π Insulation Calculator
Target R-value, blown-in bags, and added depth for your attic or floor.
Blown-In Bags to Add
Estimates for planning only. If your existing insulation is wet, moldy, or vermiculite, have it assessed before adding more.
How to Use This Calculator
Enter the attic or floor area in square feet and your best guess at the current R-value (R-19 is about 6 inches of fiberglass batts, R-30 about 10 inches). Pick your climate zone and the calculator fills in the Department of Energy recommended target, which you can override. The result tells you how many blown-in bags to add and how much depth that adds.
Measure the attic floor area in feet and multiply length by width. Measure the existing insulation depth in a few spots and convert to R-value at roughly R-2.5 to R-3 per inch for fiberglass.
Pick your climate zone from the map on the DOE website (roughly: zones 1 to 3 are the South, 4 to 6 the middle band, 7 to 8 the far North). The recommended target fills in automatically.
Read the bags to add. Bag coverage is based on about 40 square feet per bag at R-38, scaled to your target. Buy a few extra bags; attics always have obstacles that eat coverage.
Check the added depth against your attic: make sure you keep soffit vents clear with baffles, and that the hatch still opens.
The R-Value Math
R-value measures resistance to heat flow, and R-values add: R-19 batts plus R-30 of blown insulation gives you R-49. The bag math works from a standard coverage figure: one bag of blown fiberglass covers about 40 square feet at R-38, and coverage scales linearly with R-value because doubling the R-value means doubling the depth. The depth estimate uses R-2.5 per inch, typical for blown fiberglass.
Climate zone recommendations come from the US Department of Energy: R-38 for zones 1 to 3, R-49 for zones 4 to 6, and R-60 for zones 7 and 8. These are minimums for cost-effective energy savings, not maximums. If your target is below the recommendation, the calculator flags it so you can reconsider before buying.
Worked Example
A 1,000 sq ft attic in climate zone 5 with R-19 batts currently installed. Target is R-49, so R to add = 30. Bags for R-49 = 1000 x 49 / 1520 = 32.2. Existing equivalent = 1000 x 19 / 1520 = 12.5. Bags to add = 19.7, rounded up to 20 bags. Added depth = 30 / 2.5 = 12 inches of blown fiberglass over the existing batts.
Smaller job: a 600 sq ft attic in zone 3 with almost no insulation (R-11) targeting R-38. R to add = 27. Bags for R-38 = 600 x 38 / 1520 = 15. Existing = 600 x 11 / 1520 = 4.3. Bags to add = 10.7, rounded to 11 bags. Added depth = 27 / 2.5 = 10.8 inches.
Tips for the Job
Air seal first, insulate second. Spend a Saturday with cans of spray foam and caulk closing every penetration through the attic floor: plumbing vents, wire holes, recessed light housings (use proper fire-rated covers), and the gaps around the chimney (with metal flashing and high-temp caulk). This work is invisible once insulation goes in, so do it while you can see and reach everything.
Protect your ventilation. Install rafter baffles at every soffit vent bay before blowing insulation, so the new depth cannot block airflow from the eaves. Mark your target depth on the baffles or on sticks placed around the attic so you can verify coverage as you blow. And wear a real respirator, not a dust mask: blown fiberglass and cellulose fill the air with fine particles.
Frequently Asked Questions
What R-value should my attic have?
The US Department of Energy recommends R-38 to R-60 for attics in most climate zones: R-38 for warm zones 1 to 3, R-49 for mixed zones 4 to 6, and R-60 for cold zones 7 and 8. This calculator sets the recommended target automatically from your climate zone. More insulation has diminishing returns past the recommendation, but air sealing gives returns at any level.
Is blown-in or batt insulation better for an attic?
Blown-in insulation wins for attics: it fills gaps, flows around wires and pipes, and has no seams for heat to bypass. Batts work well in open, unobstructed spaces like new-construction walls and floors, but in an attic they leave gaps at every obstacle. Most pros blow attics and batt walls.
Can I add new insulation over old insulation?
Yes, as long as the old insulation is dry and in decent shape. Lay new batts perpendicular to the joists so the seams do not line up, or blow new insulation right over the old. The one exception: if the old insulation is wet, moldy, or vermiculite that may contain asbestos, do not cover it. Get vermiculite tested before you disturb it.
Does compressing insulation reduce its R-value?
Yes. Insulation works by trapping still air, and compressing it squeezes the air out. An R-38 batt stuffed into a 6-inch cavity performs closer to R-20. Buy the batt thickness that matches your cavity, and never step on or store boxes on blown insulation: every footprint is a permanent cold spot.
Should I air seal before adding insulation?
Absolutely, and it is the highest-return work you can do. Insulation slows heat flow; it does not stop air flow. Seal around plumbing and wire penetrations, top plates, recessed lights (with proper covers), and the attic hatch with caulk or foam before you insulate. A leaky attic with R-60 performs worse than a sealed attic with R-38.