An attic top-up is a subtraction problem
Almost nobody insulates a bare attic. The usual job is a top-up: some decades-old layer is already lying on the joists, and the question is how much new material to blow over it. That changes the arithmetic in a way the bag display at the store never mentions. Insulation R-values add in series, so what you are buying is not your target R-value — it is the difference, ΔR = target R minus existing R. The manufacturers themselves frame retrofits this way: the Owens Corning AttiCat install card literally carries Existing, Installed, and Total R-value lines that are meant to sum.
Get the subtraction right and everything downstream is mechanical: the coverage chart printed on the bag converts a ΔR into bags per 1,000 ft² and a depth to blow to. Get it wrong — by guessing at the existing layer, or by shopping for the full target as if the attic were bare — and you either buy a third more material than the job needs or stop short of the code target you were aiming at. So the job order is: read what you own, pick the target, price the shortfall, and prepare the attic before the first bag goes in the hopper.
Reading the insulation you already own
The existing layer is identified by eye and priced by ruler. Yellow, pink, or white fluffy blankets are fiberglass batts, worth about R-3.1 per inch. Loose gray-white fluff is blown fiberglass, about R-2.5 per inch. Gray-brown shredded paper is cellulose, about R-3.7 per inch. These are representative figures rather than product specs — the original bag is long gone — so measure the settled depth at several spots with a ruler, average them, and multiply.
Notice what that does to depth as a measure: ten inches of existing insulation is about R-25 if it is loose fiberglass and about R-37 if it is cellulose. Same ruler reading, a twelve-point swing in R — which is why "how deep is it?" alone cannot answer "how much do I need?". Two caveats before you trust the number: compressed, wet, or wind-washed insulation performs worse than the per-inch figure implies, and if the old layer is moldy, rodent-infested, or vermiculite (which may contain asbestos), that is a problem to resolve before anything gets blown on top of it.
Picking the target: a code number, not a guess
The target side of the subtraction comes from the energy code. The 2021 IECC sets prescriptive attic minimums by climate zone in Table R402.1.3, running from R-30 in the warmest zone up to R-60 across zones 4 through 8 — most of the continental US. Which zone you are in, and what the compound code options mean, is exactly what our insulation R-value calculator looks up; there is no need to memorize the table to run the top-up math.
With both sides in hand, subtract. An attic with four inches of loose fiberglass holds about 4 × 2.5 = R-10; against an R-38 target the shortfall is ΔR = 28, and that — not 38 — is the number the coverage chart gets read at. The subtraction also has a happy failure mode: if the existing layer already meets the target, the answer is zero bags. Insulation past the code target buys steeply diminishing returns, and a calculator that respects the chart will tell you you are done rather than sell you more.
How a coverage chart turns ΔR into bags
Every bag of loose-fill insulation carries a federally mandated coverage chart: for each target R-value it lists the minimum bags per 1,000 ft² of attic and the minimum thickness, and both numbers are binding — R-value depends on the mass of material and on the depth of the pile, so you must meet the bag count and reach the depth. The rows differ enormously by product. Greenfiber INS541LD cellulose (19.05 lb bags) needs 61.1 bags per 1,000 ft² to reach R-38 from bare framing; Owens Corning AttiCat expanding fiberglass (27.5 lb bags) needs 19. A bag count is meaningless without the product name attached.
The arithmetic from there is one line: bags = area × chart rate ÷ 1,000, rounded up to whole bags. A 1,000 ft² attic going from bare joists to R-38 in Greenfiber cellulose is 61.1 bags, so you buy 62 — about 1,181 lb of material, which is worth knowing before you promise your car it can carry the load. For a ΔR that falls between published rows, the chart is read by linear interpolation. Our blown-in insulation calculator encodes both verified charts, runs the existing-R subtraction, and returns whole bags plus the depths to blow to.
Settled versus installed depth: why cellulose has two numbers
Cellulose leaves the hose fluffed and then settles over the following months, so Greenfiber’s chart publishes two depths per row: an initial installed thickness to blow to and a settled thickness about 10% lower where the stated R is actually delivered. At R-38 those are 11.97 inches installed settling to 10.77 inches. Blow to the installed number; judge the job later against the settled one. AttiCat fiberglass behaves differently — Owens Corning states settling is negligible, so its installed and settled thicknesses are the same (13.5 inches at R-38).
Depth is also the practical way to supervise the work, whether you rent the blower or hire it out. Stapling a few cardboard depth markers to the rafters before blowing costs nothing and makes the second binding minimum checkable at a glance. Be ready for how deep deep is: R-60 in cellulose means blowing to about 18.4 inches so it can settle to 16.5 — at that point the joists have vanished, which is why walking an insulated attic without locating the framing first is a ceiling repair waiting to happen.
The work that has to happen before the first bag
Air sealing comes first, always. Blown insulation slows heat conducting through the ceiling but does little against air leaking through top plates, wire and pipe penetrations, recessed light fixtures, and the attic hatch — and once those leaks are buried under sixteen inches of fluff they are far harder to find. Seal them with foam and caulk, put covers with 3-inch clearance over recessed fixtures that are not IC-rated, and dam around flues and chimneys before blowing.
The second prerequisite is ventilation. If the roof breathes through soffit vents, staple baffles (vent chutes) into each rafter bay before blowing, so the new pile cannot slump into the eaves and block the intake path — a starved soffit chokes the ridge or gable exhaust, invites winter moisture, and can void shingle warranties. Whether your vent area is adequate in the first place is its own calculation; our attic ventilation calculator checks the net free area against the IRC rules. Insulation and ventilation are a system: the top-up only performs if the attic above it stays dry.