Insulation Calculator
Cavity R-17.6, effective R-12.7.
R-20 does not fit in a 5.5 in cavity with this material.
- You need 6.25 in of fiberglass batt to reach R-20, but the cavity is only 5.5 in deep.
- Options: switch to a higher R-per-inch material, add rigid board over the framing, or accept the R-17.6 the cavity gives you. Compressing a thicker batt into a shallow cavity does not get you its rated R — it gets you less.
| Figure | Value |
|---|---|
| Gross area | 1,000 ft² |
| Cavity area framing deducted | 750 ft² |
| With 10% waste | 825 ft² |
| Cavity R-value 5.5 in × R-3.2 | R-17.6 |
| Effective assembly R heat bypassing through framing | R-12.7 (28% below the label) |
| Depth needed for R-20 | 6.25 in — does not fit |
| Batts or rolls to buy 40 ft² each | 21 |
| Climate zone | Recommended attic R |
|---|---|
| Zone 1 | R-30 |
| Zone 2 | R-49 |
| Zone 3 | R-49 |
| Zone 4 | R-60 |
| Zone 5 | R-60 |
| Zone 6 | R-60 |
| Zone 7 | R-60 |
| Zone 8 | R-60 |
Effective R is the number that matters. Studs and joists conduct heat around the insulation at roughly R-1.25 per inch, so a wall never performs at its cavity R-value. This calculator uses the parallel-path method — cavity and framing side by side, weighted by area — which is why a nominally R-17.6 wall comes out at R-12.7. Framing fraction is taken as 25% at 16 in on centre and 20% at 24 in, which is typical once plates, headers, and corners are counted. Continuous rigid board over the framing is what breaks the bridge, and it is why codes increasingly require it. R-values are manufacturer figures for correctly installed material: compressed, gapped, or wind-washed insulation performs worse, and a batt stuffed around wiring can lose a third of its rating.
Find the R-value your cavity actually achieves, whether your target R will fit, and how many batts or rolls you need — with framing losses included.
How to use this calculator
- Enter the area to insulate, or the length and width if it is easier to measure that way.
- Pick the cavity you are filling. The presets are the real depths of standard framing — a 2×6 wall is 5.5 inches, not 6.
- Choose the insulation type. The R-per-inch shown next to each one is what drives everything else.
- Set a target R-value if you have one. The calculator tells you whether it physically fits in the cavity, which is the thing people discover too late.
- For batts and boards, enter the coverage per package from the bag to get a count.
The formula
Cavity R = Cavity depth (in) × R-per-inch · Depth needed = Target R ÷ R-per-inch
Effective R = 1 ÷ [ (cavity fraction ÷ cavity R) + (framing fraction ÷ framing R) ] · framing R = depth × 1.25
Worked example — a 2 × 6 wall with fiberglass batt
1,000 ft² of wall, 5.5 in cavity, standard fiberglass at R-3.2/in, framing at 16 in on centre, target R-20.
- Cavity R: 5.5 × 3.2 = R-17.6
- Framing R: 5.5 × 1.25 = R-6.9 through 25% of the wall
- Effective assembly R: 1 ÷ (0.75/17.6 + 0.25/6.9) = R-12.7
- Depth needed for R-20: 20 ÷ 3.2 = 6.25 in — more than the cavity holds
- Cavity area: 1,000 × 0.75 = 750 ft²
- Batts at 40 ft², +10% waste: 21 bags
Two things worth sitting with. The wall labelled R-17.6 actually performs at R-12.7 — a 28% shortfall you cannot see. And R-20 does not fit: to get there you either switch to mineral wool at R-4.2 per inch, or add continuous rigid board over the outside of the framing.
Thermal bridging: why the label overstates the wall
Insulation only works where it is. In a framed wall, roughly a quarter of the surface is not insulation at all — it is studs, top and bottom plates, headers, corner assemblies, and the framing around every window and door. Wood runs about R-1.25 per inch, so a 5.5 inch stud is R-6.9 while the batt beside it is R-17.6.
Heat takes both paths at once, in proportion to their areas, which is why the assembly lands well below the cavity figure. That is the number your energy bill responds to, and it is why building codes have moved steadily toward requiring continuous insulation — a layer of rigid board over the outside of the framing that the studs cannot bypass. An inch of polyiso outboard adds R-6 to every square foot of wall, studs included, which is worth far more than another inch in the cavity.
R-value per inch, and what it costs you in depth
| Material | R per inch | R in a 2×4 (3.5 in) | R in a 2×6 (5.5 in) |
|---|---|---|---|
| Blown fiberglass | 2.5 | R-8.8 | R-13.8 |
| Fiberglass batt | 3.2 | R-11.2 | R-17.6 |
| Blown cellulose | 3.5 | R-12.3 | R-19.3 |
| High-density fiberglass | 3.7 | R-13.0 | R-20.4 |
| Open-cell spray foam | 3.7 | R-13.0 | R-20.4 |
| EPS rigid board | 3.9 | R-13.7 | R-21.5 |
| Mineral wool batt | 4.2 | R-14.7 | R-23.1 |
| XPS rigid board | 5.0 | R-17.5 | R-27.5 |
| Polyiso rigid board | 6.0 | R-21.0 | R-33.0 |
| Closed-cell spray foam | 6.5 | R-22.8 | R-35.8 |
The practical reading: when cavity depth is fixed and you need more R than fiberglass gives, your options are a denser material or an outboard layer. When depth is unlimited — an attic floor — the cheapest R per dollar almost always wins, which is why blown cellulose dominates attics.
Never compress a batt to make it fit
Fiberglass and mineral wool work by trapping still air between fibres. Compress the batt and you squeeze out the air, which is the actual insulator. An R-19 batt forced into a 3.5 inch cavity delivers about R-13 — you have paid for R-19, installed it badly, and got the same result as the correct R-13 batt that costs less.
The related failure is gaps. A batt cut short, stuffed around wiring, or left bowed away from the drywall loses a disproportionate share of its rating, because air can circulate through the void. Installation quality is worth more than a point or two of label R-value: a carefully fitted R-13 outperforms a carelessly stuffed R-19.
Air sealing comes first
Fiberglass and mineral wool do not stop air. Air moves straight through them, carrying heat and moisture with it, which is why a draughty wall performs far below its rated R-value regardless of what is in the cavity.
If the house is leaky, sealing comes before adding R. Rim joists, top plates, plumbing and wiring penetrations, attic hatches, and recessed lights are where most of it goes. Caulk and canned foam are cheap, and a day spent sealing usually beats a thousand dollars of extra insulation on top of a leaky envelope. Spray foam and taped rigid board are the two products that insulate and air seal at once, which is a large part of what you are paying for.
Common mistakes to avoid
- Assuming the cavity is the nominal size. A 2×6 wall is 5.5 inches, not 6. A 2×10 joist is 9.25.
- Buying for the gross area. Roughly a quarter of a framed wall is studs. Insulate the cavity, buy for the cavity.
- Compressing batts. You lose the R you paid for.
- Comparing cavity R to whole-wall R. A contractor quoting "R-21 walls" is quoting the batt, not the assembly.
- Blocking soffit vents in an attic. Piling insulation into the eaves cuts off the ventilation path and causes ice dams and moisture problems. Use baffles.
How we calculate this
Cavity R-value is depth × R-per-inch, using published manufacturer figures. Effective assembly R uses the parallel-path method: heat flows through the cavity and through the framing side by side, so the assembly conductance is the area-weighted sum of the two, and its reciprocal is the effective R. Softwood framing is taken at R-1.25 per inch, and the framing fraction at 25% for 16 in on centre and 20% for 24 in — typical once plates, headers, corners, and rough openings are counted. Batt and roll counts work off the cavity area, not the gross area, because you do not insulate the studs. Recommended attic R-values are the Department of Energy figures by IECC climate zone.
Sources
Frequently asked questions
What R-value fits in a 2x6 wall?
A 2 by 6 wall has a 5.5 inch cavity. Standard fiberglass batt at R-3.2 per inch gives R-17.6, high-density fiberglass gives R-20.4, and mineral wool gives R-23. Closed-cell spray foam would give R-35.8 but is rarely filled to full depth. The important part: you cannot reach R-20 with standard fiberglass in that cavity — R-20 needs 6.25 inches of it.
Can I compress a thicker batt into a smaller cavity?
You can physically, but you do not get the rated R-value. Compressing an R-19 batt into a 3.5 inch cavity gives you about R-13 — better than nothing, but you have paid for R-19 and received R-13. Fiberglass works by trapping air in the space between fibres, and squeezing the air out removes most of what you were buying.
Why is my wall's actual R-value lower than the insulation's label?
Because studs conduct heat around the insulation. Wood is roughly R-1.25 per inch against fiberglass at R-3.2, and framing is about 25% of a typical wall's area once plates, headers, and corners are counted. That thermal bridging drops an R-17.6 cavity to about R-12.7 for the assembly as a whole. This calculator shows both numbers, because the effective one is what your heating bill responds to.
How much insulation do I need in my attic?
The Department of Energy recommends R-30 in the hottest zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. In blown cellulose at R-3.5 per inch, R-49 is about 14 inches deep and R-60 is about 17. Attics are the cheapest place to add insulation and usually the highest-return one, because there is no cavity depth limit — you just pile it deeper.
What is the R-value per inch of each insulation type?
Fiberglass batt is about 3.2, high-density fiberglass 3.7, mineral wool 4.2, blown cellulose 3.5, blown fiberglass 2.5, open-cell spray foam 3.7, closed-cell spray foam 6.5, EPS rigid board 3.9, XPS 5.0, and polyiso 6.0. Multiply by your cavity depth to get the cavity R-value. Higher per inch costs more per square foot, so the right choice depends on whether depth or budget is your constraint.
How many batts do I need?
Divide the cavity area — your gross area minus the framing — by the coverage printed on the bag, then add waste. A 1,000 square foot wall at 16 inches on centre has about 750 square feet of cavity, and at 40 square feet per bag that is 21 bags with a 10% allowance. This calculator does that deduction for you; forgetting it over-orders by about a quarter.
Is spray foam worth the extra cost?
It depends what you are buying it for. Closed-cell foam gives roughly twice the R per inch of fiberglass, which matters when cavity depth is fixed, and it air seals as it cures — often the larger real-world gain, since air leakage can account for a third of heating load. But it costs several times more, cannot be removed, and complicates future wiring or plumbing work. For an attic with unlimited depth, blown cellulose at a third the price usually wins.
Does insulation stop air leaks?
Fiberglass and mineral wool do not — air moves straight through them, which is why a poorly sealed wall performs far below its R-value. Spray foam and rigid board with taped seams do air seal. If your house is draughty, sealing the leaks first is almost always cheaper per unit of comfort than adding R-value on top of a leaky envelope.