Insulation and Acoustical (C-2) · Lesson 4 of 5

Spray Foam: Board Feet, Thickness and Lifts

"Spray Foam: Board Feet, Thickness and Lifts" is lesson 4 of 5 in the Insulation and Acoustical (C-2) Learning Hub. It opens with "The Board Foot Is a Foam Unit" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. The Board Foot Is a Foam Unit
  2. Thickness Comes from the R-Value
  3. Spray in Lifts, Not One Thick Pass
  4. From Area and Thickness to Sets
  5. Code: Flame Spread and the Thermal Barrier

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Thickness" through "Board feet".

What you'll be able to do

  • Explain the main idea of "Spray Foam: Board Feet, Thickness and Lifts" in plain language you can use on the exam.
  • Apply this step with confidence: "Spray in Lifts, Not One Thick Pass".
  • Apply this step with confidence: "From Area and Thickness to Sets".
  • Apply this step with confidence: "Code: Flame Spread and the Thermal Barrier".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Spray Foam: Board Feet and ThicknessExteriorInteriorstud (5.5 in. deep cavity)studLift 12 in.Lift 21.25air spaceFoam 3.25 in. = R-21.11/2 in. gypsum over the foam = thermal barrierSPRAY FOAM TAKEOFFClosed-cell: R-6.5 per inchNeed R-21 (2x6 wall): 21 / 6.5 = 3.23 in.Round up to 3.25 in. (R-21.1)Net wall area: 1,080 sq ft1,080 x 3.25 = 3,510 board ft+10% waste = 3,861 board ftSet yield 4,000 bf: 0.97 = 1 set1 set covers it: 2 in. lift + 1.25 in. lift = 3.25 in.Board foot = 1 sq ft x 1 in. thick. Cover the foam with 1/2 in. gypsum (CBC 2603.4 / CRC R303.4).Sources: 2025 CBC 2603.4, CRC R303.4, Energy Code 150.0(c)6. R per inch, lift, set yield: example valuesCC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: The Board Foot Is a Foam Unit

Spray foam is bought and priced by the board foot: 1 sq ft sprayed 1 in. thick. This is a different thing from the lumber board foot, because here the "board" is the sprayed layer. Two-component foam is sold in sets, and each set's label gives its yield in board feet. Board feet = net area (sq ft) x thickness (in.). Spray-applied foam plastic must also comply with Section 2603 and ICC 1100 (2025 CBC 2603.1.1; CRC R303.1.1), and packages must carry the label of an approved agency (CBC 2603.2).

Step 2: Thickness Comes from the R-Value

Divide the R-value required by the foam's R-value per inch to get the thickness. The required R-value comes from the plans or the Energy Code: in wood-framed walls, compliance may be shown with R-15 in 2x4 walls or R-21 in 2x6 walls at 16 in. on center (2025 Energy Code 150.0(c)6), or by the assembly U-factor (U-0.095 for 2x4, U-0.069 for 2x6, 150.0(c)1 and 2). R per inch is not printed in any California code book; the 6.5 per inch used here is an example value, and you must use the product data sheet or evaluation report. Round the thickness up so you never fall short, then check that it fits the cavity: a 2x6 wall has 5.5 in. of depth.

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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