R-value is depth times R per inch, and that is the whole calculation
Every insulation product has an R-value per inch. Multiply it by how many inches you install and you have the R-value of the insulation. There is no more to it than that, and almost every confusion about insulation comes from treating the number on the packaging as a property of the product rather than a property of the product at a stated thickness.
| Product | R per inch, typical | Inches for R-38 |
|---|---|---|
| Fibreglass batt | 3.1 - 3.4 | 11 - 12 |
| Mineral wool batt | 4.0 - 4.3 | 9 - 9.5 |
| Blown cellulose | 3.2 - 3.8 | 10 - 12 |
| Blown fibreglass | 2.2 - 2.7 | 14 - 17 |
Those are ranges because products vary between manufacturers and between grades within a manufacturer. The bag or the wrapper states the figure for the product in your hand, and that is the number to put in the calculator. Use the range above only to sanity-check what you read.
What R-value you should be targeting is a different question entirely, and it is not one this page can answer. Recommended and required R-values are set by climate zone and by the version of the energy code your jurisdiction has adopted, and they differ for attics, walls, floors, crawlspaces and basement walls within the same house. An attic target in a cold northern zone is roughly double a mild southern one. Find the figure for your address and your assembly, then come back and work out the depth.
Compression, and why it costs you more than it seems
Push a 6.25 inch R-19 batt into a 5.5 inch 2x6 cavity and something has to give. What gives is the thickness, and the R-value goes with it. Compressing insulation increases its density, which does raise the R per inch a little, because the fibres are packed tighter and there is less room for convection inside the batt. But it never raises it enough to compensate. That R-19 batt in a 5.5 inch cavity delivers roughly R-18. Push an R-30 batt, nominally about 9 inches, into the same 5.5 inch cavity and you are somewhere near R-21 for the price and bulk of R-30. You have paid for six inches of product and installed five and a half inches of performance.
This is why the correct response to "the cavity is too shallow for the R-value I need" is never a thicker batt. It is a higher R-per-inch product in the same depth, mineral wool instead of fibreglass, or continuous rigid insulation over the outside of the framing, which has the additional benefit of covering the studs.
The opposite error is quieter and just as expensive. A batt thinner than the cavity leaves an air space, and if that space connects to anything, air moves through it and carries heat around the insulation rather than through it. Gaps at the top and bottom of a bay, batts stuffed behind wiring rather than split around it, batts that do not touch the sheathing on one side and the drywall on the other — all of them degrade a wall by more than the missing area suggests, because convection is much better at moving heat than conduction through fibreglass is.
Batts count by bay, bags count by volume
These are two different calculations and it is worth knowing which one you are doing.
Batts are sized to fit between framing at a specific spacing. A batt for 16 inch centres is 15 inches wide, and it covers a strip of wall 16 inches wide because it fills the bay and the stud takes the rest. So the coverage per batt is the on-centre spacing times the batt length: 16 inches by 93 inches is 10.33 square feet. Divide your area by that and round up. The waste allowance covers short bays at corners and around openings, where you cut a batt and the offcut is rarely the right size for the next gap.
Loose fill is a volume calculation. A bag holds a known weight, and at the settled density the product reaches in service, that weight occupies a known volume. Spread that volume at your target depth and you get the area one bag covers. A 25 pound bag of cellulose at 1.4 pounds per cubic foot is about 17.9 cubic feet, which spread at 10.9 inches deep covers about 20 square feet. Manufacturers publish this as a coverage chart on the bag, listing square feet per bag at each R-value, and their chart is more authoritative than this arithmetic because it reflects their actual settled density. The calculation is here so you can see what moves the number, and so you can check the chart against what you are seeing in the attic.
The part that is not arithmetic
Insulation only works as part of an assembly, and the assembly involves air sealing, ventilation and moisture that no quantity calculation touches. Air leaking through a ceiling carries far more heat than conduction through the insulation above it, which is why sealing top plates, wire penetrations, chases and the attic hatch before insulating is worth more per hour than the insulation itself. Soffit vents have to stay clear or the roof deck stops drying. Recessed lights that are not IC-rated have to stay uncovered or they are a fire risk. In some climates and assemblies a vapour retarder belongs on a specific face of the insulation, and putting it on the wrong one traps moisture inside the wall. Knob-and-tube wiring must not be buried. If any of that describes your situation and you are not sure, that is the point to ask someone who can look at the building rather than at the numbers.
Questions people ask
How many batts do I need for a 500 square foot wall?
With framing at 16 inch centres and 93 inch batts, each batt covers the 16 inch bay across its 93 inch length, which is 10.33 square feet. Five hundred divided by 10.33 is 48.4, so 49 batts with no waste allowance and about 54 with 10 percent for offcuts and short bays around openings. At 24 inch centres with 23 inch wide batts each one covers 15.5 square feet and the same wall takes 33 before waste. Note that this is the batt count, not the bundle count — bundles hold different numbers depending on thickness, and the coverage per bundle is printed on the wrapper.
What R-value do I need in my attic?
That comes from your climate zone and the energy code your jurisdiction has adopted, and it is not a single national figure. Attic targets in cold zones are roughly double those in mild ones, and the required value for an attic is different from the one for walls or floors in the same house. Look up the requirement for your address and your assembly — your local building department or the energy code adopted in your state will state it — then use this calculator to turn that target into a depth and a bag count. Anyone who gives you one number for the whole country is giving you a number that is wrong for most of it.
Can I put new insulation on top of old attic insulation?
Usually yes, and it is the normal way to upgrade an attic. The R-values add: six inches of existing fibreglass at R-3.2 per inch is about R-19, and blowing another eight inches of cellulose over it at R-3.5 adds about R-28 for a total near R-47. Two conditions. The existing insulation has to be dry and not compacted or contaminated, because wet or matted insulation contributes far less than its nominal value and covering it hides the problem that made it wet. And if the existing batts have a paper or foil facing, the new layer must not trap that vapour retarder in the middle of the assembly — the usual advice is to remove the facing or slash it before adding unfaced material over the top.
Why is my wall colder than the R-value says it should be?
Most likely thermal bridging and air leakage, neither of which appears in a cavity R-value. The studs, plates, headers and corners of a typical wall take 15 to 25 percent of its area, and wood insulates at roughly R-1 per inch against R-3.2 or better for the insulation, so the whole-wall R is well below the number on the batt no matter how well the bays are filled. On top of that, air moving through gaps around outlets, at the top and bottom plates and around windows carries heat directly past the insulation. Adding continuous rigid insulation over the outside of the framing addresses the bridging, and air sealing addresses the leakage. Filling the bays better addresses neither.