wrap will also direct any moisture that enters through the wall onto the floor and away from the insulation.

For both the poly sheeting and exterior house wrap, make sure that the pieces do not overlap when installed. Unlike the rigid foam, they do not need to be sealed and the wood and concrete walls require the necessary spacing to breathe.

Basement Wall Insulation Tips

Basements account for up to 25% of heat loss in a home, making proper wall insulation in these areas absolutely vital. Before you begin the planning and prep for your basement insulation project, make sure that you check with local building codes in order to ensure that you are adhering to any of the required techniques or materials.

While it may not be necessary to install a moisture barrier in a warm climate, you should install one if you live in an area that experiences especially cold temperatures. Consult your local codes for specific mandates and recommendations in regard to moisture and vapor barriers.

All basement exposed joists must be insulated. Use either a spray foam material or fiberglass batting to fill the spaces where the floor and walls meet. Additionally, recessed lighting fixtures can be a major culprit for basement heat loss. Carefully place insulation in these areas to avoid a possible fire hazard.

Why Proper Basement Insulation is Important

If a basement is not appropriately insulated, warm air from inside the basement can infiltrate the foundation. Since the foundation walls are generally a cooler temperature, the warm air will hit the foundation walls, cool down, and create condensation.

This condensation in your finished basement can collect in the wood framing, insulation, and even pool on the floor behind drywall-covered walls. Because of this, your basement should not be insulated in the same way as above-grade walls and requires specialized insulation techniques.

Properly insulating a basement not only creates a dry, comfortable living area, it can also save you money on your energy bills.

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Related Questions

What is the cheapest way to insulate a basement?

While it’s not the most effective, insulating your basement with a combination of foam board and fiberglass is the cheapest method. However, this technique only works in areas that do not already show signs of water penetration. The foam board is installed first and then the frame is constructed in front of the board. Afterward, fiberglass is used to fill the stud cavities.

How do you insulate a basement that already has drywall?

Although it can be rather complicated, insulating a finished wall is possible. You may be considering this if your basement has poor, or no insulation or your heating bills are considerably high. Regardless, the best method is to cut holes into the wall, between each stud, and fill the wall with loose insulation using an insulation blower. This can be done as a DIY project or, professional services are available for tasks like this.

Summing It Up

Basement vapor barriers are harmful and can cause thousands of dollars in dry rot and mold remediation costs. Water that forms on the polyethylene barrier becomes trapped within the barrier and allows mold to thrive. The wooden sill plate will eventually rot and erode which is dangerous to your basement’s structural integrity.

Basement mold remediation costs up to $5,000 depending on how far it spreads. You can expect to spend an average of $600 per repair if you get dry rot from a basement vapor barrier. It is safe to install vapor barriers in other areas of the house in a warm wall.

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Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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