Flooring (C-15) · Lesson 1 of 7

Slab Moisture: Vapor Retarder and Capillary Break Under the Slab

"Slab Moisture: Vapor Retarder and Capillary Break Under the Slab" is lesson 1 of 7 in the Flooring (C-15) Learning Hub. It opens with "What California Prints About Slab Moisture" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. What California Prints About Slab Moisture
  2. The Vapor Retarder in the Building Code (CBC 1907.4)
  3. The Residential Code Version (CRC R506.3.3)
  4. The Capillary Break (CALGreen 4.505.2)
  5. When There Is No Retarder Under the Slab

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Is a retarder required?" through "Floor moisture limit".

What you'll be able to do

  • Explain the main idea of "Slab Moisture: Vapor Retarder and Capillary Break Under the Slab" in plain language you can use on the exam.
  • Apply this step with confidence: "The Residential Code Version (CRC R506.3.3)".
  • Apply this step with confidence: "The Capillary Break (CALGreen 4.505.2)".
  • Apply this step with confidence: "When There Is No Retarder Under the Slab".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Under the Slab: Vapor Retarder and Capillary BreakSLAB-ON-GROUND SECTION (not to scale)concrete slab4 in basesubgradeslab, 3 1/2 in min6-mil poly, 6 in laps4 in base of1/2 in or largerclean aggregateHeated R-3 addition: no exception applies,so retarder and capillary break are required.POLY SHEETS LAPPED6 in lap minimumTAKE-OFF: 28 ft x 36 ft SLABSlab area: 28 x 36 = 1,008 sq ftPoly: 10 ft wide sheets, 6 in (0.5 ft) lapseach added sheet covers 10 - 0.5 = 9.5 ft10 + 9.5 + 9.5 = 29 ft, at least 28 ft3 sheets, run along the 36 ft lengthBase: 1,008 sq ft x 4/12 ft = 336 cu ft336 / 27 = 12.4 cu yd of aggregateCALGreen 4.505.2.1 option 1: base plusretarder in direct contact with the concreteORDER 3 sheets of 10 ft poly and 12.4 cu yd of baseThe code prints no floor-moisture test limit; the flooring maker sets that numberSource: 2025 CBC 1907.3, 1907.4, 1907.4.1; 2025 CRC R506.3.3; 2025 CALGreen 4.505.2.1.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What California Prints About Slab Moisture

Flooring that is glued or sealed over a concrete slab is only as dry as the slab beneath it, so the first question is what was built under the concrete. California answers that in 2025 CBC Section 1907.4, 2025 CRC Section R506.3.3 and 2025 CALGreen Section 4.505.2: a vapor retarder and a capillary break. None of these sections prints a relative-humidity or emission-rate limit for the finished floor, so the pass/fail number for the flooring itself is the flooring or adhesive manufacturer's published limit, not a California code number.

Step 2: The Vapor Retarder in the Building Code (CBC 1907.4)

2025 CBC Section 1907.4 requires a 6-mil (0.006 inch; 0.15 mm) polyethylene vapor retarder with joints lapped not less than 6 inches (152 mm), placed between the base course or subgrade and the concrete floor slab, or other approved equivalent methods or materials to retard vapor transmission. Section 1907.3 sets the slab-on-ground thickness at not less than 3 1/2 inches (89 mm). These are the numbers a flooring contractor can check against the plans and the slab inspection record.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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