Insulation Calculator Guide: R-Values, Types & Coverage
Quick Answer
- *R-value = thermal resistance. Higher = better insulation. R-values are additive.
- *Attics need R-30 to R-60. Walls need R-13 to R-21. Depends on climate zone.
- *Calculate area: length × width, minus windows and doors for walls.
- *Fiberglass batts are cheapest; spray foam gives highest R-value per inch (R-6–7).
Understanding R-Value
R-value measures how well a material resists conductive heat flow. The “R” stands for resistance. A wall with R-13 insulation slows heat transfer much more than an uninsulated wall (about R-4 for the framing and drywall alone).
R-values are additive. If you add R-19 batts on top of existing R-11 in your attic, you get R-30 total. This is why attic insulation is the easiest upgrade — you can just pile more on top of what’s already there (as long as there are no moisture issues).
Recommended R-Values by Climate Zone
| Zone | Attic | Wall Cavity | Floor | Regions |
|---|---|---|---|---|
| 1–2 | R-30 to R-49 | R-13 to R-15 | R-13 | FL, HI, S. TX, S. CA |
| 3 | R-30 to R-60 | R-13 to R-15 | R-19 | SE US, N. CA, NV |
| 4 | R-38 to R-60 | R-13 to R-21 | R-19 to R-25 | Mid-Atlantic, Pacific NW |
| 5–6 | R-49 to R-60 | R-13 to R-21 | R-25 to R-30 | Midwest, NE US |
| 7–8 | R-49 to R-60 | R-13 to R-21 | R-25 to R-30 | MN, WI, AK |
Insulation Types Compared
| Type | R-Value per Inch | Cost/sq ft | Best For |
|---|---|---|---|
| Fiberglass batts | R-3.1 to R-3.4 | $0.30–$0.80 | Walls, attics (new construction) |
| Blown-in fiberglass | R-2.2 to R-2.7 | $0.50–$1.00 | Attics, existing walls |
| Cellulose (blown) | R-3.2 to R-3.8 | $0.40–$0.90 | Attics, retrofits |
| Open-cell spray foam | R-3.5 to R-3.7 | $1.00–$1.50 | Interior walls, sound |
| Closed-cell spray foam | R-6.0 to R-7.0 | $1.50–$3.00 | Exterior walls, basements, crawlspaces |
| Rigid foam board | R-3.8 to R-6.5 | $0.50–$1.50 | Continuous exterior sheathing |
Calculating Coverage
For attics: measure length × width of the attic floor. That’s your coverage area. For a 1,200 sq ft attic needing R-38 with blown cellulose, you’ll need about 10.5 inches of depth and roughly 30 bags (each covering about 40 sq ft at that depth).
For walls: calculate total wall area (perimeter × wall height), then subtract windows and doors. A 2,000 sq ft home with 8-foot ceilings and 200 sq ft of windows/doors has about 1,200 sq ft of wall to insulate.
For floors over unconditioned spaces (crawlspaces, garages): measure the floor area directly. Batt insulation is friction-fit between floor joists.
Air Sealing: The Missing Step
Insulation slows heat transfer, but air leaks bypass it entirely. Sealing air leaks before adding insulation can improve energy performance by 15–25%. Key areas: around plumbing and wiring penetrations, at the attic hatch, around recessed lights, and where walls meet the attic floor (the top plate).
The Department of Energy estimates that air leaks account for 25–40% of heating and cooling energy loss in a typical home. Caulk and spray foam for small gaps; rigid foam and tape for larger openings.
Calculate your insulation needs by area and R-value
Use our free Insulation Calculator →Frequently Asked Questions
What is R-value in insulation?
R-value measures thermal resistance — how well insulation resists heat flow. Higher R-values mean better insulation. R-values are additive: two layers of R-13 equal R-26.
What R-value do I need for my home?
Depends on climate zone. Attics: R-30 (Zone 1–3) to R-60 (Zone 6–8). Walls: R-13 to R-21. Floors: R-19 to R-30. Check DOE recommendations for your zone.
How do I calculate how much insulation I need?
Measure total area to insulate (length × width). Subtract windows and doors from walls. Select the right R-value for your zone. Divide total area by the product’s coverage per bag or batt.
Which type of insulation is best?
Fiberglass batts for standard framing on a budget. Blown cellulose for existing walls and attics. Closed-cell spray foam for highest R-value per inch. Best choice depends on application, budget, and existing construction.
Can I install insulation myself?
Batts and blown-in attic insulation are common DIY projects. Spray foam should be professionally installed. Always wear a respirator, goggles, long sleeves, and gloves.