# 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.

### Comment

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.