As a certified energy auditor who has extensivly studied the subject, I can assure you that there is no insulation that will provide an R-20 insulation value in a 2 x 4 wall cavity. The best faom available is polyurethane which can achieve up to a value of R-6 per inch, but that is not the kind of foam that can be used in closed wall cavities.
The type of foam USA Insulation uses can provide a maximum value of R-3.66 per inch. Simple math reveals that the maximum R value in a 3.5 inch cavity would then be R-12.81. Effective wall R value is even less than this because over 9% of the entire wall that has studs on 16-inch centers is wood, and wood has a much lower R-value per inch (about 1.25). Come on, USA Insulation, get real!
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