Recommended R-values Guide
Recommended R-values Guide
Understand cautious R-value ranges for attics, walls, roofs, floors, basements, crawl spaces, windows, and doors.
Reference
Recommended R-values by building application
This guide is educational and does not replace local building code, product documentation, or professional advice. Use it to understand why attics often need higher R-values than walls, why floors and basements have different limits, and why window performance is usually discussed with U-factor rather than insulation R-value.
Typical values are educational planning references. Always check local code, product documentation, and project-specific constraints.
Typical ranges by application
Use these ranges to choose a calculator target. They are not code requirements.
| Application | Cautious planning range | Notes | Calculator |
|---|---|---|---|
| Attic insulation | often R-38 to R-60 | large ceiling plane; keep ventilation and air sealing intact | Attic |
| Wall insulation | often R-13 to R-30 | separate nominal R from whole-wall performance with framing | Wall |
| Roof insulation | often R-30 to R-60 | sloped roofs need moisture, void, and bridge checks | Roof |
| Floor insulation | often R-19 to R-38 | conditions differ over garages, basements, and crawl spaces | Floor |
| Basement insulation | often R-10 to R-20 | check ground contact, moisture, and local rules | Required R |
| Crawl space insulation | often R-19 to R-30 | air and moisture control are central | Floor |
| Windows and doors | usually U-factor rather than R | lower U-factor is better; compare whole-window ratings | Window |
| Climate considerations | target rises in colder zones | local code and product data take priority | Required R |
Recommended R-values by building application
Example: an attic at R-19 with a planning target of R-49 needs about R-30 added before product and code checks.
Typical values are educational planning references. Always check local code, product documentation, and project-specific constraints.
Practical note
How to interpret the calculator result
Recommended R-values by building application is intended for quick option checks and technical discussion before detailed execution. The result depends on the selected units, declared material values, and chosen surface resistances, so each change in layer or thickness should be treated as a separate variant.
The calculator does not automatically verify every local rule, thermal bridge, moisture condition, structural connection, or installation tolerance. If the result is close to a requirement, treat it as a reason for deeper verification rather than a final decision.
For better comparisons, test several realistic thicknesses, check current product data sheets, and review the complete assembly. A calculated value is most useful when the assumptions are clear: material, thickness, layer order, units, and data source.
For insulation or U-value tools, layer order and correct units are especially important. For concrete, electrical, plumbing, or heating tools, the result should be read as a quick quantity or plausibility check before standards and execution conditions are reviewed.
Save the result with the date, material name, and assumptions. If the product, diameter, cable section, or thickness changes later, do not compare the numbers alone without checking which inputs changed.
For calculator pages, clear separation between inputs and result is essential. If a value looks surprising, check units and default fields first, then review the project assumptions.
Assembly thickness
Assembly thickness
Use the tables and formulas to choose a sensible starting thickness before checking the exact assembly.
U-value
U-value
Keep R-value, U-factor, U-value, lambda, k-value, and unit systems separate before comparing results.
CategoryLambdaLayerMaterialThickness
How it works
Use with care
Do not treat any table on this page as official code compliance.
Next step
Use the application calculators to estimate additional R and thickness after choosing a cautious target.
Recommended R-values Guide
Additional R needed = target R-value - current R-value, clamped at zero.
This guide is educational and does not replace local building code, product documentation, or professional advice. Use it to understand why attics often need higher R-values than walls, why floors and basements have different limits, and why window performance is usually discussed with U-factor rather than insulation R-value. Example: an attic at R-19 with a planning target of R-49 needs about R-30 added before product and code checks.
Add layerRestore default setupRemove
Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
Calculation assumptions
The calculators use visible formulas and explicit unit conversions. Treat the result as a preliminary check, not a complete building design.
- SI and US units are converted separately; R, RSI, U-value, and U-factor are not mixed without the unit factor.
- Enter positive values and compare the result with the selected product datasheet.
- Local codes, thermal bridges, fasteners, and installation quality can change the requirement.
- Last formula review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
US R-value mini-hub
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Reference FAQ
No. They are educational references for planning, comparison, and calculator input checks.
Use typical values only for early estimates. Product labels, declarations, and local rules should override them.
R-value and U-factor are common in US practice, while lambda and U-value are common in SI reporting. The pages keep unit systems explicit.
Yes. You can print the result or export it to CSV, Excel, or PDF for reports and documentation.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use Recommended R-values by building application as a first-pass reference. Before specifying anything, compare the result with the moisture condition, actual project dimensions, product data sheet, and local requirements.