How Much Insulation Do You Need?
Insulation quantity depends on three factors: the area you're insulating, the target R-value for your climate zone, and the insulation type you choose. Under-insulating leaves money on the table every month in heating and cooling costs. Over-buying means wasted material and money. This calculator nails the right amount the first time.
Understanding R-Values
R-value measures thermal resistance — the higher the number, the more heat flow is blocked. DOE recommendations vary by climate zone and application. Attics lose the most heat (hot air rises) and need the highest R-values. Walls and floors have lower targets because heat transfer is less dramatic through vertical surfaces.
The DOE divides the US into 7 climate zones. Most of the Southeast is Zones 2-3. The Mid-Atlantic, Midwest, and Pacific Northwest fall in Zones 4-5. Minnesota, Maine, and mountain states are Zones 6-7. Your zone number appears on the IECC climate zone map — or just pick the closest description in the calculator below.
Choosing Between Batts and Blown-In
Fiberglass batts are precut rolls that fit between standard studs (16" or 24" on center). They install without renting equipment and are ideal for walls with open cavities or floor joists. The tradeoff: batts leave gaps around wiring, pipes, and blocking unless you cut carefully.
Blown-in cellulose fills irregular spaces and is typically less expensive per square foot for large attics. It requires a blower machine (rentable at Home Depot or Lowe's for about $20/day with the purchase of a minimum number of bags). Cellulose is made from recycled newspaper and has good fire resistance from borate treatment.
Blown-in fiberglass is less dense than cellulose, meaning higher air permeability, but it doesn't settle over time. It's the standard choice for large open attics where you don't need to worry about gaps around obstructions.
Measuring Your Area
Attic: Measure the floor area of the attic (length × width of the house footprint below). Don't measure the sloped roof surface — insulation goes on the attic floor over the top-floor ceiling.
Walls: Measure the total wall area (perimeter × height), then subtract windows and doors (roughly 15 sq ft per door, 10-20 sq ft per window). For exterior walls only — don't insulate interior partition walls.
Floor/Crawlspace: Measure the floor area above an unconditioned crawlspace or unheated basement (length × width of the rooms above).
Materials Needed
Enter area and options above to see results
Pro Tips
- Always add 10% waste to your material estimate — this calculator includes it automatically
- In attics, you can cross-stack batts perpendicular to reach high R-values (e.g., R-30 + R-19 = R-49)
- Wear a P100 respirator, goggles, and long sleeves when handling fiberglass or blown-in insulation
- Keep attic insulation out of soffit baffles — you need 1" air gap from eave to ridge for proper ventilation
- Check your specific product's coverage chart on the bag — actual yields vary by brand and installation method
- Blown-in blower rentals are typically free with purchase of minimum bags at Home Depot and Lowe's
Frequently Asked Questions
Q: What R-value do I need for my attic?
A: The DOE recommends R-30 for warm climates (Zones 1-2), R-38 for mixed (Zone 3), R-49 for moderate and cold (Zones 4-5), and R-60 for very cold (Zones 6-7). Most US homes should target R-49 in the attic.
Q: How much insulation do I need for a 1,000 sq ft attic?
A: For a Zone 5 attic at R-49 with blown-in cellulose: about 84 bags. With fiberglass batts (cross-stacked R-30 + R-19): about 22 packs R-30 plus 14 packs R-19. Blown-in is typically cheaper per square foot for large open attics.
Q: What is the difference between fiberglass batts and blown-in?
A: Batts install without equipment but leave gaps around obstructions. Blown-in fills irregular spaces completely and costs less per sq ft for large attics. Blown-in requires a blower (rentable free with minimum bag purchase). Both are DIY-friendly.
Q: Can I add insulation on top of existing insulation?
A: Yes — in attics, just add new insulation on top of old. If the existing material is dry and undamaged, you don't need to remove it. Use this calculator's "existing R-value" field to calculate only the additional material you need.
Q: Do I need a vapor barrier with insulation?
A: In cold climates (Zones 4-7), kraft-faced batts provide a built-in vapor retarder on the warm side of exterior walls. Unfaced batts need a separate vapor retarder. Attics should not be sealed — they need ventilation. In hot-humid climates (Zones 1-3), the vapor control goes on the exterior side. Check local code.
Q: How do I insulate walls in an existing house?
A: The most common DIY option is drill-and-fill blown-in: drill holes between studs from exterior or interior, blow in dense-pack cellulose, then patch. If you're renovating with open walls, batt insulation is the simplest option. Exterior rigid foam board is the highest-performance wall upgrade but requires professional installation.