How the calculator works
The calculator answers one question: how much insulation do you need to add to reach the level recommended for your climate? It works in five steps.
- Pick the area. Attic, walls, floor, basement walls or a sloped roof. This page opens on an attic floor with blown cellulose, but you can switch to any of them.
- Measure it. Enter length × width or a total area. For walls, enter the perimeter and wall height, and the calculator subtracts doors and windows.
- Tell us what's there now. Enter the depth and type of the old insulation, or its R-value if you know it.
- Enter your ZIP code. We look up your 2021 IECC climate zone and the R-value ENERGY STAR recommends for that zone.
- Choose a material. Blown cellulose, blown fiberglass, batts, foam board or spray foam, depending on the area.
The core formula is simple:
R to add = target R − R you already have
Your existing R comes from depth × a per-inch value. ENERGY STAR publishes these for older insulation: 3.2 per inch for fiberglass batts, 2.5 for loose fiberglass, 3.7 for cellulose, 2.8 for loose rock wool and 2.7 for vermiculite. The calculator then turns the R you need into a depth and a count of bags or sheets, using each manufacturer's coverage figures. It adds 10% for waste by default. You can change that under More options.
Worked example: a 1,000 sq ft attic in zone 4A
Say your attic floor is 1,000 sq ft, your ZIP code puts you in climate zone 4A, and there's about 6 in of old loose fiberglass up there.
- Existing insulation: 6 in × 2.5 per inch = R-15
- Target for a zone 4A attic: R-60
- R to add: R-45
- Area to cover with 10% waste: 1,100 sq ft
Here's what the calculator returns for three materials:
| Material | Add | To buy | Material cost |
|---|---|---|---|
| Blown cellulose (25 lb bags) | 13.2 in, settles to 12.3 in | 72 bags | $1,151–$1,294 |
| Blown fiberglass (28.6 lb bags) | 15.2 in | 27 bags | $1,169–$1,313 |
| Fiberglass batts, 24 in wide | One layer of R-49 (14 in) | 23 bags | $2,461 |
Two things stand out. Blown fiberglass needs more depth than cellulose for the same R-value, but far fewer bags, because each bag covers much more floor. And batts only come in set sizes, so the smallest single layer that reaches the target, R-49, takes you to R-64 rather than exactly R-60.
Costs are for materials only, at typical retail prices from October 2026. They don't include tools, labor or the blower.
Reading your results
- Now, target, after. The meter shows where you start, where you need to be and where you'd end up.
- Depth. For loose-fill you'll see two depths: the installed depth and the settled depth. Blow to the installed depth.
- To buy. Bags or sheets per layer, with the coverage of each package.
- Show the math. Every step of the calculation, so you can check it.
- Copy link. Your inputs are stored in the page address, so you can bookmark a result or send it to someone.
Where to start
ENERGY STAR's rule of thumb: if the insulation in your attic is level with or below the floor joists, you probably need more. An open attic floor is also the easiest place to add insulation, because you can reach all of it.
Each area has its own page with tips for that job:
- Attic insulation calculator: adding to an existing attic floor
- Blown-in insulation calculator: bag counts, settling and blower tips
- Wall insulation calculator: what fits in 2×4 and 2×6 walls
- Floor insulation calculator: floors over crawl spaces and garages
- Basement insulation calculator: foam board and framed basement walls
Not sure what your zone calls for? See the full table of insulation R-values by climate zone.
Before you buy
- Check the bag. Every loose-fill product has its own coverage chart. Our numbers come from GreenFiber and Knauf charts. If you buy another brand, check our bag count against its chart.
- Seal air leaks first. Gaps around wiring, plumbing, chimneys and the attic hatch let warm air slip past the insulation. Seal them before new insulation covers them.
- Watch for vermiculite. Pebbly, gray-brown loose fill may contain asbestos. The EPA advises leaving it undisturbed and having a professional deal with it.
- Check your local code if the job needs a permit. Your state or city may use a different edition of the energy code, or amend it.
Sources
- ENERGY STAR: Recommended home insulation R-values
- ENERGY STAR: DIY checks and inspections
- U.S. DOE: Insulation and air-sealing essentials (2021 IECC minimum R-values)
- PNNL-33270: Guide to Determining Climate Zone by County (IECC 2021)
- GreenFiber SANCTUARY attic coverage chart
- Knauf EcoFill Wx blowing insulation product data
- U.S. EPA: Asbestos-contaminated vermiculite insulation