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Insulation Calculator

Insulation Calculator

Insulation needed to reach your target R-value — batts, blown-in, spray foam or rigid board.

How this is calculated

R to add = target R − existing R. Thickness = R to add ÷ R-per-inch of the material (batt 3.2, blown fiberglass 2.5, cellulose 3.5, mineral wool 4.0, open-cell foam 3.7, closed-cell foam 6.5, rigid board 5.0).

Enter the area, the R-value you are aiming for and what is already there. Pick an insulation type and get the thickness required, packs, bags, board feet or boards to buy, plus a material cost estimate.

Results are estimates for planning purposes only. Real-world quantities depend on site conditions, materials and workmanship. Always verify measurements, follow local building codes and confirm quantities with your supplier or a qualified professional before ordering or building.

About the Insulation Calculator

Insulation is one of the highest-return home upgrades, but it only works at the right depth and coverage. This calculator sizes batt insulation (pre-cut rolls) and blown-in loose fill for attics, walls and floors. It converts your area and target R-value into batts, bags and installed depth, so you buy enough to hit the R-value your climate zone calls for.

How to use

  1. Choose the space: attic floor, wall cavities or floor/crawlspace — each has different R-value targets.
  2. Enter the length and width in feet. For attics, measure the floor area, not the roof slope.
  3. Pick batt or blown-in: batts suit open attics and new framing; blown-in suits topping up existing insulation.
  4. Set your target R-value — R-38 is the common attic target, R-13 to R-21 for walls.
  5. For blown-in, check the bag coverage line to convert required depth into bags.

Understanding your results

Batts are counted individually or by roll, matched to 16 or 24 inch framing spacing. Blown-in shows the installed depth in inches for your R-value plus the bags to buy. The R-value line confirms what the installed insulation delivers against your climate zone recommendation.

How the calculation works

Area is length times width. For batts, area divides by coverage per batt or roll at your R-value, rounded up. For blown-in, required depth is target R-value divided by R-value per inch — about 3.2 for fiberglass, 3.5 for cellulose — so R-38 needs roughly 12 inches of fiberglass. Total volume is area times depth, divided by installed volume per bag, rounded up.

Insulation Calculator FAQs

R-38 is the standard attic target across most US climate zones — roughly 12 to 14 inches of fiberglass batts or blown-in. Hot southern zones may target R-30, while the coldest northern zones call for R-49. Check the Department of Energy climate zone map for your ZIP code, since local code may require more than the minimum.
R-value measures resistance to heat flow — higher means better insulation. It is roughly additive: two R-13 batts stacked give about R-26. But R-value only delivers its rated performance at the stated thickness and with no air gaps, which is why compressed batts and gaps around framing underperform. Air sealing matters as much as R-value.
Yes, if the old insulation is dry and in decent shape — adding blown-in over old batts is the most common attic upgrade. Do not add faced batts over existing insulation, since the second vapor barrier traps moisture; use unfaced batts or blown-in for top-ups. Remove any wet, moldy or pest-damaged insulation first.
Use faced batts (kraft or foil facing) on the first layer in walls and attics, with the facing toward the heated living space — it acts as a vapor retarder. Use unfaced batts for every additional layer on top of existing insulation. In hot, humid climates the facing direction can differ, so follow local code.
Yes — seal air leaks first, then insulate. Caulk and foam around plumbing penetrations, electrical boxes, recessed lights and the attic hatch before adding insulation. Air leaks can waste more energy than thin insulation does, and sealing after insulating means digging through it all. Many utility companies offer cheap or free energy audits that find the leaks.
Nabil Sharif, founder of CalcDune

Written by

Nabil Sharif

Founder of CalcDune — a free library of 137 online calculators for finance, health and everyday math. Every calculator shows the formula behind its result, so you can see exactly how your numbers are worked out.

More articles by Nabil →

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