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The RSI value from the label, the specification or the energy code.
Metric insulation figures arrive in three shapes and only one of them belongs here. RSI for the product as sold is what you want. Lambda — thermal conductivity in W/m·K — is a material property and has to be divided by thickness first. A metric U-value is the reciprocal, so invert it before entering. Getting the wrong one of the three produces an answer that is wrong by orders of magnitude rather than percentages.
R-value
19.87 h·ft²·°F/BTU
1 RSI = 5.678263 R, so RSI × 5.678263 = R. Exact by definition. Where a code sets the target, quote that code's own figure rather than your conversion of the other one.
- RSI entered
- 3.5 m²·K/W
- Nearest whole R-value
- 20 h·ft²·°F/BTU
- IP U-factor (the reciprocal)
- 0.05 BTU/h·ft²·°F
They open the calculator with your figures already in it
RSI to R-Value Calculator: 19.87 h·ft²·°F/BTU — 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)
- NIST SP 811, Appendix B — thermal insulance: 1 m²·K/W = 5.678263 h·ft²·°F/BTU
- ASTM C168, Standard Terminology Relating to Thermal Insulation
Inputs used
- Thermal resistance (RSI) (m²·K/W)
- 3.5
Intermediate steps
- RSI entered
- 3.5 m²·K/W
- Nearest whole R-value
- 20 h·ft²·°F/BTU
- IP U-factor (the reciprocal)
- 0.05 BTU/h·ft²·°F
Confidence note: 1 RSI = 5.678263 R, so RSI × 5.678263 = R. Exact by definition. Where a code sets the target, quote that code's own figure rather than your conversion of the other one.
What this calculation does not cover
- A declared figure converted, not a measured one. Installation quality is outside it.
- Metric assembly figures more often include surface films than IP product figures do; check what each number already contains before comparing them.
Add the equipment this sizes
This result is a specification — 19.87 h·ft²·°F/BTU — 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 2026-09-12 · v1.0.0
Regulatory standards & verification citations2
- NIST SP 811, Appendix B — thermal insulance: 1 m²·K/W = 5.678263 h·ft²·°F/BTU
- ASTM C168, Standard Terminology Relating to Thermal Insulation
Which documents these citations point at
Standards referenced: ASTM C168 (ASTM International, United States).
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Going this way is mostly a procurement problem rather than a design one. The specification is metric, the product in front of you is American, and the question is whether the thing on the pallet clears the requirement. That makes the nearest whole R-value the useful output rather than the exact one: US insulation is not sold in arbitrary increments, so an RSI requirement of 3.5 is answered by an R-20 batt and nothing finer is available to answer it with. The reciprocal is printed for the case where the specification is written as a maximum U-factor instead, which is how most commercial envelope requirements are phrased, and the exact decimal is kept in front of the rounding so that a margin of a tenth is visible rather than lost in a round-up.