R-value per inch by material

Every figure here is a range. That is the honest form, and a chart printing one number per material is claiming a precision nobody has. The last column turns each range into inches, so you can see what a target costs in depth.

Settled, aged values from published material data, carried in our reference data as ranges. Spread is the top of each range over the bottom of it. The inches column is arithmetic on those ranges, so it is an estimate rather than a measurement, and loose fill settles as it deepens. Spray foam is sprayed to a thickness by a contractor, so its inches describe what was ordered.

Three things move a material inside its own range. Density is the first: the same fiberglass blown heavier gives more R an inch. The product is the second, because two bags with the same material name are not the same recipe. Installed condition is the third, and it is the one that bites. A batt squashed around a wire or cut short of the framing loses real R-value that the label never mentions.

The widest spread in the table belongs to loose-fill mineral wool, at 32 percent between the two ends. That is what those three things add up to, priced in R-value. It is also why this site never publishes a single per-inch number.

One product is missing on purpose. A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. It is measured differently from bulk insulation and the two do not add together. It belongs in a different conversation, and the radiant barrier page runs it.

A number on a bag is not a claim the maker invented. It is a lab result, and the FTC R-value Rule (16 CFR Part 460) governs how it is measured, stated and advertised in the United States.

The rule names the test methods. For most bulk insulation those are ASTM C518, a heat flow meter, and ASTM C177, a guarded hot plate. Whole assemblies go in a hot box, ASTM C1363. Reflective products are measured for emittance instead, under ASTM C1371 or ASTM E408, which is the technical reason a radiant barrier has no R per inch to print. Materials also carry their own specifications: ASTM C739 for cellulose, ASTM C764 for mineral fiber loose fill.

The rule also fixes the condition. Tests run at a mean temperature of 75 degrees Fahrenheit, with a 50 degree difference across the sample, plus or minus 10. So a label R-value describes one material, at one temperature, in one lab. Your attic is a different place on every day of the year. That is not a scandal, it is what a standard is for. It is just not the same thing as a measurement of your ceiling.

For loose fill the rule goes further, and this is the part worth knowing before you buy. The package has to carry a coverage chart, and the chart has to show the minimum settled thickness, the initial installed thickness, the maximum net coverage, the number of bags per 1,000 square feet and the minimum weight per square foot. It has to show all of that at R-13, R-19, R-22, R-30, R-38 and R-49.

Two useful consequences. Settled thickness and installed thickness are different numbers on the same chart, which is the maker telling you the pile will drop. And the reason every bag chart you have ever read shows the same rungs is that the rule sets them. Our cellulose anchor publishes 8 rungs, the six the rule names plus R-44 and R-60.

Shop on R per inch and you are ranking materials by a spec you cannot spend. What you buy is a target R-value over a floor area, at a price. So price the materials at the same R-value and the ranking changes shape.

Material only. No labor, delivery, disposal or air sealing. These are retail shelf prices at one retailer, read on 2026-08-11, except the blown fiberglass row, whose coverage chart and price were read on 2026-08-07. A contractor buys below shelf and we have not measured that discount, so nothing here is discounted. Prices move by store and by ZIP. One product is one point, not a market.

Read the first two columns against each other. Closed-cell spray foam has the best R per inch in the table and the highest cost per square foot at R-30, about 15 times the cheapest row. Blown-in fiberglass has the worst R per inch and is the second cheapest way to the same number. The ranking by R per inch is close to upside down.

That kit price carries its own warning. The maker calls its yield theoretical board feet, which is a way of saying real yield comes in lower. Large jobs are contractor equipment rather than kits, and no installed foam price is published anywhere on this site, because none has been measured. See spray foam attic cost for the board foot arithmetic to judge a quote with.

The rule that falls out of this is short. On an open attic floor, depth is cheap and R per inch is a curiosity. Where depth is capped, R per inch is the whole question and you should expect to pay for it.

Multiply depth by R per inch and the arithmetic assumes each inch is worth the same as the one below it. For loose fill that is not true. The U.S. Department of Energy notes that settled density rises as installed thickness rises. The pile compresses under its own weight, so a deeper install earns slightly less per inch.

One maker publishes enough of its own numbers to show it. Multiply each labeled target by the coverage that bag gets at that target. If R scaled cleanly with depth, the product would be flat. It falls.

Published coverage for the Owens Corning AttiCat blown-in fiberglass, 27.5 lb bag, read 2026-08-07. We use one product because its chart is public and you can check it. The pattern is the general one DOE describes for all loose fill.

Now price the error. Take the shallow point, R-19, and scale it as if R per inch were fixed. That predicts 35.1 square feet a bag at R-60. The maker publishes 31.8. On a 1,200 square foot attic that is 34 bags against 38, so the simple method orders about 9 percent short.

Which is why the coverage chart beats the multiplication, in both directions. Reading your own attic, depth times R per inch gives an estimate that flatters a deep pile. Buying material, a fixed R per inch under-orders at high targets. The depth checker carries the ranges and the warning built in.

Here is the same data pointed at real framing. The first three columns are dressed joist depths, so a 2x6 ceiling is 5.5 inches and a 2x10 is 9.25. The last two are depths you reach by burying the framing, which is what loose fill on an attic floor is for.

Both ends round down, on purpose, so no cell overstates what a depth is worth. These are conversions from R-per-inch ranges, so they estimate R rather than measure it, and loose fill does not scale cleanly with depth. Joist depths are dressed sizes under the American Softwood Lumber Standard PS 20.

Two readings jump out of that table. A 2x10 ceiling filled flush, 9.25 inches, is R-29 to 35 in cellulose. That clears zone 1's top-up target of R-25 and straddles its bare-attic target of R-30. Every zone above asks for more. And only one material in the table reaches R-49 inside 9.25 inches: closed-cell spray foam, which needs 7 to 8.2 inches. Everything else needs 11.7 inches or more.

So on an attic floor, the joists are not the target. They are a ruler. Part the material down to the top of one, measure from the drywall, and read your depth off the table above. Then compare it with your zone target on R-value by climate zone.

One caution about filling flush. Wood carries heat faster than the insulation beside it, so bare joist tops are a shortcut through the ceiling. Loose fill blown over the whole floor beats batts of the same rated R fitted between framing. Blown-in against batts covers that trade.

This is the decision the per-inch number actually settles. It only comes up when depth is capped and you cannot add more: a cathedral ceiling, a low-slope roof, a knee wall, a floor over a garage. Find your bay depth, then read across.

The batts column uses mineral wool, the strongest per inch of the ordinary materials at R-3.3 to 4.2. Both columns round down. In a vented roof bay part of that depth has to stay as an open air channel, so the usable figure is lower than the framing suggests. See baffles.

The recommendation. If the depth is not capped, ignore R per inch and buy on cost per R. In our read that was cellulose first and blown fiberglass second, and the gap between either of them and foam is large enough that no per-inch advantage closes it.

If the depth is capped, price two options before you accept the foam quote. One, add depth: furring the bay deeper, or rigid board over the roof deck, buys inches at ordinary material prices. Two, foam it. Foam earns its price here and nowhere else, and the arithmetic to check the quote is on open cell against closed cell. Spraying the underside of the roof also changes how the roof dries and vents, which is a design call for a contractor, not a material choice.

One more, because targets are what all of this is aimed at. ENERGY STAR recommends R-60 for a bare attic across zones 4 to 8, and R-49 where 3 to 4 inches are already there. The model 2021 IECC sets a ceiling minimum of R-60 in those zones for new and permitted work. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it.

Get the capped part of the job quoted

Cathedral ceilings, knee walls and low-slope roofs are where quotes vary most. Tell us the job and your ZIP, and bring your bay depth to the call.

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We do not sell insulation and we do not install it, which is why this section can exist. For most readers, this whole page is background. An open attic floor has headroom above the joists, and depth up there is cheap. So the per-inch number stops being a choice and becomes a bag count.

Three cases where you can close the tab.

• Your attic floor is open and you can walk it. Pick on cost per R, then buy depth. The best per-inch material would cost you more to reach the same target.
• You already measure within about R-10 of your zone target. More depth is the wrong purchase whatever material it is. Air sealing is the right one. That threshold is our own estimate, not a published rule.
• The material up there is wet, moldy, rodent fouled or possibly vermiculite. Then this is a removal question before it is a depth question, and the remove-or-add-over guide runs that test first.

R per inch earns its keep in one place: a space whose depth is fixed by the building. If that is not your house, the number is trivia, and the money goes further in sealing the ceiling and evening out the depth at the eaves.

Every figure here starts with a depth you measure. Take the reading in at least five spots, away from the hatch, and work from the typical one rather than the deepest. The rest of the gear list is organized by job, and nothing on it has been tested by us.

Insulation Report earns a commission if you buy through some of the product links on this page. It costs you nothing extra and it does not change which products we recommend or what we say about them.

• Rigid ruler or depth probe

  Look for Rigid, marked in inches, long enough to reach the drywall through the deepest part.

  Depth is measured from the drywall up, in at least five places, working from the typical reading.

  Skip it if A tape measure and a steady hand will do it. This is a convenience, not a requirement.

• Laser distance measure

  Look for Anything that reads to a wall. Accuracy well beyond what this needs is standard.

  Floor area drives every bag count and every quote. Pacing it out is where estimates go wrong.

  Skip it if The attic footprint matches the floor below and you already know the square footage.

• Moisture meter

  Look for Pin or pinless, reading wood moisture content.

  Answers the question a survey cannot: whether staining on the sheathing is active or historic.

  Skip it if The deck is visibly dry and clean, with no staining, frost marks or rust on nail tips.

• Indoor hygrometer

  Look for A digital display reading relative humidity as a percentage. Accuracy to a couple of points is plenty; you are watching a band, not calibrating an instrument.

  Winter attic frost is fed by indoor humidity, and the fix is often to turn a humidifier down rather than to buy anything for the attic. Building America guidance puts the heating-season band at 30 to 50 percent. Without a reading you are adjusting by feel, and comfort intuition is exactly what gets that wrong.

  Skip it if You run no humidifier and see no frost or condensation. Then the number is a curiosity rather than a decision.

• Thermal camera or infrared thermometer

  Look for A phone-attachment thermal camera, or a spot infrared thermometer for a fraction of the price.

  Shows where insulation is missing and where air is moving, which is the difference between a depth problem and a sealing problem.

  Skip it if You have already found the answer with a ruler. This is worth buying when the depth reads fine and the house still feels drafty.

• The R-value chart: targets and materials on one screen
• R-value by climate zone, all eight zones
• How much insulation do I need? The full sequence
• Blown-in against batts
• Cellulose against fiberglass
• Radiant barrier, and why it has no R per inch
• How much insulation do I have?
• What insulation costs per square foot