Insulation Calculator
Insulating a wall, attic, floor or cathedral ceiling starts with two questions: what R-value do you need, and how much material does that take? Order too little and you stop work for another trip; over-buy and you are left storing batts and boards you cannot return.
This calculator turns each surface into the insulation to buy — batt packages, blown-in bags, or rigid board — for a target R-value you set or pick from a climate zone. It rounds every material up to real purchase units, checks whether a batt physically fits the cavity, deducts doors and windows, and adds up a whole-project shopping list. Everything runs in your browser.
Switch between metric and imperial at any time: imperial shows R-values, inches and square feet; metric shows RSI, centimetres and square metres. The physics is the same either way — only the units and labels change.
How much insulation do you need?
The calculator handles three forms, because they are bought and measured differently. Batts and rolls are bought by the package, chosen by the nominal R-value that fits your framing cavity. Blown-in (loose-fill) is bought by the bag, worked out from the settled depth needed to reach the target R. Rigid board is bought by the sheet, with each layer's R added up into an assembly total.
Each surface picks an element — wall, attic, floor or cathedral — and a form. A default project starts with one wall of batts and one attic of blown-in, mirroring the two most common jobs; add as many surfaces as your project needs and the Results panel totals them into one buy list.
Whichever form you choose, the quantity you buy fills the cavity or covers the area — it is not reduced by the framing factor. The framing factor only affects the effective R-value the assembly delivers, which is an optional secondary readout, never the purchase count.
R-value and RSI explained
R-value measures thermal resistance — how well a material resists heat flow. A higher R means better insulation. In imperial units R is measured in ft²·°F·h/BTU; in metric the same property is called RSI, measured in m²·K/W. The two are a simple constant apart: RSI = R ÷ 5.678, so an R-19 batt is about RSI 3.3.
Every material has a characteristic R-per-inch. Blown fiberglass is about R-2.9 per inch, cellulose about R-3.5, EPS about R-3.9, XPS about R-5.0, and polyiso about R-5.8 (though polyiso derates in cold weather). For loose-fill and rigid foam that value is roughly constant, so depth × R-per-inch gives the R-value directly.
Fiberglass and mineral-wool batts are the exception: they are manufactured at different densities to hit a nominal R at a fixed cavity depth, so an R-per-inch figure does not select them. That is why the calculator picks batts by nominal product (R-13, R-15, R-19, R-21, R-30, R-38) matched to the cavity, rather than deriving a depth.
Climate-zone R-value guidance
How much R-value you should aim for depends on climate. Colder zones lose more heat, so they call for higher R-values. The calculator uses the US IECC climate zones (1 warmest, 8 coldest) to suggest a recommended target R per surface, which you can override on any surface.
As a simplified single-number guide: attics run from about R-30 in Zone 1 up to R-49 in Zones 4–8; walls sit around R-13 in the warmest zones and R-20 from Zone 3 up; floors climb from about R-13 to R-38 in the coldest zones. Cathedral ceilings use the attic target but are limited by rafter depth (see the cavity-fit section).
This is recommended guidance, not a code-compliance calculation. Real building codes express wall and basement targets as combinations of cavity and continuous insulation that do not reduce to one number, and requirements vary by jurisdiction and year. Treat the zone value as a sensible starting point and verify against your local building code.
Guidance only — target R-values are typical recommendations, not a code-compliance calculation. Verify against your local building code.
Batts or blown-in?
For walls and sloped ceilings, batts are the usual choice: pre-cut lengths of fiberglass or mineral wool that press into a framing cavity. The nominal R has to match the cavity depth — an R-13 batt fills a 2×4 (3.5 in) wall, while an R-21 batt needs a deeper 2×6 (5.5 in) cavity. The two figures below show that difference: the same wall run, a shallow R-13 cavity beside a deeper R-21 one.
For open attics, blown-in (loose-fill) usually wins. It is poured to a settled depth and flows around obstructions, reaching high R-values that would need very deep framing as batts. The comparison figures below show the same target R met two ways — a batt seated in a cavity, and loose-fill piled to the depth that delivers the same R.
The calculator does not force the choice: set the element and form per surface and it computes packages for batts or bags for blown-in, so you can compare the shopping list both ways for the same job.
How blown-in depth and bag count are worked out
Blown-in insulation is modelled by volume, the same way loose material like gravel is. First the settled depth is derived from the target: depth = target R ÷ material R-per-inch. Then the volume is the insulated area times that depth, and the bag count is that volume divided by the coverage of one bag, rounded up.
The figure shows the settled fill piled above the joists with a depth ruler — the depth an installer marks on the joists is the number to hit. Manufacturer bag charts are already stated at the settled density, so no separate settling factor is applied; the bag yield is a visible, editable default you can match to your product.
Because the model is volumetric, it works for any target R and any material, not just the handful of values printed on a coverage chart. Switch the material between blown fiberglass and cellulose and the required depth and bag count update from that material's R-per-inch and bag yield.
Rigid board and assembly R-value
Rigid foam board (EPS, XPS or polyiso) is bought by the sheet and often layered — continuous board over a framed cavity, or two boards stacked. The board count is area-based, like drywall sheets, and each layer contributes its own R to a summed assembly total.
The figure below shows an R-layer stack with a gauge comparing the summed R against the target. Each layer's R is its thickness times the material's R-per-inch; a 2-inch XPS board adds about R-10, and a cavity batt adds its nominal R on top. The assembly passes when the layers together meet the target.
Every quantity comes from the net area of each surface and its target R-value. Purchase units are rounded up so you buy whole packages, bags and boards.
Batt packages
ceil( netArea ÷ coveragePerPackage × (1 + waste) )- The batt product is chosen by nominal R matched to the cavity (R-13/15 for 2×4, R-19/21/23 for 2×6, R-30/38 for attic).
- coveragePerPackage is a visible, editable default — it varies by manufacturer and batt width.
- netArea is the gross surface area minus any openings.
Blown-in depth and bags
depth = targetR ÷ rPerInch; bags = ceil( area × depth ÷ bagYield )- rPerInch ≈ 2.9 for blown fiberglass, ≈ 3.5 for cellulose (editable).
- bagYield is the settled coverage of one bag — a visible default calibrated from a manufacturer chart.
- Bag charts are already at settled density, so no separate settling factor is applied.
Rigid boards and assembly R
boards = ceil( area ÷ boardArea × (1 + waste) ); assemblyR = Σ (thicknessIn × rPerInch)- boardArea = board width × length (32 ft² for a 4 × 8 ft sheet; 2.88 m² for 1200 × 2400 mm).
- rPerInch ≈ 3.9 EPS, ≈ 5.0 XPS, ≈ 5.8 polyiso (editable).
- A cavity batt layer adds its nominal R directly to the assembly total.
Cavity fit and RSI
fits when targetR ≤ (cavityDepth − ventGap) × battRPerInch; RSI = R ÷ 5.678- The fit check uses the densest achievable batt (≈ R-4.3 per inch).
- On cathedral ceilings the vent gap is subtracted from the usable cavity depth.
- Switching to metric shows RSI (RSI = R ÷ 5.678) throughout.
Will the batt fit the cavity?
A batt can only deliver as much R as its cavity allows. This matters most on cathedral and vaulted ceilings, where a rafter of fixed depth also has to leave an air gap above the insulation for roof ventilation. If the target R needs more thickness than the rafter leaves after the vent gap, no batt will reach it.
The calculator checks this for every batt surface: the target fits when target R ≤ (cavity depth − vent gap) × the densest batt's R-per-inch. When it fails, the cross-section turns the batt red and warns that it is too thick for the cavity — the figure below shows that triggered state.
When a cavity is too shallow, the fix is not a thicker batt but a different assembly: add a continuous rigid board over the rafters, add a second insulation layer, or accept the lower R the cavity can hold. The calculator shows the largest batt that fits and flags the surface as below target so the gap is visible.
How to use the insulation calculator
Choose metric or imperial units and a climate zone at the top; the calculator remembers your choices in your browser. The zone sets a recommended target R-value on each surface, which you can override per surface.
Add a surface for each wall, attic, floor or cathedral. Pick the form — batt, blown-in or rigid board — enter the dimensions or a known area, and set the framing, spacing, material or board thickness the form needs. Add door and window openings to walls to deduct them.
Each surface shows its own quantity and the R-value it achieves against the target; the Results panel accumulates every surface into project totals — batt packages, blown-in bags and boards — plus a cost once you add prices. Open Assumptions & materials to adjust the R-per-inch values, bag yields and prices to match your product spec.
How to estimate insulation in 5 steps
- Choose units and climate zoneSelect metric (RSI, cm, m²) or imperial (R, in, ft²) and pick your IECC climate zone. The zone sets a recommended target R-value on each surface.
- Add surfacesAdd a surface for each wall, attic, floor or cathedral. Enter the dimensions or a known area, and add door and window openings to walls to deduct.
- Pick a form and materialChoose batt, blown-in or rigid board for each surface, then set the framing cavity and spacing, blown-in material, or board thickness and layers.
- Set the target R-valueUse the zone's recommended R-value or override it per surface. The calculator selects the batt product, blown-in depth or board layers to meet it.
- Read the resultsThe Results panel shows batt packages, blown-in bags and boards to buy, the R-value achieved against the target, and a cost once you add prices.
Worked examples
Three quick estimates at the default materials, coverage and pack sizes. Your own numbers will differ once you set the climate zone, target R and material to your spec, but these show how the packages, bags and boards come together.
Worked at the default materials, coverage and pack sizes. Quantities are rounded up to whole purchase units.
Garage wall — batts (imperial)
Wall · 2×6 · Zone 5- Given
- Wall 12 ft × 8 ft = 96 ft², no openings
- Zone 5 wall target R-20 → R-21 batt in a 2×6 cavity
- Coverage 77.5 ft²/package, 10% waste
- Formula
ceil( 96 ÷ 77.5 × 1.10 )- Working
- Packages: ceil(96 ÷ 77.5 × 1.10) = ceil(1.36) = 2 packages
- R achieved: R-21 ≥ target R-20 ✓
- Result
- 2 packages · R-21 meets target R-20
Attic — blown-in (imperial)
Attic · blown fiberglass · Zone 5- Given
- Attic 500 ft², Zone 5 attic target R-49
- Blown fiberglass R-2.88/inch → settled depth 49 ÷ 2.88 ≈ 17.0 in
- Bag yield ≈ 54 ft³/bag
- Formula
bags = ceil( 500 × (17.0 ÷ 12) ÷ 54.2 )- Working
- Depth: 49 ÷ 2.88 ≈ 17.0 in settled
- Volume: 500 ft² × 1.42 ft = 709 ft³
- Bags: ceil(709 ÷ 54.2) = ceil(13.1) = 14 bags
- Result
- 17 in deep · 14 bags · meets R-49
Wall — rigid board over batt (imperial)
Board + cavity · Zone 6- Given
- Wall 200 ft², Zone 6 wall target R-20
- 2 in XPS (R-5.0/in) continuous + R-13 cavity batt
- 4 × 8 ft boards (32 ft²), 10% waste
- Formula
boards = ceil( 200 ÷ 32 × 1.10 ); assemblyR = 2 × 5.0 + 13- Working
- Boards: ceil(200 ÷ 32 × 1.10) = ceil(6.88) = 7 boards
- Assembly R: (2 × 5.0) + 13 = R-23
- R achieved: R-23 ≥ target R-20 ✓
- Result
- 7 boards · assembly R-23 meets target R-20
Common mistakes to avoid
Reading the zone R-value as a code requirement. The suggested target is consumer-style guidance, not the prescriptive code table, which uses cavity-plus-continuous combinations that vary by jurisdiction. Always verify against your local code before treating a number as compliant.
Deriving a batt from R-per-inch. Batts are selected by nominal product matched to the cavity, not by dividing a target R by an R-per-inch — that would recommend a batt thickness that no product is actually made in. Blown-in and rigid foam are the forms where depth × R-per-inch applies.
Confusing the drawn cross-section with the buy count. The diagram is a schematic; the number to buy is higher because it includes your waste allowance and rounds up to whole packages, bags or boards. Order from the Results panel.
Ignoring the vent gap on cathedral ceilings. A batt that fits a bare rafter may not fit once you reserve the ventilation gap above it. Enter the vent gap so the cavity-fit check uses the real usable depth.
Frequently asked questions
How much insulation do I need?
It depends on the surface and your target R-value. For batts, the calculator divides the net area by the coverage of one package and rounds up. For blown-in, it works out the settled depth for your target R, then the bags of loose-fill to reach it. For rigid board, it divides the area by the sheet size. Add every wall, attic, floor and cathedral and the Results panel totals the packages, bags and boards to buy.
What R-value do I need?
As a guide, attics run from about R-30 in the warmest US climate zones up to R-49 in the coldest; walls sit around R-13 to R-20; floors from R-13 to R-38. Pick your climate zone and the calculator fills in a recommended target per surface, which you can override. This is consumer-style guidance, not a code-compliance figure — verify against your local building code.
How is the batt package count worked out?
Batts are selected by nominal product matched to your framing cavity — R-13 or R-15 for a 2×4 wall, R-19/R-21/R-23 for a 2×6, R-30 or R-38 for an attic — then packages = ceil(netArea ÷ coveragePerPackage × (1 + waste)). Coverage per package varies by manufacturer and batt width, so it is a visible default you can edit in the assumptions card.
How many bags of blown-in insulation do I need?
The calculator finds the settled depth for your target R (depth = target R ÷ the material's R-per-inch), multiplies it by the area for a volume, then divides by the coverage of one bag and rounds up. Manufacturer bag charts are already stated at the settled density, so no extra settling factor is added. The bag yield is an editable default — match it to your product for the closest count.
Should I use batts or blown-in?
Batts suit framed walls and sloped ceilings, where they press into a cavity; the nominal R has to match the cavity depth. Blown-in suits open attics, where it is poured to a settled depth and reaches high R-values without deep framing. The calculator computes either — set the form per surface and compare the packages-versus-bags shopping list for the same target R.
How deep does blown-in insulation need to be?
The settled depth is the target R divided by the material's R-per-inch. Blown fiberglass is about R-2.9 per inch, so R-49 needs roughly 17 inches; cellulose at about R-3.5 per inch needs roughly 14 inches for the same R. The cross-section figure shows the settled depth against a ruler — that is the depth to mark on the joists.
Will the batt fit my cavity?
A batt fits when the target R is no more than (cavity depth − vent gap) × the densest batt's R-per-inch. This matters most on cathedral ceilings, where the rafter depth is fixed and a ventilation gap is reserved above the insulation. If the target needs more thickness than the cavity leaves, the calculator flags it, shows the largest batt that fits, and turns the cross-section red — add rigid board or a second layer to close the gap.
How many rigid boards do I need?
Rigid board is counted by area, like drywall: boards = ceil(area ÷ board area × (1 + waste)), where a 4 × 8 ft sheet covers 32 ft² and a 1200 × 2400 mm sheet covers 2.88 m². Each layer's R (thickness × the material's R-per-inch) is summed into an assembly R-value shown against your target — for example a 2-inch XPS board adds about R-10.
What is RSI, and does the calculator work in metric?
RSI is the metric equivalent of R-value, measured in m²·K/W. It relates to imperial R by a constant: RSI = R ÷ 5.678, so an R-19 batt is about RSI 3.3. Switch units at the top and the whole tool shows RSI, centimetres and square metres; the underlying materials and targets are the same, correctly converted.
Are the target R-values code compliance?
No. The zone targets are simplified single numbers in the style of consumer recommendation charts. Real building codes express wall and basement targets as combinations of cavity and continuous insulation that do not reduce to one number, and requirements vary by jurisdiction and code year. Treat the zone value as a starting point and confirm compliance against your local code.
Is the calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to any server, and your project is saved only in your own browser's local storage.