Insulation Calculator
Size blown-in attic insulation from the R-value you want to hit.
What you'll need to order
- blown-in cellulose
- attic baffles
- insulation dams
- blower rental
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
bags = area × R-value to add ÷ 1080
Cellulose delivers about R-3.7 per settled inch, and a standard bag covers roughly 1,080 square-foot-R units. Fibreglass batts and loose-fill differ — check the coverage chart printed on the bag.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ASTM C739 (Cellulosic Fiber Loose-Fill Thermal Insulation) 2023 — settled density and R-value per inch for loose-fill cellulose
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With area to insulate 1000 sq ft, target r-value 38 , existing r-value 0 , this calculator returns 35.19 bags.
Insulation needed by R-value
Figures below assume area to insulate 1200 sq ft × existing r-value 0 . Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.
| Target R-value | Bags of cellulose (bags) | Depth to add (in) |
|---|---|---|
| 13 | 14.44 | 3.51 |
| 19 | 21.11 | 5.14 |
| 30 | 33.33 | 8.11 |
| 38 | 42.22 | 10.27 |
| 49 | 54.44 | 13.24 |
| 60 | 66.67 | 16.22 |
A 1,200 sq ft attic with nothing existing. Depth is for loose-fill fibreglass at roughly R-2.5 per inch.
How much insulation for an attic or wall?
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Bags of cellulose | Material cost |
|---|---|---|---|
| Small attic to R-38 | 800 sq ft × 38 × 0 | 28.15 bags | $1,126 – $1,689 |
| Average attic to R-49 | 1200 sq ft × 49 × 0 | 54.44 bags | $2,178 – $3,267 |
| Topping up R-19 to R-49 | 1200 sq ft × 49 × 19 | 33.33 bags | $1,333 – $2,000 |
| Large attic to R-60 | 2000 sq ft × 60 × 0 | 111.1 bags | $4,444 – $6,667 |
| Garage ceiling to R-30 | 576 sq ft × 30 × 0 | 16.00 bags | $640 – $960 |
Topping up existing insulation is nearly always the better value. Enter what you already have and the calculator only counts the difference.
Cost range uses $40–$60 per bag for material only.
What does attic insulation cost?
Material typically runs $40 to $60 per bag. Material only. Blown in by a contractor runs $1.50 to $3.50 per square foot, and most suppliers lend the blower free with a minimum bag purchase.
What moves the price:
- Material — fibreglass is cheapest, cellulose mid-range, spray foam several times either
- Attic access, which decides whether the job takes half a day or two
- Air sealing beforehand, which matters more than the last R-10
- Baffles and dam work around eaves and fixtures
Prices are US averages and move with fuel, season and haul distance. Always get two local quotes before budgeting.
Insulation materials and their R-value per inch
| Type | R-value per inch | Notes |
|---|---|---|
| Loose-fill fibreglass | 2.50 | Cheapest blown option; settles slightly |
| Loose-fill cellulose | 3.50 | Better air resistance, recycled content |
| Fibreglass batts | 3.20 | Only as good as the fit around obstructions |
| Mineral wool batts | 4.00 | Fire and sound resistant |
| Open-cell spray foam | 3.60 | Air seals as it insulates |
| Closed-cell spray foam | 6.50 | Highest R per inch, also a vapour retarder |
This calculator assumes loose-fill fibreglass. Air sealing before insulating usually saves more energy than the last ten points of R-value.
Frequently asked questions
How many bags of insulation do I need?
It depends on R-value and area — a 1,200 sq ft attic taken to R-49 needs roughly 60 bags of loose-fill fibreglass. Bag coverage is printed on the wrapper at each R-value.
How much insulation is already up there?
Measure the depth and divide by the R per inch of what you see. Loose fibreglass at 2.5 per inch means 8 inches is about R-20 — usually less than owners expect.
Is it worth going above R-49?
Rarely, in most US climates. Returns fall off sharply past R-49, and the money is better spent on air sealing the ceiling plane, which is where most heat actually escapes.
How many inches of insulation do I need for R-38?
About 10.3 inches of settled cellulose, or 12 to 13 inches of loose-fill fibreglass. Blow it deeper than target since cellulose settles roughly 10% in the first year.
What R-value should my attic have?
R-38 in the southern US, R-49 across the middle, and R-60 in the coldest zones. Check the current DOE map for your county — recommendations have risen over the past decade.
Can I add insulation over existing insulation?
Yes, and it is usually the cheapest efficiency upgrade available. Do not lay faced batts on top of existing insulation — the vapour barrier traps moisture. Use unfaced batts or blown-in loose fill.
Do I need baffles in the attic?
Yes, wherever insulation reaches the eaves. Baffles keep a clear air channel from the soffit vents into the attic. Insulation stuffed into the eaves blocks ventilation and causes ice dams.