Crawl space encapsulation and what the 2015 code actually requires

The sales pitch and the text

Search for crawl space encapsulation and you will find two kinds of pages. One says the work is required to protect your home. The other is the text of the building code. They do not say the same thing, so we start with the text.

We read the base 2015 International Residential Code, published by the ICC, because the Michigan Residential Code is built on it and the 2015 edition is the one in force (see which Michigan code applies). We could not open Michigan’s amendments to the under-floor chapter, so what follows is the base text. If Michigan changes a rule, your city building office can tell you.

Route one: the vented crawl space

Section R408.1 and R408.2 describe the traditional crawl space. Ventilation openings are sized at 1 square foot per 150 square feet of under-floor area, with one opening within 3 feet of each corner. The ratio can be reduced to 1 per 1,500 square feet when a Class I vapor retarder covers the ground and cross ventilation is provided. Openings are covered with corrosion-resistant mesh with a least dimension between 1/8 and 1/4 inch.

The point for homeowners: this route stays legal. Nothing in the code we read says an existing vented crawl space must be sealed. A contractor who says your house is out of compliance because the vents exist needs to name a section number.

Route two: the unvented crawl space

Section R408.3 lets you skip the vents when two conditions are met. The exposed earth is covered with a continuous Class I vapor retarder, joints overlapped by 6 inches and sealed or taped, with the edges extending at least 6 inches up the stem wall and attached. And the space has either continuous mechanical exhaust at a rate of 1 cubic foot per minute for each 50 square feet of floor area, with an air path to the living area, or a conditioned air supply at the same rate with a return path, and the perimeter walls are insulated. A plenum exception also exists.

That is the code definition of what the trade calls encapsulation. The word itself is not a defined term in the code. Companies use it for packages that usually include the liner, sealed vents and wall insulation, and sometimes a dehumidifier, a sump or drainage. The code requires the conditions above. It does not require the rest.

What the code does not name

Read R408.3 again for two words: dehumidifier and mil. Neither appears. The section sets no vapor retarder thickness, and it does not mention a dehumidifier. So pages that say “code requires a dehumidifier and a 12 to 20 mil liner” are not supported by the 2015 IRC text we read. A dehumidifier may be a sensible part of a design, and a thick liner may be sturdier underfoot, but those are choices, and you should hear them described as choices.

Under R408.4, an access opening through the floor must be at least 18 by 24 inches, and through a perimeter wall at least 16 by 24 inches, with an areaway if it is below grade. And R408.6 handles groundwater: if ground water can rise within 6 inches of the finished floor, or surface water does not drain, the under-floor grade must be as high as the outside grade unless an approved drainage system is provided. So a crawl space that floods is a drainage problem before it is a liner problem, which is why drainage and sump pump belong in the same conversation.

What federal researchers found

The US Department of Energy’s Building America program is a better source than a brochure. A 2015 report monitored two homes in New York State with sealed crawl spaces. Crawl space humidity exceeded 60 percent from April through November. In the unoccupied house without air conditioning, condensation potential on the floor vapor barrier totaled 494 hours. The authors concluded that a sealed crawl space should have air conditioning in the living space or be insulated, with the continuous vapor barrier, to limit summer condensation. Two homes is a small sample and the authors say a larger one is needed. New York is a cold climate like parts of Michigan, which is our inference and not a finding about Michigan houses.

The Building America Solution Center guide, from the Pacific Northwest National Laboratory, describes the same design: an unvented, air-sealed, insulated crawlspace is covered on the floor and walls with a polyethylene vapor retarder, with rigid foam on the perimeter walls. It lists less condensation on cold floors and rim joists, warmer floors, protection for ducts and pipes in cold climates and fewer pests as benefits, and notes that foam needs a code-required thermal or ignition barrier. The guide is several years old, so we checked it against the code edition instead of treating it as current. The Department of Energy page on the federal home improvement credit repeats the exposed-earth wording, and that page cites an edition of the code that may not be the 2015 one Michigan uses, so we count it as medium confidence.

The humidity number you will be given

Someone will tell you the sealed space has to be kept at a particular relative humidity. We found no government or extension source with a crawl space setpoint. What EPA says is that indoor humidity should stay below 60 percent, ideally between 30 and 50 percent, and that a humidity meter costs about $10 to $50 at hardware stores. That is guidance for indoor air, not crawl spaces, and the federal research above shows a sealed crawl space can sit above 60 percent in summer. If a contractor promises a number, ask where it comes from and whether the contract says what happens if the space misses it.

Radon and the crawl space floor

If you test for radon, EGLE says to avoid placing the test in a crawl space. If you have radon, EPA says the most effective method in a crawl space home is submembrane suction, which uses high-density plastic sheeting over the earth floor and a vent pipe and fan. Crawl space depressurization is less favorable. That means a liner installed for moisture may also be the first layer of a radon fix, and it is worth asking whether the installer will seal it for suction. Our radon guide explains the testing.

Who may do the work, and whether it needs a permit

Michigan has no crawl space license category. Encapsulation is improvement work on a residential structure, so a contractor charging $600 or more for labor and materials needs a LARA residential builder license or a maintenance and alteration license. The closest named crafts are insulation work, concrete and excavation, which is our inference from MCL 339.2404, and a builder license covers the work too. A dehumidifier or exhaust fan may also bring mechanical or electrical licensing into it, which we did not verify. See how to check a builder license.

On permits, section R105.1 of the base code requires a permit to alter or repair a structure and to install mechanical or electrical systems. We found no Michigan page that names crawl space encapsulation, so whether a conversion to an unvented space counts is for your building official. Sterling Heights’ plan review checklist covers crawl space ventilation and access under R408, so the code section is in use there.

Questions to ask before you sign

• Is the space being converted under R408.3, and which of the two airflow options is used?
• Who is responsible for the water if the space floods? What does the plan do about ground water?
• Does the bid include insulated perimeter walls, and is any foam covered by the thermal or ignition barrier the code requires?
• Where do the numbers in the contract, such as thickness or humidity, come from?
• Who pulls the permit, if one applies?

Our crawl space encapsulation page has the cost table. It leads with our estimate, $2,300 to $15,300 per project, a model and not a quote, and shows wages and permit fees below it.