Your attic R-value target: a six-step check

This page is the order of operations. Three other pages on this site do one step each, and this one joins them up. Work down the table, and stop at the last row, which is the only step that decides how you spend money.

One thing to fix before step 1. The number every later step rests on is the attic floor area, not the size of the house. Those are different numbers on most homes, and the gap between them is the most expensive mistake on this page. There is a section on it below.

Take a rigid ruler or a tape. Push it straight down through the material until it stops on the drywall. Read the depth where the surface of the insulation crosses it. Then do it again, four more times, spread across the attic and away from the hatch.

Five readings, not one, and the reason is money. Insulation is almost never even. It is deepest near the hatch where the installer stood, and thinnest at the eaves where the roof comes down to meet the wall. One reading taken at the deep end tells you the attic is fine when it is not. Use a typical reading, and write down the low one too. If your low reading is at the eaves, that edge is its own job, and baffles are usually part of it.

No ruler? Use the framing. Ceiling joists are dressed to a standard depth under the American Softwood Lumber Standard PS 20: 5.5 inches for a 2x6, 7.25 inches for a 2x8, 9.25 inches for a 2x10. If you can see the tops of the wood standing above the insulation, you are under the depth of a joist. In the deepest common framing that means under 9.25 inches, which in blown fiberglass is roughly R-20 to R-24 and short of the target in every climate zone in the country. A rigid ruler and a headlamp are the whole kit for this step, and the gear pillar lists what to look for.

Depth on its own does not answer anything. The same ten inches is worth very different R-values depending on what is up there. This is the step most people skip, and it moves the answer by more than a whole climate zone.

R per inch from published material data, carried as ranges rather than single numbers because it varies with the product, the density and how it went in. A label R-value is measured under the FTC R-value Rule, FTC R-value Rule (16 CFR Part 460), by a heat flow test such as ASTM C518. Cellulose has its own material standard, ASTM C739.

Telling them apart is quick. Blown fiberglass looks like pale fluffy cotton candy, often white, pink or yellow. Blown cellulose is gray, dense and made of shredded paper. Loose-fill mineral wool is darker and grittier, and often has small hard flecks. Batts come in fitted lengths between the joists. The identification guide has photographs and the two materials that mean stop.

Two materials break the arithmetic. Closed-cell spray foam runs about R-6 to R-7 an inch, and it is usually sprayed on the roof deck rather than the floor, which is a different job with different targets. A radiant barrier has no place in this sum at all. 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.

The 2021 International Energy Conservation Code splits the country into eight zones, 1 being the hottest and 8 the coldest. Your zone sets your target. Zones follow county lines, not state lines, and several states cover three of them, so a state map is not a lookup.

Use the ZIP lookup. It runs your ZIP through the U.S. Census Bureau ZIP code tabulation areas to a county, then to the county zone file published by the Pacific Northwest National Laboratory for the U.S. Department of Energy. About 1,044 of the 33,635 ZIP areas in that data straddle a real zone line, and the lookup says so when yours is one of them. If you land on a line, check both zones. They often carry the same number anyway, and where they differ, aim at the colder one.

ENERGY STAR publishes a recommended attic level for each zone, and says that guidance is based on Table R402.1.3 of the 2021 IECC. There are two columns, and picking the wrong one is common. Use the bare column if you have less than about 3 inches. Use the second column if you already have 3 to 4 inches or more, which is where most older houses sit.

That is guidance for a home that already exists, and nobody makes you follow it. Energy code is a separate thing. The model 2021 IECC sets a ceiling minimum of R-60 in zones 4 through 8, and it is a model published by the International Code Council. 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. What each state has adopted runs from a 1989 basis to the 2024 edition, which the state energy code tracker lists row by row. The zone page gives all eight zones one at a time, with the model code figure beside each retrofit target.

Now the arithmetic. Target, minus what you measured, is your gap in R-value. Divide the gap by the R per inch of whatever you plan to add, and you have the inches to blow or lay on top.

Worked once. A zone 5 attic holding 6 inches of settled blown fiberglass is about R-13 to R-16. It has more than 3 inches, so the target is R-49. The gap is about R-33 to R-36. In blown cellulose at R-3.2 to R-3.8 an inch, closing it takes roughly 8.7 to 11.3 inches over the top. The old fiberglass stays where it is, as long as it is dry and clean.

Two cautions on that number. It is an estimate, because loose-fill R-value does not climb in step with depth, which the traps section below shows in a manufacturer's own coverage figures. And it is per square foot of attic floor, so the bag count depends on the area you measured. The attic insulation calculator does the whole sum, including bags and an installed cost built from wage and coverage data. R-value per inch by material covers how a label R is tested and why per inch is a poor unit to shop on.

Everything above produces a gap. This is what to do about it. Find the row that matches what you measured and read across. Every row ends in an action.

The reason air sealing keeps appearing is that R-value only describes heat moving through material. Warm air leaking around light fixtures, pipes, the hatch and the tops of walls goes past the insulation instead. DOE attic guidance puts sealing before insulating for that reason. If you are hiring anyway, seal and insulate in one visit. The deck is only bare and easy to work on once.

If your measurement lands within about R-10 of your zone target, and the material is dry and evenly spread, you do not need more insulation. That is the most useful sentence on this page. We do not sell insulation and we do not install it, which is why it can be here.

Treat that R-10 line as our estimate, not a measured threshold. It comes from the shape of the ENERGY STAR table itself, which steps in jumps of about R-11, and from the way heat flow works. Heat moving through the ceiling falls as one over R. So going from R-11 to R-30 cuts conducted heat flow by about 63 percent, while R-38 to R-49 cuts it by about 22 percent, and R-49 to R-60 by about 18 percent. Those are figures for conduction through the insulated plane. They are not a prediction about your bill, and this site holds no energy price or degree day data, so it publishes no dollar saving.

Three more cases where the answer is to spend nothing on depth right now. If the roof, the wiring or the vents are due for work, wait. Anything that tears up insulation spends the money twice. If the attic is thin only at the edges, that is a coverage fix, not a top-up. And if your ceiling has never been sealed, sealing it makes the insulation you already own work closer to its label, usually for less than a load of new material.

Both of these change the number, not the wording. They are worth ten minutes before you sign anything.

Trap one: attic floor area is not house floor area. The attic covers what sits under the roof, which is the ceiling of the top floor. On a two-story house that is roughly the upper floor alone. Garages, porches, additions with their own roof and rooms with vaulted ceilings all move it further. We publish no average ratio between the two, because it depends entirely on the house. Measure the attic, or measure the rooms below it, and check the figure the quote is built on. Say a quote is priced on 2,000 square feet and your attic floor is 1,200. That is 800 square feet of material and labor you cannot use, on every line of the estimate.

Trap two: inches converted to R-value is an estimate. Loose fill packs down under its own weight, so the deeper the pile, the less each added inch is worth. DOE describes settled density rising as installed thickness rises. One named product shows it in the maker's own coverage table: multiply each labeled R by the coverage per bag and the product should hold steady if R scaled cleanly with depth. It does not.

Coverage for the Owens Corning AttiCat blown-in fiberglass, 27.5 lb bag, as the maker published it on the retail listing, read 2026-08-07. One product, because its coverage table is public and you can check it. The pattern is the general one DOE describes for all loose fill.

What that means for you is small but real. Your depth-to-R sum is a little kind at the deep end. Run the other way, a calculator that divides a target R by a fixed R per inch will under-order bags at high targets. Trust the bag label for the exact product you buy, and buy one bag more than the sum says.

A reader in zone 5 climbs into the attic with a tape and a headlamp. Five readings run from 4 inches at the eaves to 7 near the hatch, so the typical reading is 6 inches. The material is pale and fluffy, so it is blown fiberglass rather than cellulose.

That is about R-13 to R-16. The attic holds more than 3 inches, so the target is the second column: R-49. The gap is about R-33 to R-36. In cellulose that is roughly 8.7 to 11.3 inches on top. The attic floor measures 1,150 square feet, which is the number that goes on every quote, and the house measures more than that because of the garage.

The order of work falls out of the readings. The 4-inch spots at the eaves come first, with baffles so the vents stay clear. The ceiling gets sealed while the deck is still reachable. Then the cellulose goes on over the whole floor, deep enough to bury the joists. Nothing is removed, because the old fiberglass is dry and clean. Insulating over existing insulation covers the layering rules, including not squashing what is underneath.

The instruction this site repeats most often is measure first. Most of it needs a ruler and a torch. The rest of this list buys you answers a ruler cannot reach.

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.

Some of this is a reasonable weekend for a fit person, and some of it is not. Hire out the work when the attic has no floor to move on, when the gap is large enough to need a blower and a second pair of hands, or when the material has to come out first. Also hire when you cannot get to the eaves safely, because that is exactly where the depth is thin.

Whoever quotes, ask for four things in writing: the attic floor area, the target R-value, the settled depth in inches that target needs, and whether air sealing is included or excluded. A quote with one number and no scope cannot be compared with anything. What insulation costs shows how a price is built from wage and coverage data, so you can judge the quote you get.

• How much insulation do I have? The depth checker
• Attic R-value by climate zone, all eight zones
• R-value per inch by material
• The R-value chart, targets and materials in one place
• Air sealing, the heat loss an R-value cannot see
• How to inspect an attic properly