A wall's R-value refers to its level of insulation. Construction materials are given an R-value, and a wall's total insulation level is determined by adding up the individual R-values of the wood, drywall, insulation and other materials in the wall. R-19 to R-21 is generally considered a well-insulated exterior wall. Achieving an R-21 rating in a 2x4 wall is simpler when the wall is first being constructed so you don't need to add more insulation later on.
Things You'll Need
R-21 batt insulation sheets
Sharp utility knife
Put on gloves to protect your hands from the insulation. Measure a piece of R-21 batt insulation with a tape measure and mark the 2x4 dimensions of the sheet.
Cut the insulation with a sharp utility knife to reduce its size to 2x4. A sheet that's too large warps in the wall space, while a piece that is too small lets air through and lowers the effectiveness of the insulation.
Push the piece of insulation into the 2x4 space between the wall joists. The piece should fit inside the space tightly but comfortably without bending. The wall can then be finished once the insulation is in place.
While the surrounding materials have their own R-values that add to a wall's insulating value, that value is reduced by small leaks or gaps between those materials. Stick with the R-21 insulation and not a lower value to help guarantee the proper insulating value for your wall.
Always wear protective gloves when cutting or handling insulation to avoid cuts or skin irritation.