Insulation R-Value Calculator

Resistances add. Conductances do not.

Heat crossing a wall passes through every layer in turn, so the layers behave like resistors in series: the total thermal resistance is the plain sum of each layer's R-value. Siding, sheathing, cavity fill, drywall, and the thin films of still air clinging to each face all contribute, and the order does not matter to the total.

The U-factor is the reciprocal of that finished sum — U = 1 ÷ R — computed once, at the end. It is the rate of heat flow per unit area per degree, and it is what energy codes outside the United States are usually written in.

The trap is averaging or adding U-factors layer by layer. Because U is a reciprocal, summing the U of each layer gives a number larger than the U of any single layer, which says that adding insulation makes the wall worse. There is no reading of that result that means anything. Sum the R-values, invert once.

The number printed on the batt is not the number in the wall

A roll marked R-19 delivers R-19 through the fibreglass. It delivers nothing like R-19 through the stud sitting next to it. Softwood framing lumber is about R-1.25 per inch, so a 3.5-inch stud is R-4.4 — a quarter of the cavity next to it — and that stud is a continuous path from the drywall to the sheathing.

Framing is also more of the wall than people picture. Counted properly, including plates, corners, headers and the framing around openings, walls built at 16 inches on centre are typically 25% framing by area. Advanced framing at 24 inches with insulated headers gets it down to roughly 18%; ceiling joists are nearer 11%.

The correct treatment is the parallel-path or area-weighted method: compute the U of the cavity path, compute the U of the framing path, weight the two U-factors by their share of the area, then invert. Area-weighting the R-values instead gives a higher, wrong answer, because what conserves per unit of area is heat flow, and heat flow tracks U.

A standard 2×4 wall — vinyl siding, OSB, R-13 batt, drywall, both air films — sums to a nominal R-15.5. The framing path is R-6.9. Weighted at 25%, the wall performs at R-11.8. That 24% is not a rounding error, and almost nothing on the first page of search results accounts for it.

R per inch varies more between products than between categories

Insulation is sold by R-value, but it is manufactured with a conductivity, and the R per inch that follows differs enormously:

Ranges come from the U.S. Department of Energy insulation fact sheet and the ASHRAE Handbook of Fundamentals, chapter 26. They are starting points, which is why every R per inch in the calculator is an editable field rather than a hidden constant. Density, ageing, mean temperature and the manufacturer all move the number, and polyiso in particular loses R at low temperature — the direction you least want on a cold night.

R, RSI and the factor of 5.678

R-value is imperial: h·ft²·°F/BTU. RSI is metric: m²·K/W. They describe the same physical property and differ by 5.678. RSI 1.0 is R-5.678; R-30 is RSI 5.28.

That factor is why a Canadian label reading RSI 3.5 and an American label reading R-20 are the same product, and why reading an RSI figure as an R-value understates a wall by nearly a factor of six. This tool shows both at once, along with the metric U-value in W/m²·K, precisely so the two cannot be confused. The conversion is derived from the exact definitions of the BTU, the foot and the degree Fahrenheit, so the round trip closes.

Air films and cavities are free R-value, and they get dropped

A thin layer of still air sticks to every surface and resists heat all by itself. ASHRAE puts the inside film at about R-0.68 on a wall, R-0.61 on a ceiling with heat flowing up and R-0.92 on a floor with heat flowing down — the direction matters, because convection helps a rising flow and fights a falling one. The outside film is about R-0.17 in wind and R-0.25 in still air.

An unvented cavity between 20 and 100 mm adds roughly R-1.0. Give one face a reflective foil and it can approach R-2.8, because most of the heat crossing an air gap travels as radiation and foil refuses to emit it. Note what that means: the foil does nothing on its own. A radiant barrier pressed flat against a surface has no air space to protect and contributes almost nothing.

On a well-insulated wall these items are a small share of the total. On a bare one they can be most of it, which is exactly when leaving them out distorts the comparison you are trying to make.

What the IECC climate zones actually ask for

The prescriptive R-values in the 2021 IECC rise with the zone, and rise much faster for ceilings than for walls:

Ceilings jump to R-49 in zone 2 while the wall stays at R-13, and the reason is geometry rather than climate: an attic floor is flat, cheap to reach and has unlimited depth, so the code takes the R where the R is easy to get. The wall is expensive per unit of R.

Your jurisdiction may still be enforcing an older edition — many are on the 2018 or 2015 IECC — so treat these as the direction of travel and confirm what your building department has adopted. The target field in the calculator stays editable for that reason.

The attic first, and the reason is not insulation

In a house with an uninsulated or under-insulated attic, the attic is almost always the best dollar spent. Going from R-11 to R-49 in a ceiling removes about 78% of the conducted loss through that surface. Getting the same fractional improvement from a wall means opening it up.

But attic work is only as good as the air sealing under it. Warm air leaks upward through can lights, top plates, plumbing chases and the attic hatch, and insulation that is permeable to air — loose fill and batts both — does not stop that flow. It merely filters it. Blowing R-49 over an unsealed ceiling plane buys much less than the R-value implies, and the dirty streaks visible in old fibreglass are that air passing through it.

Doubling the R does not double the saving

Heat loss through an assembly is proportional to U, and U is 1 ÷ R. Every increment of R therefore removes a smaller slice of what is left:

• R-5 to R-10 removes half the loss.
• R-10 to R-20 removes another quarter of the original.
• R-20 to R-40 removes another eighth.
• R-40 to R-80 removes another sixteenth — an enormous amount of material for almost nothing.

Each of those steps is a doubling, each costs at least as much as the one before it, and each returns half as much. The calculator shows the next identical step alongside the current one so the shape of that curve is visible instead of implied. Somewhere on it, the money is better spent on air sealing, on the windows, or on a better heating system — and the point where that happens comes far sooner than the marketing suggests.

What a series R-value calculation cannot tell you

• Air leakage. Often a comparable share of the heating load in an older house, and completely invisible to an R-value. R measures conduction only.
• Installation quality. A batt compressed, gapped or cut short around wiring loses a substantial share of its rated performance. Grade I installation is an assumption, not a given.
• Steel framing. Steel conducts hundreds of times better than wood; simple parallel-path badly overstates it. ASHRAE's zone method or published correction factors are required.
• Thermal mass and solar gain. A steady-state R-value says nothing about how a heavy wall delays a heat pulse, and nothing about sun striking a roof.
• Moisture. Where the dew point lands inside your stack governs whether the assembly dries or rots, and it is a different calculation entirely.

Everything stays in your browser

Every figure is computed locally as arithmetic. Nothing is uploaded, nothing is stored and there is no account. The maths lives in a separate module that is verified against hand-worked examples outside the browser, including the parallel-path wall above, so the numbers are checkable rather than merely confident.