Updated: Sep 17, 2025
Spray foam insulation is a great solution for high energy bills, but using the wrong thickness can cost you thousands of dollars and make your house feel like a drafty barn.
Many contractors either use too much, thinking "more is always better," or too little to cut costs. Neither approach is effective. The correct spray foam thickness is a science, and understanding the basics can help you avoid being duped.
Why Getting the Thickness Right Matters
One of the most common mistakes homeowners make is believing all spray foam is the same. It's not even close. There are two main types: open-cell and closed-cell foam. Each has different guidelines for thickness, and mixing them up can lead to a costly disaster.
Recommended Depths: Open-Cell vs. Closed-Cell
Open-cell foam, the less expensive option, needs more thickness to perform the same job. It offers about R-3.7 per inch, so you need more of it to meet your energy efficiency goals. The benefit? It's cheaper up front and provides good soundproofing.
When Extra Foam Pays Off—And When It Doesn't
Closed-cell foam is the overachiever. It acts as its own vapor barrier and provides R-6.5 per inch. Less thickness is needed, but the cost is higher. This extra performance often justifies the price, especially in colder climates.
Staying Safe: Pass Thickness and Fire Risk
A reliable contractor will discuss your specific needs rather than just quoting their standard package. For spray foam to provide effective thermal resistance, it must reach minimum thickness levels.
Differences in Foam Materials
Closed-cell foam provides R-6 to R-7 per inch.
Open-cell foam provides R-3.5 to R-4 per inch.
Location-specific minimum thickness requirements:
Attics: 6 to 10 inches
Crawl spaces: 4–6 inches
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