Energy
Insulation R-Value Calculator
See the recommended R-value for your climate and space, and how much insulation that actually means.
Your answer
Targets follow US Department of Energy climate-zone guidance for existing homes.
What R-value means and why it changes by climate
R-value measures resistance to heat flow. A higher number means a better insulator, and R-values stack — R-13 batts over R-19 blown-in give roughly R-32 total. The right target depends entirely on climate: a home in Minneapolis loses far more heat through the attic each winter than one in Miami, so it needs a thicker resistance layer to slow that loss to an economical level.
The US Department of Energy publishes recommended R-values for existing homes by climate zone and by assembly — attic, wall, and floor each have different targets because they lose heat differently and are insulated with different practical methods.
Recommended R-values by climate zone
| Zone | Example states | Attic | Wall (cavity) | Floor |
|---|---|---|---|---|
| 1-2 | FL, S. TX, HI | R-30 | R-13 | R-13 |
| 3 | GA, LA, N. CA, NC | R-38 | R-15 | R-19 |
| 4 | VA, TN, OR, KY | R-49 | R-15 | R-19 |
| 5 | OH, IL, NY, CO | R-49 | R-20 | R-30 |
| 6 | MN, ME, MT, WI | R-49 | R-20 | R-30 |
| 7-8 | N. MN, N. Dakota, AK | R-60 | R-21 | R-38 |
R-value per inch by material
| Material | R per inch | Typical use | Notes |
|---|---|---|---|
| Fiberglass batt | R-3.2 | Walls, floors, easy attic joists | Cheap, DIY-friendly; loses value if compressed |
| Blown cellulose | R-3.6 | Attics, closed wall cavities | Fills gaps well; settles slightly over years |
| Blown fiberglass | R-2.5 | Attics | Lighter than cellulose; needs more depth |
| Closed-cell spray foam | R-6.5 | Rim joists, irregular cavities, roof decks | Air-seals as it insulates; highest cost per inch |
| Rigid foam board (XPS/polyiso) | R-5 to R-6.5 | Basement walls, exterior sheathing | Needs taped seams; good for continuous insulation |
How to check what you already have
- In an attic, measure the depth of existing insulation with a tape measure pushed to the joist, then multiply by the R-per-inch of that material to estimate existing R-value.
- In a wall, check a switch plate or drill a small inspection hole in an inconspicuous spot to see whether the cavity is filled and with what.
- For floors over a crawlspace or garage, look up from below — insulation is usually visible between joists, if present at all.
- If insulation is compressed, damp, or has settled well below the joist top, it is under-performing even if the label R-value looks adequate.
Practical installation notes
- Never block soffit vents with attic insulation — use baffles to keep an air channel from eave to ridge.
- Keep insulation clear of non-IC-rated recessed light fixtures and unrated chimney chases; check the fixture's rating first.
- Batts should fit snugly without compression — compressed fiberglass loses a meaningful share of its rated R-value.
- In cold climates, place a vapor barrier on the warm-in-winter side of wall insulation; in hot-humid climates, the logic reverses.
What does R-value actually measure?
R-value is a material's resistance to heat flow — the higher the number, the better it slows heat moving through it. It is additive, so two layers of R-13 stacked together give roughly R-26.
Can I over-insulate an attic?
Practically, no — attics almost always benefit from more insulation up to code-recommended levels. The main risks are blocking soffit vents (which causes moisture and ice-dam problems) and covering recessed lights not rated for insulation contact.
Does insulation R-value differ by climate zone?
Yes. The US Department of Energy publishes recommended R-values by zone because colder climates need much more resistance to heat loss than mild ones — an attic in Minnesota needs roughly twice the R-value of one in Florida.
Is spray foam worth the extra cost over batts?
Spray foam air-seals as it insulates, which batts do not, so it often performs better than its R-value alone suggests. It costs 2-3 times more per square foot, so it makes the most sense in hard-to-seal spaces like rim joists and irregular cavities.
Last reviewed February 2026