Manufactured Housing (C-47) · Lesson 6 of 6

Crawlspace Ventilation, Vapor Retarder and Site Drainage

"Crawlspace Ventilation, Vapor Retarder and Site Drainage" is lesson 6 of 6 in the Manufactured Housing (C-47) Learning Hub. It opens with "Three Controls Against Moisture Under a Home" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Three Controls Against Moisture Under a Home
  2. Drain the Site Away From the Home
  3. Size the Vents: 1 per 150, or 1 per 1,500
  4. Vent Coverings, the Unvented Option and Access
  5. Where the HUD and HCD Rules Fit

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Under-floor area" through "Non-vent items".

What you'll be able to do

  • Explain the main idea of "Crawlspace Ventilation, Vapor Retarder and Site Drainage" in plain language you can use on the exam.
  • Apply this step with confidence: "Size the Vents: 1 per 150, or 1 per 1,500".
  • Apply this step with confidence: "Vent Coverings, the Unvented Option and Access".
  • Apply this step with confidence: "Where the HUD and HCD Rules Fit".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Crawlspace Ventilation and MoistureUNDER-FLOOR PLAN, 28 x 52 ft HOMEClass I vapor retarderlaps 6 in, sealed or taped1,456 sq ft≤ 3 ft from each corneraccess 18 x 24 in (through floor)gold = vents; 3 needed by area, 4 by corner ruleVENT AREA, 60 sq in PER VENTNo retarder: 1 sq ft per 1501,456 / 150 = 9.71 sq ft = 1,398 sq in1,398 / 60 = 23.3, so 24 ventsClass I retarder: 1 per 1,5001,456 / 1,500 = 0.97 sq ft = 140 sq in140 / 60 = 2.33, so 3 ventsCorner rule adds 1: 4 ventsGrade falls 6 in in first 10 ft24 vents without a ground retarder vs 3 to 4 with Class ICorner vents within 3 ft; coverings 1/4 in max openingUnvented option: sealed retarder up stem wall 6 in plus exhaust or supply airSource: 2025 CRC R408.2, R408.3, R408.4 and R401.3. 60 sq in net free area per vent is given for the example.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Three Controls Against Moisture Under a Home

Once a home is skirted or set on a stem wall, the space under the floor traps ground moisture. The 2025 CRC controls it three ways: lot drainage away from the structure (R401.3), under-floor ventilation (R408.2) or an unvented crawl space (R408.3), and a ground vapor retarder. R408.1 says the space between the bottom of the floor joists and the earth must comply with R408.2 or R408.3. The CSLB C-47 Study Guide sample question on this topic asks the minimum number of skirting sides that must be ventilated; the printed answer is 2 opposite sides.

Step 2: Drain the Site Away From the Home

CRC R401.3: lots are graded so the surface falls not fewer than 6 inches within the first 10 feet away from the foundation. Where lot lines, walls or slopes prevent that, drains or swales must carry water away, and impervious surfaces within 10 feet of the foundation slope at least 2 percent away. R408.6 allows the under-floor grade at the footing bottoms, but where the groundwater table 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.

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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