Loan Insulation Estimator
Insulation is sold in packages and bags, not square feet. Tell this estimator the area and the R-value you are targeting, and it converts both into a shopping list — batts by the package, or blown-in by the bag.
For an attic, the area is the ceiling footprint below it. For walls, measure the wall area and subtract windows and doors — or use the gross area and let the offcuts fill gaps, which is what most installers do.
Coverage this estimate is built on
| R-value | Batt package covers | Blown-in bags per 1,000 sq ft |
|---|
Typical manufacturer-chart values. Your brand's bag or package prints its own coverage on the label — if it differs, buy by the label; the counts above get you within one package either way.
How it works
For batts and rolls, the estimator divides your area by the coverage printed on a typical package for that R-value — R-19 batts cover about 95 sq ft per package, R-38 about 60 — and rounds up, because partial packages are not sold. For blown-in, manufacturers rate bags per 1,000 sq ft at a given R-value and settled depth: about 22 bags per 1,000 sq ft for R-19, 40 for R-38, 52 for R-49 with typical fiberglass/cellulose products. R-value is resistance to heat flow per the whole assembly — doubling batts does not quite double performance if the cavity is compressed, which is why the coverage figures assume the insulation sits at its rated thickness, uncompressed.
Method & worked example
Method: Batts: packages = area ÷ the package coverage for the R-value (110 sq ft at R-13 down to 45 at R-49 on typical manufacturer charts). Blown-in: bags = area ÷ 1,000 × the bags-per-1,000 rate for the R-value (16 at R-13 up to 52 at R-49) at rated settled density. Both round up — partial packages are not sold.
Worked example: A 500 sq ft space insulated to R-19 with batts: 500 ÷ 95 sq ft per package = 6 packages, covering 570 sq ft. The same 500 sq ft blown in to R-38 instead would take 20 bags at 40 bags per 1,000 sq ft.
When to use this
- Topping up an attic and deciding between a blower rental weekend or batt packages.
- Insulating a new framed wall, floor or cathedral ceiling to a target R-value.
- A utility rebate requires a stated R-value — you want the package count that reaches it.
- Comparing blown-in vs batts on material count for the same space.
How to use it, step by step
- Measure the area: attic = ceiling footprint; walls = length × height minus openings.
- Pick the type: batts for open framed cavities, blown-in for attics and closed walls.
- Set the target R-value from your climate guidance or energy code.
- Buy the packages or bags shown — and one extra if a return trip is costly; unopened packages return easily.
- For blown-in, confirm the machine rental includes enough hose to reach the far corners.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Area | Surface being insulated | Attics 500–2,000 sq ft |
| Type | Batts/rolls vs blown-in — different buy units | Batts for framing, blown for attics |
| R-value | Target thermal resistance | R-38 attics most climates, R-49 cold |
Another worked example
An 800 sq ft attic floor brought to R-30 with blown-in instead: at 32 bags per 1,000 sq ft, that is 26 bags — a one-day blower rental job. The same space in R-30 batts would take 11 packages (800 ÷ 75), manageable without a machine but slower around every joist and wire.
Frequently asked questions
How much insulation do I need for a 500 sq ft attic at R-38?
Blown in, about 20 bags (40 bags per 1,000 sq ft × 0.5). In batts at R-38, packages cover roughly 60 sq ft each, so 500 sq ft takes 9 packages. The estimator above runs either version from the same two inputs.
What R-value should my attic have?
Most US climate guidance lands at R-38 to R-60 for attics, R-13 to R-21 for walls, and R-25 to R-30 for floors over unheated space. Cold northern zones sit at the top of each range. Your local energy code has the binding numbers for new work.
Batts or blown-in — which is better?
Blown-in fills odd corners and buries joists, so it usually wins on attics and retrofit work. Batts are cleaner for open wall cavities during framing and need no machine. Performance per R is the same; coverage quality is the difference.
How many bags of blown insulation cover 1,000 sq ft?
At R-38, about 40 bags of a typical product; at R-49, about 52; at R-19, about 22. Every manufacturer prints a coverage chart on the bag — bags per 1,000 sq ft at each R-value — and those numbers govern.
Can I add insulation over old insulation?
Usually yes — new batts (unfaced) or blown-in over existing attic insulation is standard practice, and the R-values simply add. Do not stack a second faced batt over a faced one; two vapor barriers can trap moisture.
🔨 Costing a bigger renovation?
Insulation is one line of a whole-home budget. The Home Renovation Estimator prices cosmetic, full and gut scopes per square foot.
How thick is R-38 in blown insulation?
About 10–11 in of settled blown fiberglass, or 13–14 in of cellulose, depending on product — the bag chart prints the depth for each R-value, and installers use depth rulers stapled to the joists to prove it. Buy by R-value and bag count, not by inches in the truck.
Should I air-seal before insulating?
Yes — the cheapest R-value is the air you stop moving. Sealing top-plate gaps, can-light boxes and plumbing penetrations before adding insulation routinely outperforms adding another R-10 on top of a leaky ceiling.