Call Now
November 4, 2026concrete

Do You Need a Vapor Barrier Under a Concrete Slab in Spokane? Shops, Garages, Barndominiums and Heated Floors (2026)

Do You Need a Vapor Barrier Under a Concrete Slab in Spokane? Shops, Garages, Barndominiums and Heated Floors (2026)

Short answer: in Washington, a heated house or barndominium slab needs a vapor retarder by code, and a detached unheated garage or shop usually doesn't. But "not required" isn't the same as "not needed." If the slab might be heated later, will get epoxy, or will have radiant heat, the sheet under the concrete is far easier to get right before the pour than after. Here's what Washington's codes say, what the concrete industry recommends, and how we handle it.

The quick answer by building type

  • House or barndominium (heated living space): yes. The Washington residential code requires a 10-mil, ASTM E1745 Class A vapor retarder under the slab
  • Detached, unheated garage or shop: not required by code. Both the residential and building codes exempt unheated accessory garages and utility buildings
  • Commercial or non-residential building: usually yes. The building code calls for a 6-mil polyethylene vapor retarder (or an approved equivalent), with an exception where moisture through the slab won't harm the building's use
  • Heated slab or radiant floor in a home: yes, plus the Washington energy code requires R-10 insulation under the entire heated slab
  • Driveways, walks and patios: no, as long as they won't be enclosed and heated later

Which code applies in Washington

Washington adopted the 2021 International Residential Code (IRC) in WAC 51-51 and the 2021 International Building Code (IBC) in WAC 51-50, both effective March 15, 2024. Houses and barndominiums used as dwellings fall under the IRC; commercial and other non-residential buildings fall under the IBC. Washington's amendments don't touch either code's slab vapor retarder section.

Houses and barndominiums: IRC R506.2.3

For a home's concrete floor on ground, Washington's residential code requires:

  • Material: a minimum 10-mil (0.010 inch) vapor retarder that meets ASTM E1745 Class A
  • Laps: joints lapped at least 6 inches
  • Location: between the concrete slab and the base course, or the prepared subgrade where there's no base course
  • The slab itself: at least 3.5 inches thick (R506.1), on a 4-inch base course of clean graded sand, gravel or crushed stone where the slab is below grade (R506.2.2), with an exception for well-drained Group I soils

The IRC exempts these from the vapor retarder:

  • Garages, utility buildings and other unheated accessory structures
  • Unheated storage rooms under 70 sq ft, and carports
  • Driveways, walks, patios and other flatwork not likely to be enclosed and heated later
  • Situations the building official approves based on local site conditions

If you've heard "6-mil plastic" for houses, that's outdated for Washington. The 2021 IRC that Washington adopted calls for 10-mil Class A. A steel-frame barndominium built as a dwelling gets the same rule as any other house.

Commercial and non-residential buildings: IBC 1907.1

Under the building code Washington adopted, a slab on ground has to be at least 3.5 inches thick and sit on a 6-mil (0.006 inch) polyethylene vapor retarder with joints lapped at least 6 inches, or another approved method that keeps moisture from coming up through the floor. The exceptions:

  • Detached structures accessory to a single-family (Group R-3) home, such as garages, utility buildings or other unheated facilities
  • Unheated storage rooms under 70 sq ft, and carports attached to Group R-3 homes
  • Buildings of other occupancies where migrating moisture won't harm the intended use
  • Driveways, walks, patios and other flatwork that won't be enclosed later
  • Where approved based on local site conditions

That third exception turns on how the building is used, so we settle it with the designer and building official.

So do you need one under a garage or shop slab?

By code, a detached unheated garage or shop on a residential lot doesn't need one. We still recommend one in most shops:

  • The exemption is for unheated buildings. Plenty of shops get a heater, an insulated ceiling or a finished office a few years later. Our metal building insulation guide covers what changes once you condition a building
  • Floor coatings. ACI 302.1R says proper moisture protection is essential where a slab will get moisture-sensitive flooring, including impermeable floor coatings and adhesives. Epoxy is an impermeable coating. See our epoxy vs. polyaspartic guide
  • What you store. ACI lists moisture-sensitive equipment, products and humidity-controlled rooms as reasons to protect the slab. Think tools, vehicles, cabinets and stored goods

Not all plastic is equal: ASTM E1745 and ACI 302.1R

ASTM E1745 is the specification for sheet vapor retarders placed in contact with soil or granular fill under concrete slabs. ASTM E1643 covers selecting, installing and inspecting them. What the industry guidance says:

  • Permeance: ACI 302.1R-15 explains that ASTM E1745 limits a vapor retarder to a permeance of no more than 0.1 perms, and that most flooring installations benefit from a material well below that
  • Ordinary poly: ACI 302.1R-15 says conventional 6, 8 and 10-mil polyethylene "does not fully conform to the requirements of ASTM E1745 and should not be considered for use as below-slab moisture protection"
  • Class matters: E1745 sorts materials into classes, and Washington's residential code specifically requires Class A
  • Choose by need and durability: ACI says thickness and permeance should be chosen from what the floor finish needs and how well the sheet will survive installation and the pour
  • Seams and penetrations: ACI calls for 6-inch laps (per ASTM E1643) or the manufacturer's instructions, with joints and penetrations sealed with the manufacturer's tape or adhesive
  • Not waterproofing: ASTM E1643 states that vapor retarders aren't meant to waterproof, and that conditions with frost heave or hydrostatic pressure are beyond its scope. A wet site needs drainage first

Where it goes: directly under the concrete

The residential code puts the vapor retarder between the slab and the base course. ACI 302.1R-15 agrees that direct contact with the slab gives the greatest protection for floor coverings and coatings. With a fill layer on top of the plastic, rain, curing water, saw-cutting slurry or compaction water can get trapped and greatly lengthen the time before the slab is dry enough for flooring, adhesives and coatings.

ACI also lists the tradeoffs: concrete placed on a vapor retarder settles more, shows larger early and drying-shrinkage movement, can curl more and open wider joints, and its surface can crust over while the concrete below stays soft, risking blistering. ACI says to offset these with slab design and construction measures and to review the location with everyone before the pour. For us that means attention to the mix, joint spacing, curing and finishing timing. Our slab thickness guide and concrete psi guide cover the rest of the slab design.

Heated floors and radiant slabs: insulation is code too

If a home or barndominium will have in-floor radiant heat, the Washington State Energy Code (residential provisions, 2021) adds requirements:

  • Under the whole slab: R402.2.9.1 requires the entire area of a heated slab-on-grade floor to be thermally isolated from the soil with at least R-10 insulation, and this applies under every compliance path
  • Soil gas systems: if a soil gas system increases air flow in the sub-slab gravel, the slab has to be isolated from that gravel layer
  • Perimeter: the energy code's prescriptive table lists R-10 slab insulation to a depth of 4 feet for Washington's climate zones, and insulation that extends away from the building has to be protected by pavement or at least 10 inches of soil

Commercial buildings follow the energy code's commercial provisions instead, which we confirm with the designer.

Test before you coat or cover

A vapor retarder slows moisture from below, but new concrete still has to dry. Before epoxy, vinyl, wood or other moisture-sensitive finishes, the slab gets tested, and the flooring or coating manufacturer sets the pass limit:

  • ASTM F1869 (calcium chloride test): measures moisture vapor emission from bare concrete, reported as pounds of moisture per 1,000 sq ft over 24 hours. It's not used over coatings, patching or leveling compounds
  • ASTM F2170 (in-situ relative humidity): measures percent relative humidity inside the slab with probes
  • Winter timing: cold slabs also affect coatings. See our guide to epoxy in winter in Spokane and winter concrete

How Raven handles it

  • Homes and barndominiums: 10-mil ASTM E1745 Class A, lapped and taped, per IRC R506.2.3
  • Shops and garages: we ask whether it will ever be heated, coated or finished. If the answer is yes or maybe, we recommend a Class A retarder under the slab
  • Radiant slabs: R-10 under the entire slab per the energy code, coordinated with your heating designer
  • Footings and frost: footings go below the local frost line. See our frost depth guide

What it costs

Slab and vapor retarder pricing depends on slab size, thickness, base prep, insulation and access, so we quote each slab individually. Building a whole shop? Raven's going rate for an installed pre-engineered metal building is about $60 to $75 per sq ft, with electrical, plumbing and utility hookups extra. The slab scope is part of your quote.

Get a slab or shop quote

Call 509-350-2016 or request a free quote with your building size, use and whether it will be heated or coated. See our concrete flatwork services and steel building services.

Sources

Frequently Asked Questions

Get a Free Quote

Privacy Policy: https://raven509.com/privacy-policy

Terms and Conditions: https://raven509.com/privacy-policy

No obligation. We typically respond within 24 hours.

Ready to Start Your Project?

Get a free estimate from Eastern Washington's most trusted concrete and construction team.

509-350-2016