Sizing the systems
Attic insulation cost
Taking a 1,200 ft² attic from R-11 to R-60 runs about $3,150 with the air sealing in it — $2.63 a square foot — and pays back in about nineteen years, or twelve if you skip the sealing and only count the fill. But R-13 to R-19 pays back in three years and R-49 to R-60 takes thirty-five, and knowing which rung you are buying is the whole thing.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-17 · Nothing you type leaves your browser
What is up there now
R-value is not a product, it is a depth — so the only two numbers that matter are what you have and what you are going to.
The floor of the attic, or the wall or floor being insulated.
Where most of the heat goes and where insulation is cheapest per point of R, because depth is free up there.
Measure the depth with a ruler. Patchy counts as the thin part, not the average.
The default attic fill. Blown in through a hose, cheap by the inch, and it does nothing at all to stop air moving through it.
Zone 4 wants R-60 here.
Leave blank to use what the Department of Energy asks for.
It is a different trade and a different line. Once the fill is down, nothing underneath gets sealed again.
Removal is needed when it is wet, contaminated, or in the way of sealing.
It changes what a saved BTU is worth.
Used only to pick a regional labour rate. It stays in your browser.
Installed
$3,154
$2.63 per ft² · 19.6 in of blown fibreglass · R-11 to R-60
Pays for itself in about 19 years
$162 a year on heating, over 1,200 ft² and 4,500 degree days. That is a far shorter payback than new windows and a far longer one than sealing the ducts.
What each rung is worth
Every inch earns less than the one under it. The first rung carries the whole installation, which is why a small top-up is poor value and a big one is not.
| Rung | Costs | Saves a year | Payback |
|---|---|---|---|
| R-11 → R-13 | $773 | $31 | 25 yr |
| R-13 → R-19 | $158 | $53 | 3 yr |
| R-19 → R-30 | $290 | $42 | 7 yr |
| R-30 → R-38 | $211 | $15 | 14 yr |
| R-38 → R-49 | $290 | $13 | 22 yr |
| R-49 → R-60 | $290 | $8 | 35 yr |
Shopping list
- Blown fibreglass
19.6 in on top of what is there, to reach R-60 — 2.5 points of R to the inch1,200 ft² - Air sealing
Every penetration through the attic floor, before anything goes on top of it1,200 ft²
Sealing, priced separately
$1,140
To seal it
before anything goes on
$40
Saved a year
a cautious estimate
29 yr
Its own payback
treat as a ceiling
Infiltration is measured with a blower door, not with a conduction sum, so this is the softest number on the page and it is deliberately cautious. What is not in doubt is the order: once the fill is down, nothing underneath gets sealed again for a generation.
What it costs
National average. Material, installation, air sealing and removal where selected. Rebates and utility programmes are not included — they change yearly and they are local.
$2.63 per ft². Before spending more on the envelope, check what the house actually needs — insulating properly often means the next boiler or heat pump can be a size smaller, which is worth more than the fuel.
Worth knowing
- R-11 now, R-60 after: 19.6 in of blown fibreglass on top. The Department of Energy asks for R-60 on attic floor in zone 4.
- The value collapses with depth, and this is the part nobody puts in a quote. Going to R-13 pays back in 25 years. The last rung, R-49 to R-60, takes 35. Same attic, same hose, same crew — each extra inch saves less than the one under it because you are dividing an ever-smaller number.
- On these numbers air sealing pays back in about 29 years and the whole job in 19. Treat the sealing figure as a ceiling rather than a floor: infiltration is measured with a blower door, not with a conduction sum, and the 20 per cent recovery assumed here is deliberately cautious. What is not in doubt is the order — once twenty inches of fibre are down, nothing underneath gets sealed again for a generation.
On this page — 7 sections
Each inch is worth less than the one under it
The short version
- R-13 to R-19 pays back in three years. R-49 to R-60 takes thirty-five. Same attic, same crew.
- But the whole job barely moves. Ten years to R-30, twelve to R-60 — the setup is paid either way.
- Seal first. Insulation does nothing about air, and after the fill is down nothing underneath gets sealed again.
- Attic insulation is the good one. Ten to twelve years for the fill, against thirty-nine for new windows.
R-value is not a product. It is a depth of a particular material, and the arithmetic behind it has a property nobody mentions when they are selling depth: what each extra inch saves collapses, because you are subtracting from an ever-smaller number.
heat lost = (1 ÷ R) × area × heating degree days × 24
- R
- the total resistance of the assembly — existing plus whatever you add
- 1 ÷ R
- the U-factor. This is where the collapse comes from: 1/13 is 0.077 and 1/60 is 0.017
- heating degree days
- how cold the year is — about 4,500 in the mixed middle, 7,500 in the cold north
Worked through — Going from R-13 to R-30 removes 0.044 of U. Going from R-49 to R-60 removes 0.004 — a ninth as much, for the same inches of material and the same crew in the same attic on the same day.
U.S. Department of Energy, Recommended insulation levels by climate zone · checked 2026-09-17
The half of the job that is not insulation
Insulation slows heat moving through still air. It does nothing whatsoever about air moving through it — and in an unsealed attic a great deal of air is moving.
Every wire hole, plumbing stack, top plate, chimney chase and recessed light is an opening between the house and the attic. Warm air goes up through them all winter, and the insulation lying on top slows it down about as much as a jumper slows wind.
What gets sealed, before anything else happens
- Top plates — the gap where every interior wall meets the ceiling, which runs the length of the house.
- Wire and pipe penetrations, each one individually, with foam rather than fibre stuffed into it.
- Plumbing vent stacks where they pass through, which are often a two-inch hole round a one-and-a-half-inch pipe.
- The chimney chase, with sheet metal and high-temperature sealant rather than foam.
- Recessed light fittings, which are effectively chimneys unless they are rated and covered.
- The attic hatch, weatherstripped and insulated — the best-value item on any attic list.
- Bathroom fans, ducted to outside rather than into the attic, which is a surprisingly common fault.
- Baffles at the eaves, so the roof still breathes once the fill is deep.
Which material, and where inches are worth paying for
| Material | R / inch | Seals air? | What it is |
|---|---|---|---|
| Blown fibreglass | 2.5 | No | The default attic fill. Blown in through a hose, cheap by the inch, and it does nothing at all to stop air moving through it. |
| Blown cellulose | 3.5 | No | Recycled paper with borate. Denser than fibreglass, so it slows air a little and fills around obstructions better — and it settles, which the depth has to allow for. |
| Fibreglass batts | 3.1 | No | Rolls cut to stud bays. The cheapest to buy and the easiest to fit badly — a batt compressed, cut short or forced round a wire loses most of the R it was sold for. |
| Mineral wool batts | 4.2 | No | Stone fibre. Holds its shape so it fits well, does not burn, and deadens sound noticeably — which is why it goes in party walls whether or not heat is the point. |
| Open-cell spray foam | 3.7 | Yes | Expands and seals as it goes, so it insulates and air-seals in one operation. Soft, vapour-open, and priced with the labour inside it because nobody sells it uninstalled. |
| Closed-cell spray foam | 6.5 | Yes | The highest R per inch there is, a vapour barrier in its own right, and it adds structural rigidity. It is also by a distance the most expensive, and in a deep attic where space is free it is the wrong tool. |
R per inch only matters where inches are scarce. On an open attic floor you can have as many as you like for the price of the material, so the cheapest R wins and that is blown fibreglass or cellulose. In a 2 × 4 wall cavity you have three and a half inches and no more, and then the 6.5-per-inch product is worth its 6× price.
| Loose fill on the attic floor | Spray foam at the roofline | |
|---|---|---|
| Cost per point of R | The cheapest there is | About 6× more |
| Seals air | Not at all | As it goes |
| Depth available | Unlimited | Limited by the rafter |
| Ductwork in the attic | Stays outside the envelope | Comes inside it |
| Storage up there | Buried unless you deck over | Attic stays usable |
| When it is right | A normal vented attic | Ducts up there, or a converted attic |
3 yr
R-13 to R-19
the best rung
35 yr
R-49 to R-60
the last one
10 yr
The whole job
to R-30
6×
Foam per point of R
against blown fibre
Owens Corning, Knauf and Greenfiber, Loose-fill and batt coverage charts · checked 2026-09-17
The order, and what goes wrong in each step
- 01Measure what is thereA ruler pushed down to the drywall, in several places. Patchy counts as the thin part, because heat goes through the thin part.
- 02Find the R your zone asks forThe Department of Energy publishes it by zone and by surface — R-49 to R-60 on an attic floor across most of the country.
- 03Seal before you insulateEvery penetration through the attic floor. After the fill is down, nothing underneath gets sealed for a generation.
- 04Fit baffles at the eavesSo the roof still breathes once the insulation is deep enough to block the gap.
- 05Work out the depth, not the bagsTarget R minus existing R, divided by the R per inch of the product. Bags follow from the depth and the area.
- 06Check each rung is worth buyingEach inch earns less than the one below it. The calculator above prices every rung separately.
A properly insulated and sealed house frequently needs a boiler or heat pump a size smaller, and that difference can be worth more than a decade of fuel. It only works in that order.
Do the insulation before sizing the equipmentFive ways this goes wrong
01Insulating over an unsealed attic floor
Insulation does nothing about air movement, and after twenty inches of fill goes down nobody is sealing anything underneath it again. The opportunity closes permanently on the day of the job.
Instead — Seal every penetration first, whatever the budget does to the depth afterwards. The order is not negotiable even when the scope is.
02A small top-up
Two inches on top of eleven costs the whole installation — crew, blower, day — for a fraction of the benefit, which is why that rung pays back in twenty-five years while the next one pays in three.
Instead — If you are getting the crew out at all, go to what the zone asks for. The material is the cheap part.
03Spray foam on an open attic floor
It costs several times as much per point of R and its advantage is R per inch — in a place where inches are free. It is an excellent product being sold for the wrong application.
Instead — Foam where inches are scarce: walls, rooflines, crawl spaces. Loose fill where they are not.
04Blowing over the eaves
It blocks the ventilation gap, the roof deck stops drying, and an attic that was fine starts to rot from the edges in. It is invisible from inside the house until the sheathing is soft.
Instead — Baffles at every rafter bay before the hose comes out, holding the channel open.
05Measuring the average depth
Heat goes through the thin places, not the average. An attic that is R-38 over most of it and R-6 over the joists is not an R-30 attic.
Instead — Measure in several places and work from the thin ones. Levelling the low spots is the cheapest R in the house.
How the calculator works it out
R-value per inch and coverage come from manufacturer charts; the R recommended for each surface in each climate zone comes from the Department of Energy, along with the heating degree days that turn a change in R into dollars. Prices are a survey collected at a target R-value rather than by the bag, because a bag of loose fill covers a very different area at R-30 than at R-60 and comparing bag prices across products means nothing. All of it is on the sources page, dated.
Three things stated plainly. The rung table puts the whole installation cost on the first rung, because that is where it falls in reality — the crew and the blower are paid whether you add two inches or twenty. The air sealing saving is the softest figure here and is deliberately cautious, since infiltration is measured with a blower door rather than calculated. And no figure includes a rebate or a utility programme: they change yearly, they are local, and building them into a national calculation is how a number stops being true.
Nothing you type is sent anywhere — the whole calculation runs in your browser.
The words on an insulation quote
Two worth knowing: R per inch, because it is only worth paying for where inches are scarce, and thermal bypass, because it is what insulation cannot fix and what sealing is for.
- R-value
- Resistance to heat flow. It is not a product and not a material — it is a depth of a particular material, and the same R comes from very different thicknesses.
- R per inch
- How much R an inch of a given product delivers. Blown fibreglass is 2.5, cellulose 3.5, closed-cell foam 6.5. It only matters where inches are scarce.
- Loose fill
- Insulation blown in through a hose rather than cut to fit. It covers awkward shapes properly, which batts between joists never quite do.
- Batt
- A pre-cut roll sized to a stud or joist bay. Cheapest to buy and easiest to fit badly — compressed, cut short or forced round a wire, it loses most of what it was sold for.
- Air sealing
- Foam, caulk and covers over every penetration in the attic floor — top plates, wire holes, plumbing stacks, the hatch, and the light fittings that are effectively chimneys. Insulation does not stop air; it only slows heat moving through still air.
- Thermal bypass
- Air moving around insulation rather than heat moving through it — up a wall cavity, through a light fitting, round a chimney. Insulation does nothing about it, which is why sealing comes first.
- Settling
- Loose fill compacting over the years, losing depth and therefore R. Cellulose settles more than fibreglass, and the installed depth is supposed to allow for it.
- Baffle
- A channel keeping the gap at the eaves open so the roof can still breathe once the insulation is deep. Blowing over the eaves without them is how attics start to rot.
- Vapour retarder
- A layer slowing moisture from passing into the assembly. Which side it goes on depends on the climate, and getting it wrong traps water inside the structure.
- Blower door
- A fan in a doorway that measures how leaky a house actually is. It is the only honest way to know what air sealing will buy — a conduction sum cannot tell you.
- Knee wall
- The short vertical wall where a sloped ceiling meets the floor in a converted attic. Almost always the worst-insulated surface in a house and almost always missed.
- Conditioned attic
- An attic insulated at the roofline rather than the floor, bringing it inside the thermal envelope. It is how you insulate an attic full of ductwork, and it needs foam rather than fill.
Common questions
- How much does attic insulation cost?
- About $2.63 a square foot all in on a 1,200 ft² attic going from R-11 to R-60 — roughly $3,150, of which about $1,140 is air sealing and $720 is the installation itself. Insulation alone, without sealing, is nearer $1.70 a square foot.
- How much insulation do I need in my attic?
- The Department of Energy asks for R-60 on an attic floor in the mixed middle of the country and R-30 in the far south. What matters more is what is there now: measure the depth with a ruler in several places, and count the thin places rather than the average, because heat goes through the thin places.
- Does attic insulation pay for itself?
- Yes, and much faster than most envelope work — around ten to twelve years for the insulation itself on a typical attic, or nineteen once air sealing is in the price, against thirty-nine or more for replacement windows. It is one of the few energy upgrades where the arithmetic genuinely supports the sales pitch.
- Is R-60 worth it, or is R-30 enough?
- Both are defensible and the reason is not obvious. Each extra rung earns less than the one below it — R-13 to R-19 pays back in three years, R-49 to R-60 takes thirty-five. But the whole job barely changes, because the crew, the hose and the day are paid for whichever depth you stop at: ten years to R-30 against twelve to R-60. Going deep is not wasteful. The last inches are just nearly free to fit and nearly worthless to own.
- Which is better, blown fibreglass or cellulose?
- Cellulose gives more R per inch — 3.5 against 2.5 — and being denser it slows air a little, which fibreglass does not do at all. Fibreglass does not settle as much and does not absorb water. In an attic where depth is free, the difference is small and either is a sound choice.
- Should I air seal before insulating?
- Always, and it is the part quotes leave out. Insulation slows heat moving through still air; it does nothing about air moving through it. Every wire hole, plumbing stack, top plate and recessed light is a hole, and once twenty inches of fill is on top of them, nobody seals them again for a generation.
- Is spray foam worth the money in an attic?
- It costs about 6 times blown fibreglass per point of R. It earns that where inches are scarce — a wall cavity, a roofline, a crawl space — because it delivers 6.5 per inch and seals as it goes. On an open attic floor, where you can have as many inches as you like for pennies, it is the wrong tool.
- Do I need to remove the old insulation?
- Usually not. New fill goes straight on top and the R-values add. Removal is needed when the existing material is wet, rodent-contaminated, or when the floor underneath has to be air sealed properly — and that last case is more common than anyone expects.
- Can I insulate the attic myself?
- Blowing loose fill, yes — the blower comes with the material at most suppliers and the work is straightforward if unpleasant. The parts worth paying for are the air sealing, which is fiddly and matters most, and getting the depth even, which is harder than it sounds from a hose in a dark attic.
- What about the attic hatch?
- It is usually a bare sheet of plywood in the middle of an otherwise insulated ceiling, and it leaks air as well as heat. An insulated, weatherstripped hatch cover costs very little and it is the single best-value item on any attic list.
- Will insulation stop ice dams?
- Insulation and air sealing together will, because ice dams are caused by heat escaping into the attic and melting snow that refreezes at the cold eaves. Insulation alone often will not — the heat is usually arriving by air leakage rather than by conduction, which is the same reason sealing comes first.
- Does more insulation mean I need less heating equipment?
- Yes, and it is frequently worth more than the fuel saved. A properly insulated and sealed house often needs a boiler or heat pump a size smaller, and that difference in equipment cost can be larger than a decade of the energy saving. It only works if the insulation goes in first and the equipment is sized afterwards.
Send this to whoever is paying
The link carries your measurements, so whoever opens it sees the same numbers you are looking at — and you can come back to it yourself tomorrow with the delivery note in front of you.
Plain links, no tracking scripts. Nothing here reports back that you shared it.
About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
More about how this site is edited