EstimatorKitInsulation bought to an R-value, not a rate
For a wall or an attic and a target R, this returns batts by the piece or bags by the count, with the depth, the coverage per bag and what is already up there all shown. Insulation is sold against a performance number rather than a thickness, so coverage per bag falls as the target R rises — the same bag covers roughly twice the area at R-30 that it does at R-60 — which is why the chart printed on the bag is the authority here and a square-foot rate is not.
- Zones 1-8
- Batts and blown-in
- Reads your bag's chart
Feet unless you say otherwise, and 12' 6 3/8" reads the way it is written. The plates come off the height automatically: a 1 1/2 in bottom plate and a doubled 1 1/2 in top plate, taken out of the floor-to-ceiling height to leave the cavity.
The batt actually made to fill a 2×6, and worth two more R than an R-19 squeezed into one. Made 5.5 in thick — the manufactured thickness of an R-21 high-density fiberglass batt.
Windows and doors come out of the insulated area but not out of the batt count: the strips over a header and under a sill are cut from the batts the openings displaced.
Add the price you were quoted per batt
Batts are racked by the bag and priced by the square foot, so the shelf label and the piece price are rarely the same number. Whichever you have, this multiplies yours.
R-21 batts for 30 bays at 16 in on center
30 batts
- With 5% spare
- 32
- Piece size
- 15 in × 93 in
- Cavity to fill
- 212 sq ft
- Framed area
- 320 sq ft
30 from the bay count, 32 with the 5% on top. The spare is not breakage — batts do not break — it is the short strips over headers and under sills, and the bay somebody discovers behind the tub surround.
How it fits the cavity
An R-21 batt is 5.5 in and this 2×6 cavity is 5.5 in — an exact fit, which in this lineup only R-13 and R-15 in a 2×4 and R-21 in a 2×6 actually manage.
The framing takes 43.6 sq ft of the 320 sq ft here — 13.62% that no batt can reach, and that is the members this layout puts in, before headers, corners and blocking. At 1.25 R per inch a 5.5 in member is about R-6.88 standing next to your R-21.
R-21 is R-9 short of the R-30 you set. There is no deeper batt for a 5.5 in cavity — which is why Zone 5 and marine 4 lists its wall as R-30, or R-20 + R-5ci, with the second figure sitting outside the framing as continuous board rather than inside it. Whether an alternative meets code is your building department’s call, not this page’s.
Bays and pieces
30 full bays of 14 1/2" clear, and the run divides evenly, so nothing gets cut to width. A 93 in piece covers a 91.5 in bay in one length, with 1 1/2" to trim.
the 15 in batt made for framing at 16 in on center. It is wider than the 14 1/2" of clear bay on purpose — the extra holds it in by friction, and a batt trimmed narrow enough to drop in loose is a batt that will sag off the top plate by the time the board goes on.
What the code asks for, by climate zone
| Zone | Ceiling | Wood-frame wall | Floor |
|---|---|---|---|
| Zone 1 | R-30 | R-13, or R-0 + R-10ci | R-13 |
| Zone 2 | R-49 | R-13, or R-0 + R-10ci | R-13 |
| Zone 3 | R-49 | R-20, or R-13 + R-5ci | R-19 |
| Zone 4 (except marine) | R-60 | R-30, or R-20 + R-5ci | R-19 |
| Zone 5 and marine 4 | R-60 | R-30, or R-20 + R-5ci | R-30 |
| Zone 6 | R-60 | R-30, or R-20 + R-5ci | R-30 |
| Zones 7 and 8 | R-60 | R-30, or R-20 + R-5ci | R-38 |
2021 IECC, Section R402.1, prescriptive path. “ci” is continuous insulation outside the framing, where it covers the studs instead of sitting between them — the reason the wall column has alternatives at all is that no batt made will put R-30 inside a 2×6. This is reference: the edition your jurisdiction has adopted, and the official who reads the plan, decide.
R-value, coverage and the bag chart
How many bags of blown-in insulation do I need for my attic?
Divide the attic's square footage by the coverage the bag's chart gives for the R-value you are adding — not the R you want to end up with. A 988 sq ft attic that already holds 6 in of old fiberglass is at about R-15, so a jump to R-60 is an R-45 layer, and reading the R-60 row instead buys a third more material than the attic can hold. Coverage figures vary enough between products that the derived number here is a starting point and the printed chart wins; whatever you type, including the price the yard quoted you, is used once in this tab and never sent anywhere.
Why does one bag cover less area at R-49 than it does at R-30?
Because a bag holds a fixed weight of material and a higher R needs more of that weight under every square foot. The thickness rises with the R and the density stays where the product settles, so the pounds per square foot rise in step and the area one bag can cover falls in inverse proportion: double the R and you halve the coverage. That is why a square-foot rate quoted with no R-value attached to it is not an answer, and it is why the federal R-value Rule makes manufacturers print a coverage chart — R-value down the side, minimum thickness, maximum coverage and pounds per square foot across — on every bag of loose fill sold.
Does squashing a batt into a shallower cavity lose R-value?
Yes, and the standard lineup makes you do it. An R-19 batt is manufactured 6 1/4 in thick and sold for a 2×6 wall, which is 5 1/2 in — pushed into that cavity it delivers about R-18, and Johns Manville's own fact sheet gives R-17 to R-18. The compression raises the density, which buys back a little R per inch, but never all of what the lost thickness cost. R-25 into a 2×8, R-30 into a 2×10 and R-38 into a 2×12 are all compression fits for the same reason. The only batts that fit their cavity exactly are R-13 and R-15 in a 2×4 and R-21 in a 2×6, which is the honest argument for R-21 over R-19.
Faced or unfaced — which one do I buy?
That is a vapor-retarder decision and not an insulation one: kraft facing changes nothing about the R-value. The facing is a Class II vapor retarder, and IRC R702.7 requires a Class I or II retarder on the interior side of frame walls in climate zones 5, 6, 7, 8 and marine 4 — so in a cold-climate wall the kraft goes toward the heated side and nowhere else. Two rules follow. Never install two retarders in one assembly, which is what kraft plus a poly sheet does: the cavity between them has no way to dry. And never lay kraft-faced batts face-up over insulation already in an attic, because the facing then sits in the middle of the assembly with cold material above it; unfaced is what that job takes.
How deep does attic insulation have to be for R-49?
It depends entirely on what you blow, which is why depth is an output here and never an input. At the middle of the range the Department of Energy publishes, cellulose at R-3.5 an inch reaches R-49 in about 14 in and loose fiberglass at R-2.5 an inch needs closer to 19 1/2 in of the same R. Neither number is a target to eyeball from the hatch: IECC R303.1.1.1 asks for one depth marker per 300 sq ft of attic, marked with the minimum initial installed thickness in numerals at least an inch high and facing the access, and the reason that rule exists is that a blower operator working alone cannot tell 11 in from 14 in by looking at it.
There is grey-brown gravel in my attic that looks like mica. What is it?
Almost certainly vermiculite, and the EPA's guidance is to assume it contains asbestos and leave it alone. Most vermiculite attic insulation sold in the US before 1990 came from a mine at Libby, Montana that was contaminated with asbestos, and the fibers are released by disturbance — walking on it, moving it aside to run a wire, or blowing anything over it. It is not a weekend job and it is not a topping-up job. Get it tested, and if it is positive the next call is a licensed abatement contractor rather than a rental blower.
Why insulation is not sold by the square foot
Every other material on this site is bought by volume or by area. Insulation is bought by performance: the number on the package is R, thermal resistance, and everything physical about the purchase — how thick the layer is, how much a bag covers, how many bags come off the pallet — falls out of that one figure afterward. For loose fill the relationship is arithmetic and unforgiving. A bag holds a fixed weight. The R you are aiming at sets the depth, the depth and the settled density set the pounds under every square foot, and the area one bag can cover is the bag weight divided by that. Double the R and you halve the coverage. This is exactly why the federal R-value Rule requires a coverage chart on the bag rather than a single number: R-value down the side, and minimum thickness, maximum coverage, pounds per square foot and bags per 1,000 square feet across. A calculator that multiplies your attic by one fixed square feet per bag has thrown that chart away, and a calculator that answers in cubic feet has answered a question nobody asked at the trade counter. The chart is the thing to type in here, and the figure this page derives from published density and R-per-inch ranges is what you use until you are standing in front of the bag.
Batts do not behave that way at all, which is why they are counted separately. A batt is a manufactured piece sized to a framing layout — 15 in wide for 16 in on center, 23 in for 24 in, both a little wider than the clear bay so friction holds them up — and the useful count is pieces, not area. What surprises people is the depth. R-13 and R-15 are made at 3 1/2 in and fill a 2×4 exactly; R-21 is made at 5 1/2 in and fills a 2×6 exactly; every other batt in the lineup is thicker than the cavity it is sold for. The famous one is R-19, manufactured at 6 1/4 in for a wall that is 5 1/2 in deep, where compressing it to fit leaves about R-18 — R-17 to R-18 on Johns Manville’s own fact sheet. That is not a defect in the batt, it is what happens whenever you trade thickness for density, and it applies just as much to a batt stuffed behind a wire or squashed by a pipe. The gap in the other direction is worse: a batt thinner than its cavity adds nothing for the space it leaves, and air loops through the void against the sheathing. None of this touches the studs themselves, which run about R-1.25 an inch by the working figure this page uses and stay cold whatever goes beside them — the reason the 2021 code offers R-20 plus R-5 continuous as an alternative to a cavity value no batt can reach. Get the cavity right before the drywall calculator closes it up, because everything here is invisible and unfixable ten minutes later.
Attics have their own set of traps, and topping up is the biggest. Almost no attic is bare, so the layer you are buying is the difference between the R already lying there and the R you want — read the chart at the total and you overbuy by a third on a typical job. Depth then has to be verified rather than eyeballed, which is why one marker per 300 square feet is a code requirement and not a nicety, and why cellulose bags print an initial installed thickness alongside the settled one. Watch two more things while the blower is running: the eaves, where loose fill pushed into an open soffit blocks the ventilation the roof depends on unless a baffle holds the air space open, and the ceiling itself, since a deep cellulose layer can put over 1.5 lb on every square foot of half-inch board. Last, the facing. Kraft versus unfaced changes the vapor retarder and not the R-value, one assembly gets one retarder and it faces the heated side, and a floor over a crawl space or an unheated garage is a thermal boundary with the same rules — the batts that go between those joists are counted in the same panel as a wall, and what lands on top of them is the flooring calculator.