Insulation Calculator

How to use the Insulation Calculator

For an attic, use Blown-In: enter the floor area and the R-value you are building up to, and you get bags, settled depth and the coverage each bag has to achieve.

For walls, floors and rafter bays, use Batts: insulation there is cut to fit between framing, so it is bought by area against the coverage printed on the bag.

Override the coverage figure whenever you can. Bags of the same nominal R-value differ between brands and bag sizes by a third or more, and between fibreglass and cellulose by two or three times, and coverage per bag is the input that decides the answer. It is printed on the bag next to the R-value chart.

Insulation Cost compares fibreglass against cellulose at the same R-value, and includes blower rental. That is often free above a minimum bag count, which is worth checking before you price the job either way.

Insulation formula

• bags = area ÷ coverage per bag at the target R-value
• settled depth = target R ÷ R per inch
• cavities = (run ÷ spacing) + 1

Coverage per bag falls as the R-value rises, because you are putting more material on the same square foot. It is not linear in practice, but it is close.

Net coverage per bag, and the depth each R-value needs. Fibreglass figures are Owens Corning AttiCat on its nominal 28.5 lb bag; cellulose is National Fiber loose fill on a 25 lb bag:

Fibreglass is taken at R-2.9 per inch settled and cellulose at R-3.6, both derived from those same charts. Cellulose reaches a given R-value in less depth but takes far more bags and weighs considerably more per square foot.

Bag size is the whole story. These two products are close in performance per pound; the coverage gap is mostly that an AttiCat bag holds 28.5 lb of a fluffier material. Always override with the figure on the bag you are actually buying, it is the input that decides the answer.

Worked example

A 40 × 30 ft attic taken to R-49 with blown fiberglass:

• Area: 40 × 30 = 1,200 sq ft
• Coverage at R-49: 38.8 sq ft per bag
• Bags: 1,200 ÷ 38.8 = 31 bags
• Settled depth: 49 ÷ 2.9 = 16.9 inches

Owens Corning publishes 25.8 bags per 1,000 sq ft at R-49, which is 31 bags for this attic, the same answer.

The same attic in cellulose is 1,200 ÷ 16.0 = 75 bags at 13.6 inches deep. Fewer inches, far more bags, and considerably more weight on the ceiling, which is why the framing matters before the R-value does.

What the result means

R-value is resistance, and it adds up. Six inches of R-19 batt over an existing R-11 gives R-30, not R-19. If there is already insulation up there, you are topping up, not starting over.

Depth is the check you can actually make. Bag counts are easy to get wrong; a ruler stuck in the finished layer is not. Every result here gives the settled depth so you can verify the job after it is blown.

Cellulose weighs about three times what fiberglass does at the same R-value. On a ceiling framed for drywall alone that can matter, check the joist size before choosing on R-value per inch.

Common mistakes

• Using the brochure coverage instead of the bag. Coverage per bag varies by a third between brands at the same R-value. Read the bag.
• Measuring depth before it settles. Blown insulation settles. The R-value on the chart is the settled figure, so blow past the line and let it drop.
• Covering soffit vents. Baffles keep the eaves clear. Blocking them trades an insulation gain for a moisture problem.
• Compressing batts to fit. A compressed R-19 batt is not R-19. Cut it to the cavity, do not stuff it.
• Forgetting what is already there. Existing insulation counts. Measure it, work out roughly what it is worth, and top up to the target.

Frequently asked questions