This post first appeared on the Green Building Advisor Website.
When I first started working in the trades, back in the mid 1990’s, I was working for my brother as an apprentice electrician. We had an agreement, he would perform any work needed in attics, I got crawlspaces. Some of the work we were performing was on seasonal cabins that were being updated for use during Minnesota’s cold winter. These cabins were constructed decades earlier, built very close to the earth. Sometimes there were even stumps from the trees that were cut down to make room for the cabin that were left under the structure, in the crawlspace. More than once, I was sure the fire department was going to have to be called to extract me from under one of these 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 present.
Times have changed, the modern crawlspaces I’m in today for the energy auditing/building investigation work I perform are sealed, conditioned spaces, though I still run into an occasional home with a dirt-floor crawlspace. My recommendations in the reports I provide are add a vapor retarder to cover the soil, add 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 chosen? Let’s start with the codes for unvented, conditioned crawlspaces.
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 of the 2021 IRC Foundation code section 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.
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.
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 covers radon control requirements.
AF103.3 Soil-gas-retarder.
A minimum 6-mil [or 3-mil cross laminated] 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.
Radon requirements covering the use of polyethylene sheeting are only required in certain areas which are listed in the 2021 IRC codes or areas that have adopted their own radon requirements. My state of Minnesota requires a radon control system installed in all new construction even though many areas are not required to by the 2021 IRC.
What we have learned in the code references is that what product is used for covering the earth in a crawlspace will depend on whether its purpose is simply for vapor control, as a soil-gas retarder, or for an earth cover before a rat slab is poured.
What and how thick of a product to use?
The requirement in R408.3 only states that a Class I vapor retarder is needed. There is no mention of thickness (mils) or any specific product (like polyethylene). 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 EVA or Ethylene Vinyl Acetate (a plastic sheet product similar to polyethylene) could satisfy the requirements of R408.3.
Polyethylene will be the most commonly used product for this application. It can be easily sourced and a cost-effective option. There are thicker versions of polyethylene sheeting that you may want to consider, especially if the crawlspace is used for storage or for the location of mechanical equipment that require periodic maintenance. 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.
Standard 6 mil polyethylene sheeting Cost- about $65 for a 10’ x 100’ roll
Pros 6 mil standard poly will be cost effective and easily sourced. 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 will be an effective class I vapor retarder. Many manufacturers to choose from.
Cons More easily damaged than thicker options. Depending on the manufacturer, they may not be available as a system, tapes and sealants that may be required may have to be sourced separately.
Standard 10 mil polyethylene sheeting Cost-about $100 for a 10’ x 100’ roll
Pros 10 mil poly will be a little more difficult to source, but still fairly common. Available in a wide range of sizes. More durable than 6 mil making it a better choice for crawlspaces that have storage or equipment that requires maintenance. Many manufacturers to choose from. An effective class I vapor retarder.
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, they may not be available as a system, tapes and sealants that may be required may have to be sourced separately. Slightly more expensive than 6 mil poly.
6 mil reinforced polyethylene sheeting Cost-about $150 for a 10’ x 100’ roll
Pros Reinforced poly is more difficult to tear but can still be punctured much like standard 6 mil poly. Easily sourced, big box store and many lumberyards stock the item.
Cons Cost is substantially more than standard 6 mil poly. Typically, not sold as a system for crawlspace encapsulation, tapes and sealants will need to be sourced separately.
15 and thicker mil polyethylene sheeting Cost-$250-$600 or more for 10’x100’
Pros Very durable product.
Cons Much harder to source. Much more expensive. The added thickness makes the product heavier, making it harder to work with, especially in tight spaces.
6, 10, 12, and 15, mil products sold as a system Costs-$150-$600 or more for 10’x100’
Pros Specifically designed for crawlspaces and sub-slab applications. Often also touted as a soil gas retarder. Very durable in the thicker options. Sold as a system; tapes, sealants and other options are often available. Some manufacturers have termite resistant and higher UV exposure ratings. Often a higher quality product.
Cons Expensive and harder to source. Fewer manufacturers of polyethylene crawlspace encapsulation systems.
A few brand names of crawlspace encapsulating systems are Stego, Spirecover, and Viper.
A case study
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 1970’s home was not constructed to handle that type of humidity during the heating season. All the new 2-pane windows were acting as dehumidifiers and when outside temperatures moderated, water would drip through the ceiling from the condensation that was accumulating in the attic (the attic was vented). 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. The addition of a vapor retarder on the dirt floor, and a dehumidifier in the crawlspace cured the problem (I also recommended an HRV be installed).
Crawlspaces aren’t uncommon in my area, though slab on grade and basements are the norm. Sometimes crawlspaces are identifiable by the odors inside the home as you enter. 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. If the space is used often and the budget is high enough, pour a rat slab. It makes life so much easier for those who crawl around in these spaces.