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Right Mil Thickness For Your Crawlspace Vapor Barrier

Right Mil Thickness For Your Crawlspace Vapor Barrier

Crawlspaces beneath older houses are usually vented, meaning they have small openings to let air pass freely between the crawlspace and outside. In practice, however, these vents don’t do a good job of circulating air, so whatever moisture makes it under the house will stay there. In humid climates, vented crawlspaces are especially damaging. Moisture-laden air enters the crawlspace and condenses into water droplets on cool pipes and concrete walls, which promotes mold, fungus, mildew, and bacteria.

Fortunately, vented crawlspaces can be sealed with the proper installation of a vapor barrier. Vapor barrier thicknesses range from 6 mil to 20 mil, with 6 mil being the bare minimum and 20 mil being the most heavy duty and puncture-resistant.

Why a Crawlspace Vapor Barrier?

Sealing off your crawlspace can prevent rodents, bugs, or other pests from making their way into your home. Moisture creates a hospitable environment for undesirable guests, and if left unaddressed, moisture can destroy the subfloor of your home — and along with it, the foundation. Depending on the extent of the damage, the cost to clean and/or repair a crawl space can be considerably high, which is why the preventive measure of installing a vapor barrier is the preferred course of action.

Sealing an open crawlspace with a vapor barrier can also prevent moisture accumulation in your floors and living space, saving your home from wood rot and mold. Plus, you’ll realize significant energy savings.

Crawlspace Vapor Barrier Materials

Reinforced polyethylene plastic sheeting (poly) comes in a variety of thicknesses and strengths. A 6 mil thick poly is commonly used as a vapor barrier and offers short-term savings to the homeowner. However, those savings may likely be lost in the long run if someone walks or crawls on the poly, causing micro holes in the vapor barrier. Once there are holes in the vapor barrier, however small, it will no longer function as a true vapor barrier. For this reason, depending on soil conditions, Americover recommends 10 mil and higher for long-term vapor protection.

Another concern is that polys stocked at local hardware stores are often thinner, non-reinforced, and could be made out of recycled or “regrind” poly. Regrind poly may contain impurities such as dirt or moisture. While thinner plastic or regrind may be okay for short-term use, only reinforced virgin poly should be used for long-term applications like crawlspace vapor barriers. Properly installed virgin reinforced poly should last 20 years or more in a crawlspace.

Choosing a Crawlspace Vapor Barrier Thickness

There are two measurements you should be familiar with when choosing your poly for your crawlspace vapor barrier. ‘Mils’ is the measurement of the poly’s thickness, and perms represent permeability to moisture.

  • Mils: Polyethylene sheeting is measured in ‘mils.’ A mil is equal to 0.001 inch of thickness. Codes for residential applications often cite a 6 mil (0.006-inch thick) minimum reinforced poly vapor barrier. However, Americover recommends 10 mil or higher, for crawl space applications. Americover offers thicknesses all the way up to 20 mil.
  • Perms: The perm rating represents how readily water vapor will pass through a material. The lower the perm rating, the less permeable a material is. To classify as a vapor barrier, a material must have a perm rating of 0.1 or below. Poly sheeting that is 6 mils (0.06 perms) and thicker meets this bare minimum.

Choosing a Crawlspace Vapor Barrier

The thickness of poly you choose will mainly depend on two factors:

  • Utilization of the crawlspace (storage, maintenance, etc.)
  • Soil or material on the floor of the crawlspace

First, you will need to figure out how much poly sheeting you will need to cover your crawlspace. Remember to get enough material to overlap 6-12 inches at the seams. A 6-foot-wide roll will cover most foundation walls effectively, but consider a wider roll for the floor. Here’s how to calculate the total square footage you’ll need to cover:

  • Wall = 2 x (Length + Width) x Height
  • Floor = Length x Width (Don’t forget to figure in the 6 to 12-inch overlap.)
  • Total = Wall + Floor

If the crawlspace is going to be entered regularly for maintenance, or if items are going to be stored in the crawlspace, it is a good idea to get a thicker poly, such as 12 mil. The minimum 6 mil is also inadequate if the floor of the crawlspace has rocks, roots, or concrete chunks. Remember that one tear or puncture in your vapor barrier will allow moisture to pass through and render the barrier less effective.

To save cost without sacrificing durability, you can use a thinner poly, such as a 6 mil or 8 mil, on the walls of the crawlspace, while using a thicker poly on the floor.

List of Americover Crawlspace Liners

All of Americover’s vapor barriers are string-reinforced 100% virgin poly.

6 mil String-Reinforced Crawlspace Liner

String-reinforced 6 mil is an economical choice when the overriding concern is cost. However, 6 mil is not recommended for areas where there will be activity or there are rough surfaces.

8 mil String-Reinforced Crawlspace Liner

String-reinforced 8 mil works well over soils made of dirt, mud, clay, or sand. Our 8 mil is frequently used on the walls in conjunction with a heavier material on the floors.

10 mil Pro Crawl Barrier

Pro Crawl Barrier is a 10 mil, Class 1 Vapor Barrier with a high gloss white finish. This option offers low permeability for excellent moisture protection. Because the 10 mil Pro Crawl Barrier is manufactured with virgin polyolefin-based resins, it has a high tensile strength and superior puncture resistance to small rocks.

12 mil String-Reinforced Crawlspace Liner

Because of its puncture resistance, string-reinforced 12 mil is great for soils that are rough or contain small rocks or tree roots, or for crawlspaces where there will be light or moderate activity. This film is an excellent choice when you need to strike a balance of economy and quality.

Antimicrobial 12 mil String-Reinforced Crawlspace Liner

Although all polyethylene is inherently antimicrobial, this crawlspace liner is specially treated to prevent the growth of harmful bacteria, fungi, mold, and algae. Tuff-Scrim TS12-AM’s antimicrobial additive is blended into the poly to make it non-migratory, wash-resistant, non-toxic, and environmentally safe. Tests verify that fungi will not grow on, near, or around TS12-AM.

20 mil String-Reinforced Crawlspace Liner

String-reinforced 20 mil is one of the thickest and toughest films on the market. It is great for commercial applications where an extra heavy-duty product is required. Its puncture resistance makes it great for use over soils that are hard or contain large rocks or broken concrete. Our 20 mil is the best product to use where there will be heavy activity.

Seal the Job with Vapor Barrier Tape

Tape is an essential part of the vapor barrier installation process, as you’ll need it to overlap panels and seal the material to the wall. We generally recommend two types of tape for crawlspace applications:

  • Vapor Tape can be used to create a waterproof seal and a clean finish. The general rule of thumb with vapor tape is one roll for every 1,000 square feet of vapor barrier. Make sure to allow for extra if you have multiple pylons or piers, which will require skirting, and therefore more vapor tape.
  • Butyl Tape is an aggressive, self-healing, double-sided tape used to adhere the crawlspace liner to the wall and seam the overlapping vapor barrier panels (where applicable). The amount of butyl tape you’ll need is based on the perimeter of the entire space, including any columns. We highly recommend a reinforced Butyl tape, so it does not stretch during application. And like Vapor Tape, allow for extra if you have multiple pylons or piers that require skirting.

In addition to tape, you’ll need fasteners to keep the vapor barrier attached to the foundation wall. We recommend our Christmas Tree Ratchet Fasteners — they can be easily installed with a drill bit every 18-24 inches through the butyl tape, creating an airtight seal around the hole.

Do you have questions about deciding which crawlspace vapor barrier is right for your project? Call to speak with an Americover representative today at 760-388-6294 or email us at

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As an Americover customer, you will receive the personalized support of a dedicated account manager to simplify the ordering process and ensure the best solution for every one of your projects. Our friendly, knowledgeable staff is here to answer your questions and fulfill orders Monday-Friday from 6 a.m. to 6:30 p.m. Pacific Standard Time. If you would like to speak with a representative, call us at 760-388-6294 or contact us.