Insulation Calculator — How Much Insulation Do I Need?
Enter your area, insulation type, and target R-value to get the bags or packages needed.
How to Calculate Insulation Needed
Insulation coverage isn't just about square footage — it's about square footage at a specific thickness (R-value). The same bag of blown-in insulation covers far more area at R-30 than it does at R-49, because higher R-values require a thicker, denser layer.
Calculate your area (length × width), choose your insulation type and target R-value for your climate zone, and the coverage-per-bag figure for that specific combination determines how many bags or packages you need.
Recommended R-Values by Area
| Area | Recommended R-Value |
|---|---|
| Attic (most US climates) | R-38 to R-60 |
| Exterior walls | R-13 to R-21 |
| Floors over unheated space | R-25 to R-30 |
| Basement walls | R-10 to R-19 |
Colder northern climate zones need the higher end of each range; warmer southern climates can typically use the lower end. Check your local energy code for the exact requirement in your area.
Coverage Per Bag by R-Value
Coverage per bag drops significantly as your target R-value rises, since a thicker layer means less material stretches across the same bag.
| R-Value | Blown-In Fiberglass | Blown-In Cellulose |
|---|---|---|
| R-30 | ~32.5 sq ft/bag | ~27 sq ft/bag |
| R-38 | ~25.7 sq ft/bag | ~21 sq ft/bag |
| R-49 | ~19.7 sq ft/bag | ~16 sq ft/bag |
| R-60 | ~16 sq ft/bag | ~13.5 sq ft/bag |
Example: a 1,200 sq ft attic targeting R-38 with blown-in fiberglass needs about 1,200 ÷ 25.7 ≈ 47 bags. The same attic at R-49 needs about 61 bags — 30% more material for the higher target.
Common Insulation Mistakes
- Compressing batt insulation to fit a shallower cavity. R-value depends on the insulation reaching its full loft (thickness) — cramming an R-19 batt into a 2x4 wall cavity it wasn't designed for crushes the fibers and cuts real-world performance well below the rated value.
- Skipping air sealing before adding insulation. Insulation slows conductive heat transfer, but it does nothing to stop air leaking through gaps around wiring, plumbing stacks, and the attic hatch. Sealing those gaps first is often more cost-effective per dollar than extra insulation depth.
- Covering soffit vents with attic insulation. Blown-in or batt insulation pushed all the way to the eaves can block the soffit vents that let outside air into the attic, causing moisture buildup and reduced ventilation. Use baffles to keep the vent path clear.
- Buying by bag count instead of coverage area at your target depth. Blown-in insulation bags list coverage at a specific depth — the same bag covers less area at a higher R-value (deeper fill). Check the bag's coverage chart for your target depth, not just the total square footage of your attic.
- Ignoring vapor barrier placement in the wrong climate. A vapor barrier belongs on the warm-in-winter side of the wall — installing it backward (or adding one where your climate and wall assembly don't call for it) can trap moisture inside the wall cavity instead of keeping it out.
Frequently Asked Questions
What R-value do I need for my attic?
Most US climate zones recommend R-38 to R-60 for attic insulation. Colder northern climates need the higher end; warmer southern climates can use R-30 to R-38.
How many bags of blown-in insulation do I need for an attic?
Coverage per bag depends on your target R-value and insulation type — a bag of blown-in fiberglass covers roughly 32 sq ft at R-30, but only about 20 sq ft at R-49, since more material is needed per square foot at higher R-values.
Is blown-in or batt insulation better for an attic?
Blown-in insulation fills gaps and irregular spaces around joists and wiring more completely, making it the more common choice for attics. Batt insulation is easier to install without special equipment and works well for open, regularly-shaped wall cavities.
Do I need to remove old insulation before adding new insulation?
Not usually. New insulation can typically be added directly on top of existing insulation, as long as the old material is dry and not damaged by mold or pests.