Cellulose insulation will settle and compress over time reducing the original R-Value performance. As an air-permeable¬ insulation, air movement through cellulose reduces effective R-Value by as much as 50%. Convective currents caused by air movement in wall cavities can cause other problems. Reaching dew point inside the cavity makes condensation a real possibility, in fact, mold found inside envelope walls is often the direct result of bulk water spurred by convective currents present in air-permeable exterior walls.