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The total insulation R-value the wall assembly needs to achieve.
This is the R-value contributed by the EIFS foam board itself — check your local energy code or design target for the required continuous insulation R-value.
The insulation board's rated thermal resistance per inch of thickness.
Check the manufacturer's technical data sheet for the specific foam type — EPS, graphite-enhanced EPS, and XPS all carry different rated R-values per inch.
Required insulation thickness
3.33 in
R-value per inch varies by foam type (EPS, graphite-enhanced EPS, XPS) and manufacturer — use your specific product's rated value, not a generic assumption.
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EIFS Insulation Board Thickness Calculator: 3.33 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Required thickness = target R-value ÷ the foam board's rated R-value per inch (commonly ~4.5-5.0 for EPS/XPS insulation board)
Inputs used
- Target R-Value
- 15
- Board R-Value per Inch
- 4.5
Confidence note: R-value per inch varies by foam type (EPS, graphite-enhanced EPS, XPS) and manufacturer — use your specific product's rated value, not a generic assumption.
What this calculation does not cover
- There is a ceiling on the answer that the arithmetic never sees. An EIFS system is approved for a maximum insulation thickness in its code report — commonly around 4 in (102 mm) for the tested assembly — and on a building that needs an NFPA 285 wall, the foam thickness is fixed by the assembly that actually passed the test rather than by an R-value target. A thickness above that is not a thicker version of the same system; it is a different system, or none.
- Rated R and delivered R drift apart over the life of the wall. XPS is published at its fresh value and gives some of it back as the blowing agent diffuses out, which is why an aged long-term figure is quoted for it at all, while EPS holds its value and runs slightly better in cold weather, when the wall is being asked to work hardest. Sizing the layer on the number printed on the board leaves the assembly short of it years later.
- Thickness is a detailing decision as much as a thermal one. Every extra inch pushes the finished face further off the sheathing, and the window and door returns, the head flashing, the base track above grade and the roof edge all have to grow with it. A board thick enough for the R-value but thicker than the openings were framed for leaves nowhere to flash the head — which is where these walls leak.
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This result is a specification — 3.33 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Required thickness = target R-value ÷ the foam board's rated R-value per inch (commonly ~4.5-5.0 for EPS/XPS insulation board)
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