Back in the mid 1990s, when I first started working in the trades, it was as an apprentice electrician with my brother. We had an agreement: he would perform any work needed in attics, and I was responsible for crawlspaces. Some of the work was on seasonal cabins that were being updated for use during Minnesota-cold winters.

These cabins were constructed decades earlier and were built very close to grade; sometimes there were even stumps from trees that had been cut down to make room for the cabin. (More than once, I was sure the fire department was going to have to be called to extract me from under one of those cabins.) Most were vented crawlspaces, some you couldn’t tell if they were vented or unvented, and very few had any type of vapor retarder or soil gas mitigation system.

Crawlspaces today

Times have changed. The crawlspaces I’m in today for the energy auditing/building investigation work I perform are sealed, conditioned spaces. I still run into an occasional home with a dirt-floor crawlspace, and when I do, my recommendations are to add a vapor retarder to cover the soil, install a dehumidifier, and monitor interior humidity levels in several areas around the home.

The process of encapsulating a crawlspace isn’t hard, though the work is unpleasant. In designing these assemblies, how do you select a vapor retarder? Is the standard 6-mil poly good enough? Should a 10, 15, or even a 20-mil product be used? What do the codes say? Let’s start there—with the IRC requirements for unvented, conditioned crawlspaces. Here are the relevant sections:

2021 IRC-R408.3 Unvented Crawlspaces

For unvented under-floor spaces, the following items shall be provided:

Exposed earth shall be covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches and shall be sealed or tapes. The edges of the vapor retarder shall extend not less than 6 inches up the stem wall and shall be attached and sealed to the stem wall or insulation.

One of the following shall be provided for the under-floor space.

2.1 Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute for each 50 square feet of crawl space floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.10.1 of this code.

2.2 Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute for each 50 square feet of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.10.1 of this code.

2.3 Plenum in existing structures comply with Section M1601.5, if under-floor space used a plenum.

2.4 Dehumidification sized in accordance with the manufacturer’s specifications.

One additional reference from Chapter 4 is relevant:

R408.5 Removal of debris

The under-floor grade shall be cleaned of all vegetation and organic material. Wood forms used for placing concrete shall be removed before a building is occupied or used for any purpose. Construction materials shall be removed before a building is occupied or used for any purpose.

Author’s note

The codes in Chapter 4, Foundations, do not specify the type or thickness of the vapor retarder, just that a Class I vapor retarder is needed.

In addition to Chapter 4, code references regarding vapor retarders are also found in Chapter 5, Floors, specifically in R506, Concrete Floors (On Ground).

R506.2.3 Vapor Retarder

A minimum 10-mil vapor retarder conforming to ASTM E1745 Class A requirements with joints lapped not less than 6 inches shall be placed between the concrete floor slab and the base course or the prepared subgrade where a base course does not exist.

Exception: The vapor retarder is not required for the following:

Garage, utility buildings and other unheated accessory structures.

For unheated storage rooms having an area of less than 70 square feet and carports.

Driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.

Where approved by the building official, based on local site conditions.

Author’s note

Vapor retarders required when using an under-slab system do call out a required thickness, (10 mil in the 2021 IRC, this was increased from 6 mil in earlier codes), but the code reference is not specifically for crawlspace applications. I mention this section in case a “rat slab” or thin layer of concrete is poured for the floor of a crawlspace. Will the AHJ require the R506.2.3 code or 408.3 code? You might want to have that discussion with the code official.

One final place we see crawlspace floor coverings mentioned is in Appendix AF, which addresses radon control requirements:

AF103.3 Soil-gas-retarder

A minimum 6-mil [or 3-mil cross-laminated, high-density] polyethylene or equivalent flexible sheeting material shall be placed on top of the gas-permeable layer prior to casing the slab or placing the floor assembly to serve as a soil-gas-retarder by bridging any cracks that develop in the slab or floor assembly, and to prevent concrete from entering the void spaces in the aggregate base material. The sheeting shall cover the entire floor area with separate sections of sheeting lapped not less than 12 inches. The sheeting shall fit closely around any pipe, wire or other penetration of the material. Punctures or tears in the material shall be sealed or covered with additional sheeting.

AF103.5.2 Soil-gas retarder

The soil in crawl spaces shall be covered with a continuous layer of minimum 6-mil polyethylene soil-gas-retarder. The ground cover shall be lapped not less than 12 inches at joints and shall extend to all foundation walls enclosing the crawl space area.

Author’s note

Radon requirements covering the use of polyethylene sheeting are only required in certain areas, which are listed in the 2021 IRC or in areas that have adopted their own radon requirements. Minnesota, for instance, requires a radon control system installed in all new construction, even the 2021 IRC doesn’t require it.

Choosing a product

What we have learned in the code references is that the product used for covering soil in a crawlspace will depend on whether its purpose is for vapor control only, or as a soil gas retarder too (or in preparation for a rat slab).

So, what is the right material to use?

The requirement in R408.3 states only that a Class I vapor retarder is needed. There is no mention of thickness or any specific product. My interpretation is there are several products that can be sealed and extended up the foundation wall and meet the requirement of a Class I vapor retarder. A product like ethylene vinyl acetate (EVA), a plastic sheeting similar to polyethylene, could satisfy the requirements of R408.3.

Polyethylene is the most commonly used product for this application. It is easily sourced and cost-effective. There are thicker versions of polyethylene sheeting that you may want to consider, especially if the crawlspace is used for storage or mechanical equipment that require periodic maintenance. In this case, a 10-mil or thicker product may be a better choice.

Let’s go over the pros and cons of a few polyethylene vapor retarder products currently on the market.

6-mil standard polyethylene sheeting

Pros: 6-mil standard poly is cost effective and easily sourced. It’s available in a wide range of sizes. The product can be clear, translucent white, solid white, or black. (I would choose a light color to help brighten a dark crawlspace.) Some manufacturers may offer other colors. 6-mil is an effective class I vapor retarder. There are many brands from which to choose.

Cons: It’s more easily damaged than thicker options. Depending on the manufacturer, it may not be available as a system, meaning tapes and sealants would be sourced separately.

Cost: about $65 for a 10’ x 100’ roll

10-mil standard polyethylene sheeting            

Pros: 10 mil-poly is a little more difficult to source, but still fairly common. It’s also available in a wide range of sizes, but is more durable than 6 mil, making it a better choice for crawlspaces that have storage or equipment that requires maintenance. Again, there are many manufacturers’ products to choose from.

Cons: Standard 10-mil poly may not have the ASTM E1745 certification needed for under-slab vapor barrier requirements that were put in place in the 2021 IRC codes. Depending on the manufacturer, it may not be available as a system, and it is slightly more expensive than 6-mil poly.

Cost: about $100 for a 10’ x 100’ roll

6-mil reinforced polyethylene sheeting                       

Pros: Reinforced poly is more difficult to tear but can still be punctured much like standard 6-mil poly. It’s easily sourced through big-box stores and many lumberyards.

Cons: The cost is substantially more than standard 6-mil poly. Typically, it’s not sold as a system for crawlspace encapsulation, so tapes and sealants will need to be sourced separately.

Cost: about $150 for a 10’ x 100’ roll

15- and thicker mil polyethylene sheeting                   

Pros: It’s a very durable product.

Cons: It’s much harder to source and much more expensive. The added thickness makes the product heavier, making it harder to work with, especially in tight spaces.

Cost: $250-$600 for a 10’x100′ roll

6, 10, 12, and 15-mil products sold as a system                       

Pros: They are specifically designed for crawlspaces and sub-slab applications; and often also touted as a soil gas retarder. The thicker options are very durable. They are sold as a system; tapes, sealants, and other options are often available. Some manufacturers have termite-resistant and higher UV-exposure ratings. It’s often a higher-quality product.

Cons: These products are expensive and harder to source because there are fewer manufacturers of polyethylene crawlspace encapsulation systems.

Costs: $150-$600

There are manufacturers of products specifically made for crawlspaces, such as StegoCrawl. In addition to Stego, a few other brands make crawlspace encapsulating systems including Spirecover and Viper.

Why encapsulate, by example

A couple years ago, I was called in to perform a building investigation on a home with a wintertime condensation issue. When I arrived, the interior humidity level was nearing 50% with an outside temperature of -10°F. The 1970s home was not constructed to handle that type of humidity during the heating season. All the new double-pane windows were acting as dehumidifiers; when outside temperatures moderated, water would drip through the ceiling from the condensation that was accumulating in the vented attic.

The source of the high indoor relative humidity? A recent renovation improved the airtightness of the home, but the dirt floor in the sealed crawlspace was not addressed. Installing a vapor retarder over the dirt floor, and adding a dehumidifier in the crawlspace cured the problem (I also recommended an HRV be installed).

Sometimes crawlspaces are identifiable as a problem by the odors inside the house, which are especially noticeable upon entering. Changing these spaces to provide a healthier indoor environment is not hard, but the work is usually not fun. Choose the product best suited for how the space is used. If you are never in the space, a simple 6-mil poly may work perfectly. If the space is occasionally occupied, a thicker product may be a better choice.

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Randy Williams is a builder and energy auditor in Grand Rapids, Minn. Photos courtesy of the author, except where noted.