Home Insulation R-Value Target Calculator

It depends on your climate zone. The Department of Energy and ENERGY STAR recommend roughly R-30 to R-49 in the warmest parts of the country, R-49 to R-60 across the middle, and R-60 for most of the northern half of the United States. Walls and floors have their own targets, generally around R-13 to R-21 in the wall cavity plus continuous exterior sheathing in colder zones. The calculator applies the target for the zone you pick and shows all three assemblies together, because a well-insulated attic over uninsulated walls is a common and expensive imbalance.

It varies by material, which is exactly why depth alone is a poor way to judge an attic. Blown cellulose delivers about R-3.5 per inch, so R-49 takes roughly 14 inches and R-60 about 17. Blown fiberglass is closer to R-2.6 per inch, needing about 19 inches for R-49. Closed-cell spray foam reaches R-6.5 per inch and gets there in well under half the depth, at several times the price. The calculator does this conversion for whichever material you pick and shows all six side by side so you can trade depth against cost.

In most cases yes, and it is usually the cheapest route. Existing fiberglass or cellulose keeps working as long as it is dry, reasonably flat and not full of rodent damage or debris, so you simply add over the top and the R-values stack. The important exception is vapor barriers: never lay faced batts over existing insulation, because the second barrier traps moisture in between. Remove and replace only if the material is wet, contaminated, or if you find vermiculite that needs asbestos testing before anyone disturbs it.

Usually, though the payback depends heavily on how thin your attic is now, how cold your winters are, and what you pay for heat. Going from a nearly bare attic to the recommended level in a cold climate with expensive fuel can pay back in just a few years; topping up an already decent attic in a mild climate can take considerably longer. Two things the payback number misses: the calculator counts heating savings only, so summer cooling savings come on top, and comfort improves immediately — fewer drafts, warmer floors upstairs, and less ice damming in snowy regions.

For an open attic floor, blown cellulose and blown fiberglass are the value picks — both are inexpensive per R, go down fast, and fill around framing better than batts. Cellulose packs slightly more R into each inch and resists air movement a little better; fiberglass is lighter and does not absorb water the same way. Batts make sense where you need a tidy fit between accessible joists. Spray foam costs several times more per R and is best reserved for places where depth is limited or you need the air seal and moisture control it provides, such as a conditioned attic at the roofline or a rim joist.

The federal position changed, so confirm your own eligibility with a tax professional before counting on anything. Insulation and air sealing were covered by the Energy Efficient Home Improvement Credit (Section 25C), worth 30% of materials up to $1,200 a year. The IRS now takes the position that the credit is unavailable for property placed in service after December 31, 2025, and the 2025 Form 5695 instructions state that plainly — though some members of Congress have publicly disputed that reading of the law. Separately, many states, utilities and weatherization programs run their own insulation rebates that are unaffected by the federal question, so check with your utility before you write off an incentive.