SettingsSettings for this calculationUS
The R-value of your main insulation layer, printed on the product.
This is usually your batt, blown-in, or rigid foam insulation — the biggest contributor to total R-value.
The R-value of a second layer, like exterior sheathing.
Set to 0 if this layer doesn't apply to your assembly.
The R-value of interior finish material, like drywall (typically about R-0.45 for 1/2 in).
Set to 0 if this layer doesn't apply to your assembly.
Total assembly R-value
15.8 R-value
- Sum of material layers
- 14.95 R
- Air film allowance
- 0.85 R
They open the calculator with your figures already in it
R-Value Calculator: 15.8 R-value — 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)
- Thermal resistances (R-values) in a layered assembly add directly: total R = sum of each layer's R-value
- A combined interior + exterior air film allowance of approximately R-0.85 is commonly included in total assembly R-value calculations
Inputs used
- Layer 1 R-Value (e.g. batt insulation)
- 13
- Layer 2 R-Value (e.g. sheathing)
- 1.5
- Layer 3 R-Value (e.g. drywall)
- 0.45
Intermediate steps
- Sum of material layers
- 14.95 R
- Air film allowance
- 0.85 R
What this calculation does not cover
- Layers are summed straight through the insulated cavity. Studs, joists, plates, headers and rim areas conduct several times faster than the insulation between them and are not deducted here, so the real whole-wall or whole-ceiling figure is lower than this total. Run the Thermal Bridging Effective R-Value Calculator on the same wall to see by how much.
- The figures are imperial R-values in hr·ft²·°F/BTU, and so is the fixed R-0.85 air film allowance. Metric RSI values in m²K/W are about 5.68 times smaller, so entering those makes the film allowance alone roughly six times too generous. Convert before you type.
- The air film allowance is one fixed number written for a wall: still indoor air with sideways heat flow, and an outside face exposed to wind. It does not change for heat flowing up or down, for a surface facing a vented attic or an enclosed crawl space, or for a reflective low-emissivity face, all of which shift the film values.
- Product R-values are lab ratings for material at full thickness with no gaps. Batts compressed under wiring, voids at plates and corners, and loose fill that has settled all deliver less than the printed number, and nothing here downgrades the total for installation quality.
- R-value covers conduction only. It says nothing about air leakage, wind washing through the insulation, or moisture in the assembly, and this total is not a code compliance check: the required figure depends on climate zone and on which element you are building, and codes are frequently verified against a whole-assembly U-factor that includes the framing this sum leaves out.
Add the equipment this sizes
This result is a specification — 15.8 R-value — 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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations2
- Thermal resistances (R-values) in a layered assembly add directly: total R = sum of each layer's R-value
- A combined interior + exterior air film allowance of approximately R-0.85 is commonly included in total assembly R-value calculations
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