Start by finding out what is already up there
The top-up calculation is a subtraction, and the number being subtracted is the one nobody measures properly. Push a ruler down through the existing insulation until it touches the ceiling drywall, in at least half a dozen places spread across the attic. You will get a range rather than a number, and the low end of that range matters more than the average, because heat leaves fastest through the thinnest part.
Then work out what the material is worth per inch. This is not the same as what it was worth when it was installed. Loose fill settles, old fibreglass mats down, anything that has been walked on is compressed, and anything that has been wet is worth substantially less than its label and probably has a cause that needs finding. The figures in the field hint are reasonable starting points for material in ordinary condition, and if the layer is visibly compacted the honest move is to take the low end.
Multiply the depth by the R per inch and you have what you have. Subtract that from the target and you have the shortfall. Divide the shortfall by the R per inch of the new product and you have the inches to add. Everything else on this page is about converting those inches into bags.
The coverage figure on the bag is a trap unless you read the depth next to it
A bag of loose fill contains a fixed volume of settled material. The coverage chart on the bag reports that volume as an area at a series of depths, and the numbers fall as the depth rises, because it is the same volume being spread thinner or thicker. A bag stated to cover 40 square feet at 6 inches contains 20 cubic feet, and that same bag covers about 20 square feet at 12 inches and about 13 at 18.
| Depth blown | Coverage from a bag holding 20 cu ft | Bags for a 1,200 sq ft attic, before any overblow allowance |
|---|---|---|
| 4 in | 60 sq ft | 20 |
| 6 in | 40 sq ft | 30 |
| 10 in | 24 sq ft | 50 |
| 14 in | 17.1 sq ft | 70 |
Reading the top line of a coverage chart and ordering against it is how people arrive at the attic hatch with a third of the material they need. That is why this calculator takes the coverage and the depth it is quoted at as two separate inputs, and reports the coverage at the depth you are actually blowing to as its own line.
Manufacturer charts are more authoritative than this arithmetic, because they reflect the settled density their product actually reaches. Use the chart as the input and let the calculation carry it to your depth.
Over the joists, not between them
Filling to the top of the joists and stopping feels finished and is not. A 2x6 joist every 16 inches is a strip of wood at roughly R-1.25 per inch running the entire length of the ceiling, and it bridges straight from the warm drywall to the cold attic. Blowing over the joist tops covers that bridge, which is a large part of why attics are the cheapest R-value in a building and why the recommended depths are as deep as they are.
The cost is that the attic stops being a place you can walk or store things. Depth markers set before the blowing starts are what keep the finished job honest, and a raised walkboard on standoffs preserves access without crushing the fill flat along the route. If storage genuinely has to happen up there, it happens on a platform raised above the insulation, not sitting on it.
The whole-assembly effect of joists and rafters is the same physics the assembly R-value calculator works through for walls, and it is worth running the attic through it once to see what the joist fraction is doing before and after the top-up.
The order of operations, which is not negotiable
Air sealing first, then baffles at the eaves, then insulation. Sealing the top plates, wire and pipe penetrations, chases, dropped soffits, the attic hatch and any recessed light housing does more per hour of work than the insulation that follows it, and once the fill is down you cannot find any of it. Baffles keep the soffit vents open so the roof deck can still dry, which matters more once the attic runs colder. The attic ventilation calculator handles the intake and exhaust side, and the home energy audit guide covers how to find the bypasses in the first place.
One question belongs before all of it. In a house built before the late 1970s, old ceiling texture, old vermiculite loose fill and painted surfaces can contain asbestos or lead, and blowing insulation is a disturbing operation. Whether any of that is present is a testing question for someone qualified to answer it, and it is answered before the work starts rather than during it.
Questions people ask
Can I put new blown insulation over old batts?
Usually yes, and it is the normal way an attic gets upgraded. The R-values add: six inches of settled old fibreglass at about 2.5 per inch is roughly R-15, and ten inches of cellulose at 3.5 over it adds about R-35 for a total near R-50. Two conditions matter. The existing layer has to be dry and not matted flat, because wet or compressed material is worth far less than its label and covering it hides whatever is causing it. And if the old batts have a paper or foil facing pointing up, that facing must not end up sandwiched in the middle of the assembly — the usual advice is to remove or slash the facing before adding unfaced material over the top.
How many bags of insulation do I need for a 1,200 square foot attic?
It depends entirely on the depth, which is why there is no single answer. A bag holding 20 cubic feet of settled material covers 60 square feet at 4 inches and 24 square feet at 10, so the same attic takes 20 bags in the first case and 50 in the second. Work out the depth from the shortfall between what you have and what you are targeting, then read the coverage at that depth off the manufacturer chart rather than off its top line. Add an overblow allowance of around 10 percent, because coverage is never perfectly even and the eaves eat material.
Do I need the same type of insulation as what is already there?
No. Cellulose over fibreglass, fibreglass over cellulose and either over old batts are all ordinary combinations, and the R-values simply add. What matters more than the material match is that the new layer is unfaced, that no vapour retarder ends up buried in the middle of the stack, and that whatever goes down is kept out of the soffit path and off anything that gets hot. If the existing material is vermiculite, stop and get it tested before disturbing it, because some vermiculite fill contains asbestos and that changes the job from a weekend project into something with a specialist and a containment plan.
Why does my attic insulation not seem to be working?
The usual answer is air, not depth. Insulation resists conduction and does nothing to stop air moving through it, so a ceiling with open chases, unsealed top plates, a leaky hatch and recessed lights venting into the attic loses heat by convection straight past the fill. A second common answer is coverage rather than average depth: insulation thins toward the eaves and around anything that was worked on, and a bare strip costs far more than its share of the area. A third is the joists themselves, if the fill was stopped level with their tops. Blower door and infrared work will rank these for your specific house, which is what the home energy audit guide describes.
What R-value should my attic be?
That comes from your climate zone and from the version of the energy code your jurisdiction has adopted, and it is not one national figure — attic targets in cold zones are roughly double those in mild ones. It is also not the same as the requirement for walls or floors in the same house, and the building official is the one who decides what applies to your job. Look up the figure for your address and your assembly, then use this page to turn it into inches and bags. Treat any single number offered without a climate zone attached as wrong for most of the country.