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Attic Insulation Calculator — How Many Bags for R-49?

Attic Insulation Calculator — Bags to Reach Your R-Value

Enter your attic and the R-value you are aiming for. It accounts for the insulation already up there and tells you the depth to add and the bags to buy.

By The OneTripDIY Team · Updated

Measure the space

L-shaped room or more than one area? Add a section for each rectangle and we'll total them.

in.

Measure with a ruler pushed down to the ceiling drywall. Zero if the attic is bare.

R

The US Department of Energy recommends R-49 to R-60 for most attics, R-30 to R-49 in the warmest zones.

Bags to buy — bags
Depth to add — in.
You already have — R-value
Still to add — R-value
Total volume — cu ft

Estimates only. Check against your supplier's stated coverage before you buy.

Take this list to the store

We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.

We send the list you asked for. Anything else only if you tick the box, and you can unsubscribe in one click. What we do with it.

Floor, wall and perimeter are three different numbersInsulation is sold to cover an area at a depth.WALL AREAperimeter × heightLengthCeiling heightWidthOne room, three different numbersThis one needs wall area.Not floor or perimeter.

How to work it out

  1. Measure the attic floor

    Length × width of the insulated area — the ceiling below, not the roof above. Split it into sections if the attic is an awkward shape.

  2. Measure what is already there

    Push a ruler down through the existing insulation until it touches the ceiling drywall. Do it in three or four places and take the lowest reading — old insulation settles unevenly and the thin spots are what cost you.

  3. Pick your target

    The Department of Energy recommends R-49 to R-60 for attics in most of the US, dropping to R-30 to R-49 in the warmest southern zones. Higher is not wasted, but the returns flatten above R-60.

  4. Rent the blower with the bags

    Most home centers lend the machine free when you buy a minimum number of bags — often around ten. Two people makes it far easier: one feeding the hopper, one on the hose in the attic.

Cellulose needed per 1,000 sq ft

Blown cellulose at R-3.5 per inch, over a bare attic floor.

TargetDepth neededBags per 1,000 sq ftPer 1,500 sq ft
R-308.6 in.45 bags67 bags
R-3810.9 in.57 bags85 bags
R-4914.0 in.73 bags110 bags
R-6017.1 in.90 bags134 bags

If you already have insulation, you only need the difference — enter what is there and the calculator subtracts it.

R-value per inch by material

MaterialR per inchNotes
Blown cellulose3.2 – 3.8Denser, fills gaps well, made from recycled paper.
Blown fiberglass2.2 – 2.7Lighter, does not settle as much, more bags for the same R.
Fiberglass batts3.1 – 3.4No machine needed, but gaps around wiring cost real performance.
Closed-cell spray foam6.0 – 7.0Highest per inch and an air seal, but a professional job.

Settled cellulose loses a little depth in the first year. Blow it about 10% deeper than the target to allow for it.

How many bags of insulation for R-49?

Reaching R-49 from bare joists needs about 14 inches of blown cellulose. Over 1,000 sq ft that is roughly 1,167 cubic feet, or 73 bags at 16 cu ft each.

If you already have 4 inches of old fiberglass up there, that is worth about R-13 — so you only need another R-36, or about 10 inches, which cuts it to roughly 53 bags.

Air seal before you insulate

This matters more than the last few inches of depth. Before blowing anything, seal the holes air escapes through: around recessed lights, plumbing stacks, the attic hatch, wiring penetrations and the top plates of interior walls.

Insulation slows heat moving through a surface. It does almost nothing about air pouring through a gap. An attic with R-60 and an unsealed hatch still leaks heat all winter.

Do not bury these things

  • Soffit vents — fit baffles at the eaves so insulation cannot block the airflow. Blocked soffits cause ice dams and damp sheathing.
  • Recessed lights — only fixtures marked IC-rated may be covered. Anything else needs a clearance box around it.
  • Chimneys and flues — keep the clearance the appliance calls for, usually 2 to 3 inches, using metal flashing.
  • The attic hatch — insulate and weatherstrip the hatch itself rather than piling insulation on top of it.

Cellulose or fiberglass?

Cellulose gives more R per inch, packs into awkward gaps better, and costs less per R. It is heavier and it settles, so blow it deeper than target. Fiberglass is lighter, settles less, and does not absorb water — but you need far more of it for the same R-value, which is why the bag counts differ so much above.

For an attic retrofit over existing insulation, cellulose is usually the better value. Either is a large improvement over nothing.

What R-value do you actually need where you live

The Department of Energy splits the US into climate zones and recommends attic values by zone.

  • Zones 1–2 (southern Florida, south Texas, Hawaii) — R-30 to R-49
  • Zone 3 (Georgia, Arizona, southern California) — R-30 to R-60
  • Zones 4–5 (most of the middle and north-east) — R-49 to R-60
  • Zones 6–8 (Minnesota, Maine, Alaska) — R-49 to R-60

Notice how little the top of the range moves. Above R-60 the extra inches buy very little anywhere, which is why almost every zone tops out there.

Walls are different and far harder to retrofit — an existing 2×4 wall is limited to about R-15 by the cavity depth. Attics give you the most improvement for the least money, which is why they come first.

Insulating a crawl space or basement

Attic insulation is straightforward. Below the floor is where people go wrong.

The modern approach to a crawl space is to seal and insulate the perimeter walls rather than the floor above, and to lay a heavy vapour barrier across the ground. That brings the crawl space inside the building envelope, keeps the pipes in it from freezing, and stops damp air being drawn up into the house.

The old approach — batts pushed up between the floor joists — tends to fail. The batts sag away from the subfloor over a few years, leaving an air gap that does nothing, and any moisture in the crawl space gets trapped in them.

If the crawl space is damp, fix the water before adding any insulation at all.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

  1. existing R = existing depth × R per inch of that material Old blown fiberglass is about R-3.2 per inch; cellulose about R-3.5.
  2. R still needed = target − existing R You only pay for the difference.
  3. depth to add = R needed ÷ R per inch of the new material Cellulose 3.5 per inch, blown fiberglass 2.5.
  4. volume = area × depth ÷ 12 Inches to feet, giving cubic feet.
  5. ÷ coverage per bag, rounded up About 16 cu ft for a cellulose bag, 40 for blown fiberglass.

Common questions

How many inches of insulation do I need for R-49?

About 14 inches of blown cellulose, or 20 inches of blown fiberglass. Fiberglass gives less R per inch, so it needs considerably more depth for the same performance.

Can I blow new insulation over old?

Yes, and it is the normal way to do it. Blown insulation goes straight over existing batts or old loose-fill. The one thing not to do is lay faced batts over existing insulation — the paper facing traps moisture inside the layer.

How do I measure what is already up there?

Push a ruler down through it until it touches the ceiling drywall. Check three or four spots and use the lowest figure — insulation settles unevenly and the thin patches are where the heat actually leaves.

Do I need to rent a blower?

For loose-fill, yes. Most home centers lend the machine free when you buy around ten bags or more. Budget half a day and two people — one feeding the hopper on the ground, one on the hose in the attic.

Should I air seal first?

Always. Sealing the gaps around lights, pipes, wiring and the attic hatch does more for your heating bill than the last four inches of depth. Insulation slows heat through a surface; it does nothing about air rushing through a hole.

Is more than R-60 worth it?

Rarely. The savings flatten out — going from R-19 to R-49 makes a large difference, R-49 to R-60 a modest one, and beyond R-60 you are usually better spending the money on air sealing or windows.

Will insulation make my attic damp?

Not if the ventilation is intact. The common mistake is burying the soffit vents at the eaves, which stops the airflow and causes damp sheathing and ice dams. Fit baffles before you blow.

How much does cellulose settle?

Roughly 10 to 20% over the first year. Blow it about 10% deeper than the target depth so it settles to where you want it rather than below.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

  • US Department of Energy insulation guidance — recommended attic R-values by climate zone
  • ASTM C739 and C764 — loose-fill cellulose and fiberglass thermal values and settled density
  • Manufacturer coverage charts — bags per 1,000 sq ft at each R-value
  • International Residential Code R806 — attic ventilation and required soffit clearance

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.