Insulation R-Value Calculator

How this is calculated

R-value is thermal resistance, and it is ADDITIVE with thickness: double the inches of the same material and you double the R. So the thickness needed is simply the target R divided by the material's R per inch. R PER INCH, for common materials: Fibreglass batt 2.9 to 3.8, with about 3.2 typical for standard density and 3.8 for high density. Blown fibreglass about 2.8. Blown cellulose 3.2 to 3.8, commonly cited at 3.7 for attic applications. Open-cell spray foam about 3.5. Closed-cell spray foam 6.0 to 7.0 - the highest of the common materials. Polyisocyanurate rigid board 5.6 to 6.5. ⚠️ THE CAVITY IS THE CONSTRAINT, NOT THE R-VALUE YOU WANT. This is the thing worth understanding before buying anything. A 2x4 stud wall has a cavity 3.5 inches deep. A 2x6 has 5.5. Those numbers do not move. Work it through and the consequence is sharp: R-13 needs 4.06 inches of standard-density fibreglass at 3.2 per inch, and a 2x4 cavity is 3.5 inches. **Standard batt physically cannot reach R-13 in a 2x4 wall.** The R-13 batts sold for 2x4 walls achieve it by being denser: fitting R-13 into 3.5 inches requires exactly 3.71 per inch, which is what those batts are built to deliver. The general high-density figure in the material list above is 3.8, slightly better again - it reaches R-13.3 in the same cavity. Either way, the density is the point, and it is why high-density batts exist and cost more. So in a fixed cavity your only lever is the material. At 3.5 inches you get about R-11 from standard batt, R-13 from high density, and R-23 from closed-cell foam. Same space, twice the performance, and a very different price. ATTICS ARE THE OPPOSITE CASE, and that is why they are where the value is. Depth is usually free there - nothing is stopping you adding more - so the cheap materials win on cost per R. R-49 takes about 13.2 inches of blown cellulose at 3.7, which is simply a matter of blowing more in. The same R-49 in a wall would be impossible. ⚠️ INSTALLATION QUALITY CAN COST MORE THAN THE MATERIAL CHOICE. Batts compressed to fit, gaps at the edges, insulation stuffed around wiring, and missed spots behind pipes all reduce real performance well below the rating on the packet. A carefully installed R-13 outperforms a badly installed R-19. Compressing a batt to fit a shallower cavity does NOT give you its rated R - it gives you roughly the R of that thickness of that material, and often less because it disturbs the air pockets doing the work. WHAT THIS DOES NOT COVER: air sealing, which frequently matters more than adding R-value and is far cheaper; thermal bridging through the studs themselves, which is why continuous exterior insulation exists; and vapour control, which is climate-specific and gets buildings into trouble when guessed at. Recommended R-values also vary by CLIMATE ZONE and by where in the building you are - your local code and energy office publish the figures for your area.

Common questions

Why can't standard batt reach R-13 in a 2x4 wall?

Because the arithmetic does not fit. R-13 at 3.2 per inch needs 4.06 inches, and a 2x4 stud cavity is 3.5 inches deep. That is not a rule of thumb - it is a subtraction, and no amount of pushing changes it. The R-13 batts sold for 2x4 walls achieve it by being DENSER: R-13 in 3.5 inches needs exactly 3.71 per inch, which is what the calculator's 'R per inch you'd need' output returns and what those batts are built to. The general high-density figure in the list is 3.8, which reaches R-13.3 in the same space. In a fixed cavity your only lever is the material.

Can I just compress a thicker batt to fit?

No, and this is the single most common insulation mistake. Compressing a batt does not give you its rated R-value - it gives you roughly the R of that thickness of that material, and often somewhat less, because compression disturbs the trapped air pockets that are doing the actual work. Stuffing an R-19 batt into a 2x4 cavity gets you something in the region of an R-13, at R-19 prices, with a worse fit. Buy the batt made for the cavity.

What R-value should I be aiming for?

It depends on your climate zone and on where in the building you are, and this calculator deliberately does not guess. Recommended values differ substantially between an attic, a wall, a floor and a basement, and between a cold climate and a mild one - your local building code and energy office publish the figures for your area. What the calculator does is answer the next question: given that target, what thickness does it need and will it fit.

Why is the attic where the value is?

Because depth is usually free there. Nothing is stopping you adding more, so the constraint that dominates walls simply does not apply - and that means the cheap materials win on cost per R. R-49 takes about 13.2 inches of blown cellulose at R-3.7 per inch, which is a matter of blowing more in rather than a design problem. The same R-49 in a stud wall would be impossible at any density. If you have a fixed budget and both are unfinished, the attic almost always returns more.

Is a higher R per inch always better?

Only when space is the binding constraint, and that is worth being clear about because the expensive materials are sold on this number. Closed-cell foam at R-6.5 per inch is genuinely twice the performance of standard batt in the same cavity, which matters enormously in a 2x4 wall and hardly at all in an open attic where you could simply add depth. Pay for R per inch when inches are scarce; pay for cost per R when they are not.

What matters more than the R-value?

Two things, and both are routinely skipped. Installation quality: batts compressed to fit, gaps at the edges, material stuffed around wiring and missed patches behind pipes all drag real performance well below the number on the packet - a carefully installed R-13 outperforms a badly installed R-19. And air sealing, which frequently matters more than adding R-value and costs far less. Insulation slows heat moving through a surface; it does nothing about air moving through a gap.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Insulation R-Value Calculator at https://www.bfcbrilliance.com/tools/insulation-calculator.

What I entered:
- Target R-value (R): ___
- Material: ___
- Space you have (in): ___
- Area to insulate (sq ft): ___

What it calculated:
- Thickness needed: ___
- Cavity it would fill: ___
- Best R this space can give, with this material: ___
- Short of target by: ___
- R per inch you'd need to fit it: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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