Before you start, you’ll want: a tape measure, and one trip into the attic with a ruler to check how deep the existing insulation is. Five minutes. That second number is what stops you paying twice for R-value you already own.
The whole job is two numbers: how many bags, and how deep it ends up.
Everything below is working those two out — and then what they cost.
What Does the Blown-In Insulation Calculator Estimate?
A 1,000 sq ft attic taken to R-49 needs about 72 bags of cellulose or 25 bags of blown fiberglass, finishes 13.7 or 17 inches deep, and costs $1,000 to $2,800 installed. Our blown insulation calculator works that out for any attic size, material and target.
Every figure it returns is a number you can act on. The bag count is what you buy. The settled depth is what you measure the finished job against. The cost per square foot is what you compare quotes with.
| Output | What it means |
|---|---|
| Bag count | Whole bags of your chosen material, rounded up |
| Settled depth | Finished thickness once the material stops compacting — this sets your real R-value |
| Blow-to depth | What the installer aims for on the day, so it settles to the right place |
| Additional R-value | What you’re adding on top of whatever’s already there |
| Estimated total | Material, plus labor or equipment, plus removal and prep if you select them |
| Cost per sq. ft. | Total divided by area — handy for comparing quotes, misleading on small attics |
What the bag count means
There is no single bag count. It depends on the weight of the bag and how much insulation you are going for.
A 25 lb bag of cellulose covers about 14 square feet at R-49. A 27.5 lb bag of blown fiberglass covers about 40 square feet at the same R-value. Nearly three times the area, same finished performance.
The reason is weight. Blown fiberglass is about a third as dense as cellulose, so it fills more space per bag. You need far fewer bags — and each one costs more.
The number always rounds up, and it’s a minimum. Install fewer bags than the chart on the bag calls for and you get a lower R-value than you paid for. Buy a few more than the calculator says and return what you don’t open — running out mid-attic means stopping, driving to the store, and finishing a job that should have been continuous.
What the settled depth means
Here’s something most calculators skip: cellulose shrinks after it goes in — about 10%.
Not dramatically, but enough to matter. It’s why every bag prints two depth numbers: how thick it goes in, and how thick it ends up. The R-value on the label is the settled figure.
Blown fiberglass barely moves — under 1% over twenty years, which makes no practical difference.
So a cellulose attic has to be blown deeper than its target. A fiberglass attic doesn’t. Our calculator does that conversion for you.
What the cost range includes — and what it doesn’t
Included: material, installation labor or DIY equipment, old insulation removal, and attic prep — where you’ve selected them.
Not included: structural repairs, roof leaks, electrical work, permits, and mold or asbestos removal. Those get priced after someone looks, and any of them can move a job by thousands.
One honest limitation: the calculator can’t see your attic. It doesn’t know whether you’ve got a pull-down stair or a small hatch above a closet, or whether you can stand up in there. Those move the labor side of a real quote more than square footage does.
How to Use the Blown-In Insulation Calculator
Six inputs, about thirty seconds. Two of them — what’s already up there and your target R-value — do most of the work.
1. Enter your attic area
Multiply length by width for each rectangular section of attic floor and add them up. The tool also accepts length and width directly if you’d rather it did the math.
Don’t use your home’s total square footage. Rooms under a second story, a vaulted ceiling or the garage aren’t sitting under attic, and including them inflates everything downstream.
2. Tell it what’s already up there
Push a ruler down through the insulation until it hits the ceiling. Take that reading in four to six spots across the attic, then average them and enter the average.
One reading in the middle is the single most common reason an estimate comes out wrong. Attic insulation is almost never even — thinner at the edges where it was awkward to reach, thicker near the hatch where the installer was standing.
If you can see the tops of the wooden joists, you’ve got less than joist depth — and the attic is well short of any current recommendation.
Not sure what material you’re looking at? The identification guide further down covers it.
3. Choose your material
Cellulose or blown fiberglass. Both work fine over an attic floor.
Undecided? Run it both ways — the tool remembers your other inputs, so it’s a single click to compare.
4. Set your target R-value
Pick “recommend for my climate” and the calculator sets it from your state. Or enter your own number if you already know what you’re aiming for.
The Department of Energy recommends R-49 to R-60 for an uninsulated attic in the southern US, or R-38 to R-49 if you already have three or four inches up there (DOE). ENERGY STAR gives single figures for the same region — R-49 uninsulated, R-38 over existing (ENERGY STAR).
Both are recommendations rather than code. Code minimums are lower.
5. Choose DIY or professional installation
This doesn’t just scale the number — it swaps a labor line for an equipment line. The DIY comparison further down has the real figures, including where DIY stops making sense.
6. Add removal or air sealing if you need them
Tick removal if the existing material has to come out. Tick air sealing if it’s never been done. Neither is automatic, and the optional-inputs section further down explains when each one applies.
Then read your result. If you want to check the arithmetic yourself, the next section shows every formula the tool uses.
Blown-In Insulation Formulas This Calculator Uses
Six short steps, and that is the whole calculation. If you want to know how to calculate blown-in insulation by hand — or check a contractor’s numbers — this is all of it.
They run in the order you filled the form in: your area, then what you already have, then what that leaves to add, then how deep that is, then how many bags it takes. Nothing happens in here that you did not tell it.
1. Attic area
Measure each rectangular section separately and add them. Irregular attics are usually several rectangles rather than one awkward shape.
2. What you already have
Six inches of settled cellulose is about R-22. Six inches of blown fiberglass is about R-17. Same reading on the ruler, different answer — which is why the tool asks what’s up there, not just how deep.
3. What you still need
If this lands at or below zero, your attic already meets the target and topping it up returns very little. We’d rather tell you that than sell you a job.
4. How deep it needs to go
This is the finished thickness — what your R-value actually depends on, and the number to measure a completed job against.
5. How deep it has to be blown
This is the one that gets published wrong all over the internet. You divide. You don’t multiply.
Take a real coverage chart: at R-49 it lists 15.2 inches going in and 13.7 inches after settling.
- Divide: 13.7 ÷ 0.90 = 15.2 — matches the chart
- Multiply by 1.10: 15.07 — short, on every row
For blown fiberglass, settling is effectively zero, so blow-to depth equals settled depth.
6. How many bags
Coverage per bag is printed on every bag, and it falls as your target R-value rises. The tool has the common ones built in.
Estimators sometimes work from density instead: bags = (area × settled depth ÷ 12 × density) ÷ bag weight. Same answer, more steps.
Working backwards
Run backwards, this answers the question most people actually start with: what have I got already? The depth table further down does it for you.
The cost side
How Deep Should Blown-In Insulation Be?
Blown-in insulation should finish about 10.8 inches deep for R-38, or 13.7 inches for R-49, if you’re using cellulose. Blown fiberglass needs more — 13.5 inches for R-38 and 17 inches for R-49.
Cellulose gets there in less depth because it insulates better per inch. That matters if your attic is tight at the edges.
Blow-in insulation depth chart for attics, by R-value
| Target R-value | Cellulose, blow to | Cellulose, settles to | Fiberglass, installed and final |
|---|---|---|---|
| R-19 | 6.2 in | 5.6 in | 7.0 in |
| R-30 | 9.6 in | 8.6 in | 10.75 in |
| R-38 | 12.0 in | 10.8 in | 13.5 in |
| R-49 | 15.2 in | 13.7 in | 17.0 in |
| R-60 | 18.4 in | 16.5 in | 20.5 in |
Bookmark that blow-in insulation chart — it’s the fastest way to check whether a finished job hit its number.
What R-value do I already have?
Run the chart backwards. Measure what’s up there, find your depth, read across.
| Measured depth | Cellulose | Blown fiberglass | Fiberglass batt |
|---|---|---|---|
| 3 inches | R-11 | R-8 | R-10 |
| 5 inches | R-19 | R-14 | R-16 |
| 6 inches | R-22 | R-17 | R-19 |
| 8 inches | R-30 | R-22 | R-26 |
| 10 inches | R-37 | R-28 | R-32 |
| 12 inches | R-44 | R-34 | R-39 |
| 14 inches | R-52 | R-39 | R-45 |
How to tell what you’re looking at:
- Cellulose — gray, fluffy, looks like shredded newspaper
- Blown fiberglass — pale yellow, pink or white, looks like cotton candy
- Batts — rolls or blankets laid between the joists rather than blown loose
Settled depth vs. blow-to depth
These are two different numbers, and confusing them is the most common way a well-calculated attic ends up under-insulated.
The R-value printed on a bag of cellulose is the settled figure — federal labeling rules require it. Cellulose settles about 10%, so it has to go in deeper than that.
Blown fiberglass is the opposite. Its settling is negligible, and the bag prints the same number for installed and final depth.
What this means for you: if your installer blows cellulose to the target depth rather than above it, you finish about 10% short. On an R-49 attic that’s roughly R-44. Not a disaster — but it’s R-value you paid for and didn’t get, and it only shows up a year later when the depth markers read low.
How deep for R-30, R-38 and R-49 in an attic
An R-49 cellulose job buries typical ceiling joists by about eight inches. Fiberglass buries them by nearly a foot.
That’s a good thing — covering the framing is what stops heat escaping through it — but it also means you can no longer see where to step.
Which is why code requires depth markers: at least one per 300 square feet, fixed to the joists, numbers an inch high, each facing the hatch so you can read them from the opening.
If your attic doesn’t have them, your installer skipped a code requirement — and you’ve got no way to check the depth you paid for.
Materials Available in the Calculator
Cellulose and blown fiberglass both work over an attic floor, and both reach any R-value given enough depth. Cellulose needs less depth; fiberglass handles moisture better.
Our insulation calculator for attic projects runs both. Which suits you depends on your attic — and since we install both and sell neither, we’ve no reason to push you either way.
Which should you choose?
| If this is true of your attic | Choose |
|---|---|
| Tight clearance at the edges, or a low roof | Cellulose — hits R-49 in 3½ inches less depth |
| Drafty, with leaks you can’t fully seal | Cellulose — denser, resists air movement better |
| Damp, or a humid climate | Fiberglass — doesn’t absorb water or grow mold |
| Older home, ceilings you’d rather not load | Fiberglass — about a third the weight |
| You want the depth to read the same in ten years | Fiberglass — doesn’t settle |
| Pest problems in the past | Cellulose — the borate treatment deters insects and rodents |
The specs side by side
| Blown cellulose | Blown fiberglass | |
|---|---|---|
| R-value per inch | 3.2–3.8 | 2.5–3.0 |
| Weight installed | About 1.5 lb per cubic foot | About 0.5 lb per cubic foot |
| Settling | ~10% | Negligible |
| Depth for R-49 | 13.7 in settled | 17.0 in |
| Coverage per bag at R-49 | ~14 sq ft per 25 lb bag | ~40 sq ft per 27.5 lb bag |
| Bags per 1,000 sq ft at R-49 | ~72 | ~25 |
| Made from | Up to 85% recycled paper | Glass fiber, partly recycled |
| Fire treatment | Borate | Doesn’t burn |
| In damp conditions | Absorbs and releases moisture | Doesn’t absorb |
| Installed cost | $1.20–$2.80 per sq ft | $1.00–$2.80 per sq ft |
Blown cellulose
Cellulose is recycled paper — up to 85% — treated with borate.
That borate does two jobs. It’s the fire retardant, and it’s why cellulose deters insects and rodents in a way glass fiber doesn’t.
Its real advantage is depth. Cellulose reaches R-49 in under fourteen inches where fiberglass needs seventeen. In an attic with a low roof or tight edges, those three and a half inches are the difference between hitting your target and not.
It also handles drafts better, because it goes in about three times denser.
The trade-offs are real. It settles, so it has to be over-blown. It’s dustier to install. And at the same R-value it puts roughly three times as much weight on your ceiling — worth a thought on an older house.
Blown fiberglass
Blown fiberglass doesn’t settle, doesn’t absorb water, and won’t grow mold if your attic gets damp. In a humid climate that carries real weight.
The catch is depth. Seventeen inches for R-49 is a lot of attic, and in a house with a shallow roof you may not have it.
The other is shelf price. A bag of blown fiberglass costs several times what a bag of cellulose does — but it covers nearly three times the area, so the gap per square foot is far smaller than it looks in the aisle.
Not seeing batts here? They aren’t a loose-fill product and this tool doesn’t price them. Our whole-home insulation calculator does batts and rolls, along with every other area of the house.
What This Calculator Can’t See in Your Attic
The calculator prices what you type. A quote prices what somebody finds once they are up there — and on most attics the gap between the two is access, condition and the job minimum, not square footage.
Everything above this point is arithmetic, and the tool does it well. This is the other half: eight things that move a real number and that no calculator can reach from a screen. None of them are edge cases. We meet most of them most weeks.
| What the tool can’t see | What it does to your number |
|---|---|
| How you get in — a pull-down stair, or a hatch above a closet every bag has to pass through | Labor only, but it can add hours to a job the estimate treats as a morning |
| Working height — whether a crew can stand, stoop, or only crawl | Labor, and coverage. Low attics finish thin at the edges no matter who blows them |
| What the existing material actually is — you pick a type from a dropdown; only a look confirms it | Nothing, if you picked right. Everything, if it turns out to be vermiculite |
| Whether it is dry — staining, a musty smell, a slow leak above it | Turns an add-over job into a removal, plus finding and fixing the water |
| Recessed lights and fixtures — how many, and whether they are IC-rated | Prep labor per fixture, and coverage lost around each one |
| Baffles at the eaves — whether there are any | Prep cost now, or blocked soffit vents and roof condensation a season later |
| Knee walls and shallow bays — area that isn’t flat attic floor | Quoted separately, in a different material, and not in the figure on your screen |
| The job minimum — the floor underneath every quote | Decisive below about 700 sq ft, where it sets the price instead of the area |
The first two catch people hardest. Two attics of identical square footage — one with a walk-up stair, one with a hatch in a bedroom closet — are not the same job, and nothing you type into a calculator will separate them.
The one input the calculator has to take your word for
Attic area you can measure. Material and target you choose. But how much insulation is already up there is a reading, and the tool has no way to check it. Get it wrong and every figure downstream is wrong with it.
It is worth real money. Five inches of settled cellulose is R-18 you don’t have to buy again. Enter too little and you overbuy; enter too much and you finish short of the target you were aiming for.
Take a ruler, push it down to the ceiling, and read it. Then do it again in four to six places and average them.
Don’t measure once. Attic insulation is uneven almost by default — thinner at the edges where it was awkward to reach, thicker near the hatch where the installer was standing, disturbed wherever an electrician or AC tech has been since. A single reading in the middle typically overstates the average by two to four inches.
Enter the average. Not the deepest, not the one nearest the hatch.
What the house was sold as is not a measurement
Loose-fill that has been down fifteen or twenty years sits lower than it went in. That is normal, and it is not a fault — but it means the original spec stopped being true a long time ago.
An attic specified R-30 in 1998 often reads R-19 to R-22 today. The calculator cannot know that. It only sees the number you type, so type what the ruler says rather than what the paperwork claims.
Know too that the thinnest patch sets real performance for that part of the attic. An attic averaging ten inches with four-inch bald spots at the edges performs worse than its average suggests — and an average is all the tool is working from.
Whether the old insulation can stay — the call no tool can make
Tick removal and the estimate jumps by $1 to $2 per square foot. Leave it unticked and it doesn’t. The calculator will accept either answer without question, which makes this the input most worth getting right before you plan around a number.
Add over the top when the existing material is dry, uncontaminated and identifiable. This is the default, and it is much cheaper. Loose-fill goes over loose-fill, and over batts, without any problem.
Age by itself is not a reason to remove. Insulation that is dry and in place is still working at forty years old. It is just working less well than it did new, because it settled — and adding over it recovers that.
Four situations where removal is not a judgment call:
- Moisture damage — dark staining, a musty smell, visible mold. Wet insulation has lost R-value permanently and is holding that damp against your ceiling. Tick removal, and know the estimate is provisional until someone finds the water: roof leak, bathroom fan venting into the attic, blocked vents. Insulating over an active leak buries the problem and guarantees a repeat.
- Rodent or bird contamination — nesting, droppings, tunnels. A hygiene problem before it is an insulation one.
- Fire or smoke damage — the material holds the smell and the soot.
- Suspected vermiculite — gray-brown, pebbly, roughly the texture of aquarium gravel, in an attic built before about 1990. A lot of it contains asbestos. Don’t disturb it, don’t push a ruler through it, don’t add over it. Test first. If it comes back positive this stops being an insulation project and becomes a removal one at several times the price — our guide to asbestos in insulation covers how to identify it.
There is a fifth, softer case: material so flattened that adding on top means insulating over a layer contributing almost nothing. That is a judgment call, and it belongs in the attic rather than on a calculator.
Recessed lights: the label decides, and only you can read it
Older recessed lights need three inches of clearance from insulation. Newer ones marked IC-rated can be covered directly. The tool asks about neither, because there is no sensible way to guess how many you have or which kind they are.
What it does to the real number: every fixture that can’t be covered needs a box built around it — that is prep labor — and the boxed area doesn’t get insulated. On an attic with a dozen of them, that is real lost coverage and a real line on your quote. Check the label inside the fitting before you assume.
Knee walls and shallow edges: square footage that isn’t attic floor
Knee walls are vertical, and loose-fill won’t stay on them. The shallow bays at the edge of the attic can’t take full depth, and they are where drafts strip performance.
What it does to the real number: this calculator prices flat attic floor, and the area you entered is what it priced. Knee walls need batts or boards and get quoted separately. Edges need baffles and realistic expectations. Neither changes the figure on your screen — both belong on the quote.
And the one that surprises everybody: the job minimum
Below about 700 square feet, what you actually pay is set by the contractor’s minimum charge rather than by your attic. The calculator shows you the arithmetic; the minimum sits underneath it like a floor, and no input you can change will move it.
The cost section below explains why it exists, and what to do about it if your attic is small.
The Optional Inputs: Removal, Air Sealing and Who Installs It
Three toggles on the tool are optional to tick and rarely optional in practice. Together they move an attic estimate by more than the material choice does.
Removal
Tick it only in the four situations listed above — wet, contaminated, fire-damaged or suspected vermiculite. Clean removal adds $1 to $2 per square foot. Contaminated removal costs considerably more, and no calculator can price it until someone has looked.
Air sealing
Adds roughly $250 to $750. Tick it unless it has been done and you know when.
The reason is timing, not preference. Once loose-fill is down, the gaps are buried and you can’t reach them — the cheapest chance you will ever get to seal an attic is the ten minutes before the blower starts. The EPA recommends sealing before insulating for exactly that reason. What gets sealed: the tops of interior walls, gaps around pipes, wiring and ductwork, chimney chases, and the attic hatch — usually the biggest single leak up there.
Preparation and baffles
Usually inside the installation labor rather than a separate line. Tick it if there are no baffles at the eaves, if recessed lights need boxing out, or if bathroom fans vent into the attic.
Baffles are the rigid channels that keep an air path clear from the eave vents up over the insulation. Without them, loose-fill blown toward the edges blocks the vents — and that shows up as roof condensation a season later, once the attic is full and hard to get back into.
DIY or professional
On a 1,000 sq ft attic at R-38, DIY runs about $850 to $1,050 against $1,000 to $2,800 installed.
Materials cost the same either way — you buy the same bags. What DIY removes is $0.70 to $1.70 per square foot of labor. Blower rental is usually free with a minimum bag purchase, so it adds little; below that minimum, budget $100 to $200 a day.
Two things make the saving thinner than it looks. Labor is the smaller half of the bill, so you are removing well under half the cost. And the penalty for a patchy install is severe — miss 5% of the coverage and you can lose over half the R-value, with the misses landing predictably at the edges, the hatch, and under storage boards.
Our honest take: switch to DIY on a straightforward attic with good access, no removal, no contamination and a second person free for a weekend. Leave it on professional for anything involving removal, suspected contamination, awkward access, or an attic you can’t stand up in.
Attic Insulation Cost Calculator: What Your Job Should Come To
Blown-in attic insulation costs $1.00 to $2.80 per square foot installed — about $1,000 to $2,800 for a typical 1,000 sq ft attic. Cellulose sits marginally higher than fiberglass.
Our attic insulation cost calculator builds your number from those rates and adjusts for what you enter. For the wider pricing picture, see our guides to attic insulation cost and blown-in insulation cost. This section is about what the tool does.
What the cost estimate includes
- Material — bag count × price per bag
- Labor — $0.70 to $1.70 per square foot
- Removal — $1 to $2 per square foot when selected
- Preparation — air sealing and baffles, typically $250 to $750
Switch to DIY and the blow in insulation calculator swaps the labor line for equipment: roughly $0.35 to $1.50 per square foot in materials, plus blower rental.
Estimated cost by attic size
| Attic area | Typical installed total | Effective cost per sq ft |
|---|---|---|
| 500 sq ft | $1,500 – $2,000 | $3.00 – $4.00 |
| 800 sq ft | $1,500 – $2,240 | $1.88 – $2.80 |
| 1,000 sq ft | $1,000 – $2,800 | $1.00 – $2.80 |
| 1,200 sq ft | $1,200 – $3,360 | $1.00 – $2.80 |
| 1,500 sq ft | $1,500 – $4,200 | $1.00 – $2.80 |
| 2,000 sq ft | $2,000 – $5,600 | $1.00 – $2.80 |
| 2,500 sq ft | $2,500 – $7,000 | $1.00 – $2.80 |
Look hard at the first two rows.
A 500 sq ft attic doesn’t cost half what a 1,000 sq ft attic costs. It costs nearly the same.
Most insulation contractors have a minimum project charge of $1,500 to $2,000, because travel, setup, equipment and disposal cost the same whether your attic is 400 square feet or 1,400.
That’s not gouging. It’s the reason small jobs get quoted at what looks like triple the per-foot rate — and it’s the biggest single source of sticker shock on a quote.
Practical advice: if your attic is small, bundle the job with air sealing or a removal you needed anyway. You’re paying the minimum either way, so get more for it.
What the estimate leaves out — and what a quote adds
A quote prices what a calculator can’t see. Attic access. Working height. Whether the existing material is contaminated. Whether the edges need baffles. Whether light fixtures need boxing out. Disposal.
Contamination is where estimates and quotes diverge hardest. Clean removal is $1 to $2 per square foot. Mold or asbestos work runs several times that, with a decontamination minimum on top.
Attic floor vs. attic roof deck
This calculator prices attic floors — insulation laid over the ceiling, measured per square foot.
Insulating the roof deck instead is a different job: it brings the attic inside your heated and cooled space, usually with spray foam, and it’s priced differently. Loose-fill doesn’t do that job.
If that’s your project, this isn’t the right tool — our spray foam insulation calculator is, and it prices by the board foot rather than the square foot.
Blown-In Insulation Calculator Examples
Four worked scenarios. If you want to see what the blown in insulation calculator for attic work looks like end to end, this is it.
500 sq ft attic, blown fiberglass to R-49
Inputs: 500 sq ft · nothing existing · blown fiberglass · R-49 · professional
- R-value needed: R-49
- Depth: 17 inches
- Bags: 500 ÷ 40 = 13 bags
- Calculated at $1.00–$2.80 per sq ft: $500–$1,400
Estimated total: $1,500 – $2,000
The calculated figure lands below the minimum charge, so the minimum applies. That works out around $3.00 to $4.00 per square foot — triple the headline rate, for reasons that have nothing to do with this attic.
1,000 sq ft attic, adding cellulose over existing
Inputs: 1,000 sq ft · 5 inches existing cellulose · adding cellulose · R-49 · professional
- What you already have: 5 inches ≈ R-18.5
- Still needed: 49 − 18.5 = R-30.5
- Depth to add: 8.2 inches settled, blown to 9.2 inches
- Finished depth: 13.2 inches
- Bags: 39
Estimated total: $1,000 – $2,400
Those five existing inches take roughly $600 to $1,000 off this job. It’s the biggest saving available on any attic — and you only get it if you measure and enter it.
1,500 sq ft attic, full replacement
Inputs: 1,500 sq ft · 7 inches of contaminated insulation · remove and replace with cellulose · R-49 · professional
- Removal: $1,500 – $3,000
- Depth: 13.2 inches settled, blown to 14.7 inches
- Bags: 107
- Install: $1,500 – $4,200
Estimated total: $3,000 – $7,200
Removal roughly doubles this job. Which is exactly why “is my old insulation genuinely contaminated, or just old?” deserves a careful answer before anyone quotes it.
Same attic, DIY versus professional
Inputs: 1,000 sq ft · nothing existing · cellulose · R-38
- DIY: 53 bags ≈ $850 · blower rental often free with a minimum bag purchase · 2 people, 1–3 days
- Professional: bags, blower and labor all included
DIY $850 – $1,050 vs Pro $1,000 – $2,800
On paper DIY saves up to $1,750. In practice you’re only saving the labor line — the bags cost the same either way — and you’re buying it with one to three days of two people’s time.
Frequently Asked Questions
How much blown-in insulation do I need for 1,000 square feet?
About 53 bags of cellulose or 19 bags of blown fiberglass to reach R-38. At R-49 it’s roughly 72 bags of cellulose or 25 of fiberglass. Counts vary a little between products because bag weights differ.
How deep should blown-in insulation be in an attic?
For R-38, about 10.8 inches of cellulose or 13.5 inches of blown fiberglass. For R-49, about 13.7 inches of cellulose or 17 inches of fiberglass. Cellulose needs less depth because it insulates better per inch.
How many inches of blown-in insulation do I need for R-30?
About 8.6 inches of settled cellulose — blown to roughly 9.6 inches — or 10.75 inches of blown fiberglass.
How much does a 25 lb bag of blown insulation cover?
About 26 square feet at R-30, or 14 square feet at R-49. The deeper you go, the less ground each bag covers. Always check the chart printed on the bag you’re buying.
What R-value is 12 inches of blown insulation?
Roughly R-44 for cellulose or R-34 for blown fiberglass. The depth table above covers other depths.
Is it better to remove old attic insulation before adding new?
Usually not. If it’s dry, clean and identifiable, adding over the top is cheaper and works fine — and what’s already there counts toward your target. Remove it only if it’s wet, moldy, rodent-contaminated, fire-damaged or possibly vermiculite. Age alone isn’t a reason.
Does blown-in insulation settle?
Cellulose does, by about 10%, which is why it has to be blown deeper than the target. Blown fiberglass settles negligibly.
Can you put too much insulation in an attic?
You can reach a point of poor return, and you can cause problems by blocking vents or covering light fixtures that need clearance. Past about R-60, the extra saving per inch gets very small in most climates.
Do I need to air seal before insulating?
The EPA recommends it, and the reason is practical as much as thermal: once loose-fill is down, the gaps are buried and you can’t reach them. It’s the cheapest it’ll ever be in the ten minutes before the blower starts.
How long does blown-in insulation last?
Indefinitely, if it stays dry. Cellulose settles in the first year or two and then holds. Blown fiberglass barely changes. What ends an attic’s working life is water, pests or someone disturbing it — not age.
Why is my quote for a small attic so expensive per square foot?
Because most contractors have a minimum charge of $1,500 to $2,000. Travel, setup and disposal cost the same on a small attic as a large one. Under about 700 square feet, the minimum sets the price rather than the area.
Something has been nesting in my insulation — can I add over it?
No. That’s a hygiene problem before it’s an insulation one. The material needs removing, the area cleaning, and the entry points sealing — or you’ll be doing it all again.
We added insulation and our bills went up. What went wrong?
Usually one of three things. The attic was never air sealed, so new insulation is sitting over an open ceiling. The eave vents got blocked. Or the depth is short of what was quoted. Read the depth markers first — that one’s actually checkable.
How do I check the finished job matches my estimate?
Read the depth markers against the settled depth this calculator gave you. Code requires markers at one per 300 square feet, so they should be there. Short of your number? You didn’t get what you paid for. No markers at all? You have no way to know.
How accurate is a blown-in insulation calculator?
Accurate on bag count, which is just arithmetic. Approximate on cost, which depends on access and attic conditions no tool can see. Treat the bag count as reliable and the price as a solid planning range.
Related Insulation Calculators and Guides
- Insulation Calculator — compare materials and costs across any area of the home, including walls, floors and crawl spaces
- Spray Foam Insulation Calculator — foaming the roof deck instead of the attic floor? Board feet, thickness and crew hours
- Insulation R-Value Chart — R-values for every common material, in both directions
- Attic Insulation Cost — what a full attic job runs, beyond the calculator estimate
- Blown-In Insulation Cost — pricing broken down by material and job type
- Does Your Insulation Have Asbestos? — identifying vermiculite before you disturb it