Measure the height and width of any windows or doors in the wall. Multiply the measurements to find the area of each window or door, and subtract them from the total square footage of the wall. This gives you the square footage of insulation you need for that wall.
How many insulation bags do I need?
On the insulation manufacturers label on every bag you'll find that for an R-25 you'll need 28 bags per 1000 sq ft. So if you have 2100 sq ft, take 28 x 2 = 56 bags for 2000 sq ft. For the last 100 sq ft, take 1/10 x 28 bags = 3 additional bags rounded up.Dec 1, 2013