Attic Insulation Calculator
Heat leaves a house through the ceiling in proportion to area divided by R-value. Add R, cut the loss, and the payback is arithmetic — which is why the starting R-value matters more than the brand of insulation.
Quick answer
How much does it cost to insulate a 1,200 sq ft attic, and does it pay back?
Going from R-11 to R-49 takes about 10 inches of blown cellulose — 41 bags, roughly $554 in material and $2,170 installed with air sealing. That saves about $146 a year on gas heat in a mixed climate, so the payback is 15 years. The same upgrade on an attic with almost nothing in it pays back in 3 to 4 years — the starting R-value decides this, not the insulation you buy.
Attic insulation calculator
What each target R-value buys
| Target | Depth | Bags | Installed | Saving per year | Payback |
|---|
Adding R never stops helping and it does stop being worth the money. Compare the last two rows: the increment from R-49 to R-60 usually takes longer to repay than the whole job did to get to R-49.
Insulation types for this job
| Type | R per inch | Depth | Bags | Material |
|---|
What R-value actually buys
Heat loss through a ceiling is area divided by R-value, multiplied by the temperature difference and the hours. Nothing else in the equation is under your control: the area is the house, the temperature difference is the climate, the hours are the winter. Insulation is the only term you can change, and it sits in the denominator — which is why the first inches do the most work and the last ones do the least.
On a 1,200 square foot ceiling in climate zone 4, going from R-11 to R-49 cuts the ceiling's heat loss by about 78 percent — 8.8 million BTU a year, or roughly 98 therms of gas before the furnace's own efficiency is counted. At $1.30 a therm that is about $127 a year, plus about $20 of cooling electricity in summer. The installed job is around $2,170, so the simple payback is close to 15 years.
That number disappoints people who have been told insulation pays back in three years, and the reason is almost always the starting point. An attic with R-3 in it loses nearly four times as much heat as one with R-11. The same upgrade on that attic saves about $650 a year and pays back in three and a half years. The insulation did not get better; the baseline got worse.
Depth is the decision, not the R-value on the bag
Blown cellulose is R-3.7 per inch, blown fiberglass R-2.5, mineral wool R-3.1. To add R-38 you need 10.3 inches of cellulose or 15.2 inches of fiberglass — a difference of five inches of depth, which matters when a roof truss is 10 inches deep and the soffit vents are at the edge of the same cavity.
Depth also has to be measured after settling. Cellulose settles 15 to 20 percent in the first years, and the bag's coverage chart is already stated at settled density, so the installed depth should be measured against the chart rather than eyeballed against a ruler the day after installation.
Air sealing comes first, and it is not optional
Insulation slows conduction. It does nothing about the holes: the top plates of every partition wall, the plumbing penetrations, the chimney chase, the recessed lights, the attic hatch. A 1,200 square foot attic typically has the equivalent of a two square foot hole in the ceiling, and air moving through it carries far more heat than conduction through the insulation beside it.
Sealing those paths with foam and caulk before the blower arrives costs a few hundred dollars and is the difference between the calculated saving and the one on the gas bill. It has to happen first because once 10 inches of cellulose is on the floor, finding the holes means moving it.
What the installer has to work around
- Soffit vents. Every rafter bay that vents from the eave needs a baffle to keep the insulation from blocking the airway. Blocked soffits turn an attic into a moisture problem and a hot roof deck.
- Recessed lights. Old non-IC fixtures cannot be covered. They have to be replaced with sealed IC-rated units or boxed, or the insulation goes around them and the hole stays.
- Knob-and-tube wiring. It cannot be buried in insulation. It has to be assessed by an electrician first, which is a real cost on a 1920s house.
- Ducts in the attic. Insulating the ceiling does nothing for a duct run that leaks and radiates in the same attic. Seal and insulate the ducts, or the saving is shared with the attic.
- Moisture. A bath fan or dryer venting into the attic wets the new insulation from below. Fix the venting before adding any.
DIY or contractor
Blowing cellulose is a two-person job: one feeding the hopper, one in the attic with the hose. The blower is usually free for 24 hours with a minimum bag purchase from the same store. Material for this job is about $554; a contractor charges about $1.00 per square foot of labour on top, plus air sealing.
Doing it yourself saves the labour and costs you a day, two respirators, and the risk of a job that is uneven at the eaves and thin around the hatch. Doing it badly costs more than hiring it: air sealing is the part that benefits most from experience, and it is also the part most often skipped by both.
What this does not count
Rebates. The federal Section 25C credit for insulation ended for work placed in service after December 31, 2025, so a 2026 job gets nothing from the IRS; state and utility programs still exist in many states and are worth a phone call before signing. Also not counted: removing old insulation, repairing a roof, adding a radiant barrier, or the value of a cooler second floor in July, which is real and does not appear on a utility bill.
Frequently asked questions
How much does it cost to insulate an attic?
About $1.80 per square foot installed for blown cellulose in an accessible attic, or roughly $2,170 for 1,200 square feet at R-49 from R-11, including air sealing. Material alone is about $554, or $0.46 per square foot.
How many bags of cellulose do I need for 1,000 square feet?
Thirty-four 40 lb bags to add R-38 at 10.3 inches, or 41 bags for 1,200 square feet. The bag's coverage chart is stated at settled density, so it already accounts for the 15 to 20 percent the material loses as it settles.
What R-value do I need in my attic?
R-49 in climate zone 4, R-60 in zones 5 to 7, and R-38 or less in zones 1 to 3. Those are the DOE recommendations for a vented attic; the cost of going higher rarely pays back at today's fuel prices, and going lower is what most older houses already have.
Can I just add insulation over the old insulation?
Usually yes, as long as the old material is dry, not vermiculite, and not hiding knob-and-tube wiring. Blown insulation goes right over fiberglass batts. What you cannot do is cover the soffit vents, bury non-IC recessed lights, or leave air paths unsealed underneath.
Do I need a vapour barrier under the new insulation?
Not in a vented attic in most of the US. The paint on the ceiling is the vapour retarder, and adding a second one on the attic floor can trap moisture between them. In a hot-humid climate the detail changes; ask locally before adding any.
How long does attic insulation take to pay for itself?
From R-11 to R-49 on gas heat in a mixed climate, about 15 years at $1.30 a therm. From almost nothing, about 3 to 4 years. In a cold climate, or with propane or oil heat, the same job pays back two to three times faster.
Is blown insulation better than batts?
For a retrofit, usually. Blown insulation fills around obstructions, joins and truss webs without cutting, which is where batts leave gaps. Batts are better where the cavity is open and regular, and they do not settle.
Are there rebates for attic insulation in 2026?
The federal 25C credit ended for work placed in service after December 31, 2025. Many states and utilities still run their own programs, and some require an audit first, so check with your utility and your state energy office before booking the job.
Disclaimer: This calculator estimates insulation depth, bag count, installed cost and annual energy saving from your inputs and a simplified degree-hour model. Actual savings depend on air sealing, duct location, moisture, the condition of the existing insulation and local fuel prices. It is not an energy audit.
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