4.3.7 Determining the Number of Rolls
In order to determine how much the DIY choice will cost, I must determine how many rolls of insulation are needed in the attic. In this case, drawing a diagram might help. After counting how many joists there were in the attic, I sketched out a rough plan:
Note: the attic door is in the floor and will be covered by insulation. Then, the insulation will later be cut to allow the door to open.
As the insulation fits perfectly between the joists, as well as the frame and outer joists, the amount of insulation needed will be easy to calculate. Recall that one roll of insulation is 300 inches long. The length of the attic minus the frame at each end is inches = 324 inches. One roll between the joists is not quite enough to do the job—there is an extra inches that need to be covered between any two joists.
Activity: How Many Rolls So I Need to Buy?
Let’s determine how many rolls of insulation I will need to properly insulate the attic.
How many gaps are there between the joists? Between the frame and outer joists? Determine how many inches, in total, will be left over, and then see how many lengths of 300-inch rolls will be needed to cover the gaps.
First, we can count that we will need 16 rolls to lie between the joists and two rolls between the frame and outer joists. So far, we will need at least 18 rolls of insulation.
However, we know that each roll leaves an extra 24 inches that need to be covered. This happens 18 times, so we are short inches. Since one roll is only 300 inches, we will need to buy two more rolls of insulation. Thus, we need a total of 20 rolls of insulation.