Attic Insulation Calculator

What is an attic insulation calculator?

An attic insulation calculator turns a thermal goal into a shopping list. You give it the size of your attic floor, the R-value you are aiming for, and the R-value already up there; it returns the settled depth of loose-fill (blown-in) insulation you need to add and the number of bags you have to buy to get it.

R-value is the standard US measure of thermal resistance: the higher the number, the more the material resists heat flow. Insulation is sold by the bag, but specified by R-value, so the arithmetic that connects the two — depth per point of R, square feet per bag — is exactly the part that is easy to get wrong at the store.

How does it work?

The calculator works in three steps.

1. Attic area. A rectangular attic floor $L$ long and $W$ wide has area

$$A = L \times W$$

2. Required depth. Every insulation material adds a fixed amount of R-value per inch of settled depth, written $R_{\text{inch}}$. To climb from the R-value already in the attic, $R_{\text{existing}}$, to your target, $R_{\text{target}}$, you need

$$d = \frac{R_{\text{target}} - R_{\text{existing}}}{R_{\text{inch}}}$$

inches of new material. If the attic already meets or beats the target, $d$ is zero — you need nothing.

3. Bags. Each bag of loose-fill covers a stated number of square feet at one inch of settled depth. That figure, the bag yield $Y$ (in ft²·in), is what appears on the coverage chart printed on the bag. The bag count is the total volume of insulation divided by one bag’s yield, rounded up, since bags come whole:

$$\text{bags} = \left\lceil \frac{A \times d}{Y} \right\rceil$$

Material figures used

The R-values per inch are the standard published figures for loose-fill attic material (blown fiberglass sits in the R-2.2–2.7 band, blown cellulose in the R-3.2–3.8 band). The bag yields are typical values for a standard bag of each material. Treat the bag count as an estimate: bag weights and coverage charts differ between manufacturers, so once you have picked a product, check the coverage chart on its bag and adjust.

Worked examples

A 40 ft × 30 ft attic to R-38 with cellulose. The area is $40 \times 30 = 1200 \text{ ft}^2$. Starting from bare joists ($R_{\text{existing}} = 0$), the depth is $38 / 3.5 = 10.857$ inches. The bag count is

$$\left\lceil \frac{1200 \times 10.857}{36.6} \right\rceil = \left\lceil 355.97 \right\rceil = 356 \text{ bags}$$

The same attic with blown fiberglass. Fiberglass is a weaker insulator per inch, so it needs a deeper layer: $38 / 2.5 = 15.2$ inches. But its bags cover far more ground, so the count drops:

$$\left\lceil \frac{1200 \times 15.2}{110} \right\rceil = \left\lceil 165.82 \right\rceil = 166 \text{ bags}$$

Topping up an attic that already has R-11. Only the shortfall matters. With cellulose, $(38 - 11) / 3.5 = 7.714$ inches of new material closes the gap, and

$$\left\lceil \frac{1200 \times 7.714}{36.6} \right\rceil = \left\lceil 252.93 \right\rceil = 253 \text{ bags}$$

Practical notes

• Settled depth, not blown depth. Loose-fill compacts after installation. The depth above is the settled depth that actually delivers the R-value; blow slightly deeper so it settles to target, and use the depth markers stapled to the joists to check as you go.
• Estimate what you already have. If you do not know your existing R-value, measure the depth of the old insulation and multiply by that material’s R-per-inch — the same table above works in reverse.
• Match the target to your climate. R-38 is a common attic target for mixed climates; colder zones typically call for R-49 to R-60, warmer ones less. Check the recommendation for your region before you buy.
• Do not bury what needs air. Keep insulation clear of recessed lights not rated for contact, and keep soffit vents open with baffles, or you trade an energy saving for a moisture problem.
• Pair it with related tools. Confirm your floor area with the square footage calculator, and plan the rest of the room with the drywall calculator or the paint calculator.

Frequently asked questions

How deep is R-38? It depends on the material. In blown cellulose (R-3.5 per inch), R-38 is $38 / 3.5 = 10.857$ inches. In blown fiberglass (R-2.5 per inch), the same R-38 needs $38 / 2.5 = 15.2$ inches.

Fiberglass or cellulose? Cellulose packs more R-value into each inch, so it suits attics with limited headroom. Fiberglass covers far more area per bag and does not absorb moisture as readily. Both reach the same R-value — they just get there at different depths and bag counts.

Can I add new insulation on top of old? Yes. Loose-fill can be blown straight over existing insulation, and R-values add up: the calculator’s existing R-value field is exactly for this, so it only sizes the material needed to close the gap.

Why does the bag count round up? You cannot buy part of a bag. A 355.97-bag job means 356 bags — and the fraction left over is a useful margin, since real-world coverage rarely matches the chart exactly.