You think you need to add insulation, but you don’t know how much to add to achieve the recommend R-value for your area. Here’s how to figure it. Let’s use an example where we have 6″ of cellulose covered by 6″ of fiberglass batts in the attic. We take the R-value of cellulose, which is 3.5 and multiply it by 6 to get 21.0. We then take the R-value of fiberglass batts, which is 3.2 and multiply that by 6 to get 19.2. Since the insulation is layered one on top of the other, we add them together 21.0 + 19.2 to get 40.2.
If we live in a region where the recommended R-value is 38, we already have 40.2, so we do not need to add insulation. What happens though, if we live in a region that recommends 49–we need to add some insulation, but how much? That’s easy too! Take the recommended R-value, which is 49, and subtract what we have already, which is 40.2 (49 – 40.2 to get 8.8). We need to add an R-value of 8.8. The R-value of an inch of fiber glass batts is 3.2. Divide the amount we need to add, 8.8, by the R-value per inch, 3.2, to get 2.75. Batt and blanket insulation comes in several thicknesses. One of these is 3-1/2″. So one layer of 3-1/2″ fiber glass batt insulation added to what we have will give us a little more than what we need. It is always ok to add more insulation than is recommended. Just remember not to pack it too tightly because packing it can reduce its effectiveness.