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Insulation R-Value Calculator
Adds up an assembly R-value then corrects it for the studs, giving the whole-wall R and U-value that a nominal batt rating never shows.
Calculators
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No sign-up
Free, unlimited
A 2×4 is 3.5 in, a 2×6 is 5.5 in. Leave at 0 to match the insulation.
About 25% at 16 in centres once plates, headers and corners are counted; 20% at 24 in.
Board outside the studs, which no stud passes through.
OSB or plywood, R-1.25 per inch.
Plasterboard, R-0.9 per inch.
Optional — says how much continuous board would get you there.
Enter your figures and press Calculate.
In short
Insulation R-Value Calculator is a free online tool on inputtesthub.com that adds up an assembly R-value then corrects it for the studs, giving the whole-wall R and U-value that a nominal batt rating never shows. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the Insulation R-Value Calculator
- Pick the cavity insulation and enter its thickness.
- Set the stud depth and the framing factor for your wall.
- Add sheathing, lining and any continuous board outside the studs.
- Compare the effective R against the nominal figure on the packaging.
About the Insulation R-Value Calculator
The Insulation R-Value Calculator adds up an assembly R-value then corrects it for the studs, giving the whole-wall R and U-value that a nominal batt rating never shows. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Finding out what an R-21 batt actually achieves once the studs are counted.
- Deciding how much exterior board it takes to hit a target R.
- Converting between R-value and the metric RSI used outside the US.
Details
| Parallel paths | Add in U-value, never by averaging R-values |
|---|---|
| Softwood framing | About R-1.25 per inch — a 2×6 stud is only R-6.9 |
| Framing factor | Around 25% of a wall at 16 in centres, once plates and headers count |
| Metric | RSI = R × 0.1761, so R-20 is RSI 3.52 |
| Air films | R-0.68 inside and R-0.17 outside, on every assembly |
The maths behind it
This tool uses each path's R is the sum of its layers; the paths combine in U, not R, so U = framing fraction ÷ stud-path R + cavity fraction ÷ cavity-path R, and the whole-wall R is 1 ÷ U. Every intermediate value is shown on screen, so the result can be checked by hand.
The two figures this returns are both honest and they are not the same thing. The nominal R is what the packaging claims and what most calculators stop at. The effective R is what the wall does. Energy codes increasingly specify the second, which is why continuous exterior insulation has gone from optional to routine.
Where this tool stops
A one-dimensional parallel-path calculation, which is the method energy codes use and is good to a few percent for a normal stud wall. It does not model windows, corners, junctions, air leakage or moisture, and air leakage often costs more than thermal bridging does.
Frequently asked questions
Why is my R-21 wall only R-17?
Because the studs are not insulated. Softwood is about R-1.25 per inch, so a 2×6 stud is R-6.9 where the batt beside it is R-21, and at 16 inch centres roughly a quarter of the wall is timber once you count plates, headers, corners and the framing round openings. Heat takes the easy path, so the assembly performs much closer to the stud than the arithmetic average suggests.
How do I get the lost R back?
Continuous insulation on the outside of the studs, because it is the one layer nothing bridges. An inch of XPS adds R-5 to every part of the wall equally — to the stud path as well as the cavity — and recovers most of what the framing costs. It also warms the sheathing, which is what keeps condensation out of the cavity.
Why does polyiso have a warning on it?
Its R-value falls as it gets colder. A board sold at R-6 per inch can be closer to R-5 at freezing, which is exactly the weather you bought it for. Foil-faced polyiso is still an excellent product; it simply should not be specified at its laboratory number in a cold climate.
Are there any usage caps on the Insulation R-Value Calculator?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the page.
Is what I paste into the Insulation R-Value Calculator stored?
No. The Insulation R-Value Calculator runs entirely in your browser using JavaScript. Nothing is transmitted, logged or stored, which you can verify yourself in the Network tab of your browser's developer tools.
Can I check the maths myself?
It uses each path's R is the sum of its layers; the paths combine in U, not R, so U = framing fraction ÷ stud-path R + cavity fraction ÷ cavity-path R, and the whole-wall R is 1 ÷ U. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What do people use the Insulation R-Value Calculator for most?
In practice it gets used for three things: finding out what an R-21 batt actually achieves once the studs are counted. Deciding how much exterior board it takes to hit a target R. Converting between R-value and the metric RSI used outside the US. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Is there anything the Insulation R-Value Calculator cannot do?
A one-dimensional parallel-path calculation, which is the method energy codes use and is good to a few percent for a normal stud wall. It does not model windows, corners, junctions, air leakage or moisture, and air leakage often costs more than thermal bridging does. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What figures does the Insulation R-Value Calculator work from?
Parallel paths is Add in U-value, never by averaging R-values. Softwood framing is About R-1.25 per inch — a 2×6 stud is only R-6.9. Framing factor is Around 25% of a wall at 16 in centres, once plates and headers count. These are printed on the page itself, so you do not have to take them on trust.
Is the number rounded?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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