How Much Insulation Do I Need?

If it is single storey, the attic is about 1,500 sq ft; if it is two storeys, the attic is about 750 because it only covers the top floor. That distinction changes the answer by half and is the most common mistake in DIY estimates. At the R-49 typical of much of the country, 1,500 sq ft of attic needs roughly 13.6 inches of settled cellulose. Put your own figures in above for the exact numbers.

Around 1,000 sq ft of attic for a single storey, or 500 if it is spread over two. Walls are the other half of the job and work out at roughly 4 times the square root of the footprint, times wall height — for a 1,000 sq ft single storey that is about 1,000 sq ft of gross wall before you deduct windows and doors.

Both, depending on the material, and mixing them up is how people over-order. Batts and boards are sold by area, so square feet is the unit. Loose fill is sold by bag, and a bag's coverage depends on the R-value you are aiming for — the same bag covers far less area at R-60 than at R-30, because you are laying it deeper. The calculator gives whichever unit applies to your choice.

Measure the settled depth in the attic with a ruler pushed down to the ceiling drywall, then divide by the R per inch of whatever is up there. Old fibreglass that has compressed over decades performs closer to its actual depth than its original rating, so measure what is there rather than trusting what was installed. You only need to add the difference between that and your target.

No, and the recommendations differ a lot. Attics get the highest target because heat rises and attic space is cheap to fill deeply; walls are limited by cavity depth so their target is lower; floors sit in between and matter most over an unheated crawlspace or garage. The zone table above shows all three for every zone.

The attic, almost always. It is the cheapest surface per unit of R-value because you can simply add depth without opening anything up, and it is where the largest temperature difference is. Walls need the cavity opened or drilled and are limited to what fits, which makes them a much larger job for a smaller gain.