Manual J load calculator
Sizing & LoadA room-by-room worksheet that follows the ACCA Manual J method, printable.
BTU/h · room by room
Attic insulation R-value calculator
Pick your climate zone, say what is up there and how deep, and the calculator returns the attic insulation R-value ENERGY STAR recommends, the R-value you already have, and the inches it takes to close the gap.
Pre-filled with a zone 4A attic holding 4 inches of old loose fiberglass, topped up with blown cellulose.
By county — see the map on ENERGY STAR's R-value page.
in
Measure in several spots; use the low ones, not the middle of the attic.
R/in
Defaults to ENERGY STAR's figure; the bag's coverage chart overrides it.
Existing against R-60
Add R-50: about 13.5 in of blown cellulose on top of what is there.
What attic insulation R-value means
R-value is resistance to heat flow through a layer: double the R and you halve the heat that crosses it. The attic insulation R-value that matters is the total of everything on the attic floor, and the level worth reaching depends on climate. ENERGY STAR publishes the table below for retrofitting existing wood-framed houses, built on the 2021 International Energy Conservation Code. The first column is what to add to a bare attic, which makes it the total to aim for.
| Zone | Attic, uninsulated | Attic, 3–4 in already | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 6, 5, and 4C | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
Your zone is set by county, not by state. Many states span more than one zone, so look up the county on the map on ENERGY STAR's page rather than guessing from a neighbour.
Sources: ENERGY STAR, Recommended Home Insulation R-Values (read 2026-10-18)
How to use the calculator
- Choose the climate zone for the house's county.
- Say what is on the attic floor now and measure its depth with a tape at several points, including near the eaves. If the batts have an R-value printed on them, enter that instead.
- Choose what you will add. The R per inch fills in; overwrite it with the bag's figure.
- Read the R to add and the inches to add. Print it or copy it into the job notes.
Measuring what is already there
ENERGY STAR's method is a tape measure and a multiplier. Identify the material, measure the depth, multiply. The quick version: if the insulation is level with or below the floor joists you probably need more, and if you cannot see the joists you probably have enough.
| What you see | What it probably is | R per inch |
|---|---|---|
| Loose fibers, light yellow, pink or white | Fiberglass | 2.5 |
| Dense gray or near-white, black specks | Rock wool | 2.8 |
| Small gray flat pieces, from newsprint | Cellulose | 3.7 |
| Light-weight granules | Vermiculite or perlite | 2.7 |
| Batts, yellow, pink or white | Fiberglass batts | 3.2 |
Safety — Vermiculite and the walk across the joists
Vermiculite (light, pea-sized, flaky gray granules) may contain asbestos. Do not disturb it until an approved lab has tested it, and topping it up means working in it. Walk on joists or truss chords, never on the drywall between them.
Sources: ENERGY STAR, What to Look For: DIY Checks and Inspections (read 2026-10-18)
Worked example
A zone 4A house, four inches of old loose fiberglass across the attic floor, topped up with blown cellulose. These are the calculator's defaults.
| Step | Arithmetic | Result |
|---|---|---|
| Existing R | 4 in × 2.5 | R-10 |
| Zone 4A level | ENERGY STAR, uninsulated attic | R-60 |
| R to add | 60 − 10 | R-50 |
| Cellulose to add | 50 ÷ 3.7 | 13.5 in |
ENERGY STAR's own shortcut for an attic holding 3–4 inches in that zone is to add R49, which is the same answer rounded. The depth is a minimum: blown material settles, which is why the bag quotes two thicknesses.
Read the bag, not a blog
Per-inch figures differ between products, so treat the default as a starting point. Federal labelling rules make every bag of loose-fill carry a coverage chart: the minimum settled thickness, initial installed thickness, coverage area, bags per 1,000 square feet and minimum weight per square foot at R-13, 19, 22, 30, 38 and 49. Install to the initial thickness and count bags against the chart; a short bag count means a short R-value however deep it looked on the day.
Sources: FTC R-value Rule, 16 CFR 460.12 (eCFR) (read 2026-10-18)
Seal first, then insulate
EPA recommends air sealing the attic before adding insulation: top plates, plumbing and wiring penetrations, the chase around the flue. Insulation slows conduction; it does little against air pouring through a hole. EPA estimates that air sealing plus insulation in attics, floors over crawl spaces and basements saves homeowners an average of 15% on heating and cooling.
Sources: ENERGY STAR, Attic Insulation Project (read 2026-10-18) · ENERGY STAR, Rule Your Attic! (read 2026-10-18)
What it does to the equipment size
The ceiling is one line of a load calculation, and a big one in a single-storey house. Take the new R-value into the Manual J load calculator (pick the matching ceiling row) or the envelope setting of the BTU calculator, before anyone quotes a condenser. A load run on the old ceiling sizes the system for an attic that no longer exists; that is the oversizing trap in how to size an AC unit and in sizing a furnace. It is also why square-foot rules in tonnage per square foot drift so far from a real calculation.
The tax credit is gone
The federal 25C credit that used to cover attic insulation applies only to improvements made through 2025-12-31. Check utility and state programmes instead; the HVAC tax credit guide covers what is left.
Sources: IRS, Energy Efficient Home Improvement Credit (read 2026-09-04)
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