How Much Insulation Do I Need?

If you are asking how much insulation do i need, the honest answer is a depth, not a bag count. Bags are only how that depth is sold. Every insulation job reduces to three numbers: the R-value you are aiming for, the R-value already in place, and the R-per-inch of the material you plan to add. With those three, the arithmetic takes about a minute. This guide walks through the method, gives you the R-per-inch figures for the common materials, and works a full attic example end to end. If you would rather skip the maths, the insulation calculator does the same calculation from your area, your target and your material.

The one-line method

The whole method fits in one line: depth needed = (target R − existing R) ÷ the material’s R-per-inch. Say your attic should reach R-38, it currently holds about R-11 of old insulation, and you plan to blow in cellulose at R-3.7 per inch. The gap is 27 R-points, and 27 ÷ 3.7 is about 7.3, so you need roughly 7.3 inches of new cellulose, at its settled depth, across the whole attic. Bags come from the volume that depth creates: area × depth. Seven inches or so over 1,000 square feet is roughly 608 cubic feet of material, and the coverage chart printed on each bag turns that volume into a bag count for the depth you chose. The insulation calculator runs both steps for you. The rest of this guide explains where each number comes from, so you can sanity-check a quote or a bag label before you buy.

R-value per inch by material

Materials earn their R-value at different rates, so the same target demands very different depths. These are the standard planning figures, and they are the ones used in every example below.

Material	R-value per inch
Fiberglass batts	3.2
Blown-in fiberglass	2.7
Blown-in cellulose	3.7
Mineral wool	4.2
Open-cell spray foam	3.7
Closed-cell spray foam	6.5

Two lessons hide in that table. Blown-in fiberglass is the bulkiest route to any target: it needs about a third more depth than cellulose for the same R-value, which matters in shallow spaces. Closed-cell spray foam sits in a different league per inch, which is why it gets used where depth is limited, even though it costs far more per R-point installed. For open attic floors, where depth is cheap, the blown materials usually win on price.

Worked example: a 1,000 sq ft attic to R-38

Take a 1,000 sq ft attic that is empty today, in a climate where R-38 is the target. That target comes from the recommended attic levels in our insulation R-value chart: the middle climate zones land at R-38 to R-60, and R-38 is the entry point. With blown-in cellulose at R-3.7 per inch, the depth needed is 38 ÷ 3.7, which is 10.3 inches. That depth spread over 1,000 square feet is about 856 cubic feet of material, and it works out to roughly 80 bags of 30 lb cellulose. That is the whole job: one depth, one volume, one bag count.

Run the identical attic in blown-in fiberglass instead, at R-2.7 per inch, and the depth grows to 14.1 inches, while the bag count falls to about 36, because each fiberglass bag covers far more volume than a dense cellulose bag. Same attic, same target, completely different shopping list. That is why a bag count quoted without a material and a settled depth means very little, and why installers should always tell you the depth they are blowing to, not just the number of bags on the truck.

Topping up what you already have

Most attics are not empty, and you only ever buy the difference. Measure what you have first: the method is in the first FAQ below, but in short, depth × that material’s R-per-inch gives you its approximate R-value. Subtract it from your target and divide by the new material’s R-per-inch, exactly as before. To lift an attic from R-19 to the R-49 recommended for colder zones in the insulation R-value chart, the gap is 30 R-points, and in cellulose that is 30 ÷ 3.7 = 8.1 inches more — noticeably cheaper than insulating from scratch. You can mix materials freely: cellulose blown over fiberglass batts is one of the most common top-ups in existing homes. The one rule is that no vapour barrier should end up sandwiched in the middle of the stack, where it can trap moisture between layers. The painted ceiling below remains the control layer that matters.

Before you add depth

Depth is the last step, not the first. Air-seal the attic floor before anything goes down: foam or caulk the gaps around pipes, wires, duct boots and the attic hatch. Insulation slows heat moving through materials, but it does almost nothing to stop warm air leaking straight through those gaps, and insulation laid over air leaks underperforms its label for the life of the house. Two safety checks belong in the same visit. Keep insulation clear of recessed light fittings unless they are rated for direct contact, because a buried fitting that is not rated can overheat. And if the house still has knob-and-tube wiring, do not bury it: that wiring sheds its heat into open air by design, covering it is a fire risk, and an electrician should assess it first. Once the floor is sealed and safe, add your depth, check it with a ruler as you go, and confirm the final figures in the insulation calculator.

Frequently asked questions