Insulation Calculator

How it works

R-value measures how well a material resists heat flow. Higher is better. Each type of insulation gives a fairly steady R-value per inch, so the depth you need is the target R-value divided by the R-value per inch of your product.

thickness (in) = target R-value ÷ R-value per inch

bags or bundles = area ÷ coverage per package at that R-value (rounded up)

Manufacturers print a coverage chart on every bag of loose fill and every bundle of batts. It tells you how many square feet one package covers at each R-value. That chart is the most reliable figure for your product, so the calculator asks you for it.

Worked example

You want R-49 over a 1,000 sq ft attic floor with blown-in cellulose. At about R-3.5 per inch, R-49 needs 49 ÷ 3.5 = 14 inches (35.6 cm) of settled insulation, roughly 1,167 cu ft. If the bag chart says each bag covers 15 sq ft at R-49, you need 1,000 ÷ 15 = 66.7, so 67 bags. In metric terms, R-49 is about RSI 8.6.

Typical R-value per inch

Product labels always win over these averages. Batts are sold by their rated R-value, so for batts the bundle label tells you both the R-value and the thickness.

Depth needed for common targets

Blown insulation settles over time. Coverage charts list an initial installed depth and a minimum settled depth. Blow to the initial depth so you still have the full R-value after it settles. Attic rulers stapled to the joists make the depth easy to check.

Batts or blown-in?

Batts and rolls suit open attics with regular joist spacing and few obstructions, and need no machine. They must be cut neatly around wires, pipes and framing; gaps and compressed areas lose a large share of their rated R-value. Blown-in loose fill flows around obstructions and into awkward corners, which makes it a better fit for attics with lots of wiring or irregular framing and for topping up existing insulation. It does need a blowing machine and a second person feeding the hopper.

Topping up existing insulation

Measure what is there and estimate its R-value, then insulate for the difference. For example, if the attic has old 3.5 in batts rated about R-11 and you want a total of about R-49, add about R-38: roughly 10.9 in of blown cellulose. In cold climates the current recommendation is a total near R-60, so you would add about R-49 instead: 14 in of cellulose. Lay new batts across the old ones at right angles, and use unfaced insulation on top so you do not trap moisture between layers.

Practical tips

• Seal air leaks around plumbing stacks, light fixtures, wiring holes and the attic hatch before insulating. Insulation does not stop air.
• Install baffles at the eaves so insulation does not block soffit vents.
• Machine rental for blown insulation is often free or cheap with a bag purchase. It is a two-person job.
• Insulation in walls is limited by stud depth: a 2×4 wall holds R-13 to R-15 batts and a 2×6 wall holds R-19 to R-21.

Safety and code

• Wear a well-fitting dust mask or respirator, eye protection, gloves and long sleeves.
• In the attic, step only on joists or boards laid across them. A foot through the ceiling causes injuries and costly repairs.
• Keep insulation clear of chimneys, flues and recessed lights unless the fixture is rated for insulation contact (IC). Follow the clearances in local code.
• If you find old knob-and-tube wiring, or gray-brown pebbly vermiculite that may contain asbestos, stop and call a professional.
• Minimum R-values are set by local energy codes. Check them before you buy.

Frequently asked questions

Last updated November 8, 2026. See how we check our numbers.