Choosing the right R-value is the decision that actually matters. The bag count is trivial once you know whether you are insulating a 2x4 wall, a floor above a crawl space, or an attic. Pick the wrong R-value and you either waste money on insulation the cavity cannot hold, or you buy a thin batt that lets half the heat escape anyway.
Insulation bags are labeled by the area they cover at a given R-value. An R-13 bag spreads across about 40 square feet, an R-19 bag across 50, and a thick R-30 bag across 65. That is the figure the whole estimate rests on.
R-value for walls, floors and attics
Walls in a standard 2x4 frame take R-13 to R-15. Step up to a 2x6 wall and the cavity opens up for R-19 to R-21, which is why newer energy-conscious homes frame with 2x6 studs. Floors over crawl spaces and unheated garages usually land around R-19.
Attics are where the biggest gains live. Heat rises, so the ceiling is the single place you lose it fastest. Most climates want R-30 or better in the attic, and cold northern regions push all the way to R-49. If you only do one insulation upgrade in an old house, the attic is the one.
How much a bag covers
A bag of unfaced fiberglass batts prints its coverage right on the label. R-13 batts, the thin 3 and a half inch panels, cover about 40 square feet per bag. R-19, at 6 inches thick, covers about 50. The thick R-30 attic batts, around 10 inches, cover about 65 square feet per bag.
Take a 20 by 8 foot wall, 160 square feet, filled with R-13. At 40 square feet a bag, that is 4 bags, and a little waste around electrical boxes and plumbing bumps makes it 5. The math stays this simple because the manufacturer already did the density work for you on the bag.
Batts versus blown-in
Batts are the pre-cut panels that fit neatly between studs and joists. They are the right call for new construction and any wall you can see into, because you can press each panel in snug and see the gaps you missed. They are cheap, DIY-friendly, and they come faced or unfaced depending on whether you need a vapor barrier.
Blown-in insulation, either loose fiberglass or cellulose, is the call for existing walls where the drywall is already up, and for attics with odd angles. A hose blows the material into the cavity or across the attic floor, and it settles around obstructions that a batt would leave as a gap. The tradeoff is that you usually need a rental blower and a helper, and the coverage is measured in bags of a certain weight rather than square feet per bag.
Waste and awkward spots
Insulation wastes around things. Electrical boxes, recessed lights, vent pipes and the odd cross-brace all cut a batt short, and those offcuts add up. Build in a small waste factor, around 5 to 10 percent, and you will not run short on the last bay.
Recessed lights that are not rated for contact need clearance around them, which means cutting the batt short and leaving a gap. Do not jam insulation against an unrated fixture, because it is a real fire hazard.
What the upgrade looks like
Picture a 20 by 8 foot wall filled with R-13. The 160 square feet divides into about 4 bags, plus one spare for the boxes and corners, so 5 bags cover the wall and leave a little extra. Now flip to the attic above the same house, where R-30 at 65 square feet a bag covers a typical 1000 square foot attic in around 16 bags.
That attic upgrade is the one that pays you back every month. Heating and cooling bills typically drop 10 to 20 percent once the ceiling is properly filled, and the bags themselves are one of the cheapest materials in the whole building envelope.
See also the drywall calculator for the panels that go on after the cavities are filled, and the paint calculator for the finish on the other side.
Insulation Calculator
An R-13 bag covers about 40 square feet, an R-19 bag about 50 and an R-30 bag about 65, so pick the R-value before counting bags.
How to use
- Measure the wall or attic area in square feet
- Choose the R-value, such as R-13 for 2x4 walls or R-30 for attics
- Pick batts for new work or blown-in for existing cavities
- Adjust the waste factor for awkward cuts around fixtures
- Read the number of bags you need
Supported formats and limits
- An R-13 bag covers about 40 square feet
- An R-19 bag covers about 50 square feet
- An R-30 bag covers about 65 square feet
- Attics commonly call for R-30 or higher
Examples
A 20 by 8 foot wall with R-13 insulation needs about 4 bags, plus one spare for waste.Common errors
- Wall area must be a positive number
- R-value should match the cavity you are filling
- Waste factor should stay under 20 percent
Privacy and browser support
All calculations run locally in your browser and nothing is uploaded.
FAQ
What R-value do I need for walls?
R-13 to R-15 fits a 2x4 wall, and R-19 to R-21 fits a 2x6 wall.
What R-value do I need in the attic?
Most climates call for R-30 or better, with cold northern regions pushing toward R-49.
How much does one bag cover?
An R-13 bag covers about 40 square feet, R-19 about 50, and the thicker R-30 bag about 65.
Batts or blown-in insulation?
Batts suit new framing where you can fit panels neatly. Blown-in fits existing walls and attics with no drywall open.
Does insulation actually cut bills?
Yes. A properly insulated shell typically trims heating and cooling costs by 10 to 20 percent.
Can I put batts over old insulation?
Yes, as long as the old material is dry and unfilled gaps remain, but keep the whole cavity flat for the drywall to sit on.