Porosity is defined as the amount of holes and inter-fiber pores in a sheet. Paper air resistance is an important property for coated papers; the more porous a paper, the greater is the amount of coating required [16]. Besides fiber morphology and pulp beating, the addition of CNFs has a great influence on the development of this property. Fig. 1d shows the variation of air resistance of extracted and un-extracted samples. No significant differences were observed among the porosity of unbeaten pre-extracted and un-extracted papers (Fig. 1d). However, the beaten sheets presented considerably higher air resistance values compared with the unbeaten ones. The improvement effect of CNFs is much greater than that achieved with pulp beating. It was expected that CNFs would be able to fill pores and voids in the sheet. Air resistance of the unextracted paper filled with 10 wt% CNFs was increased approximately by 46% compared to the corresponding sample without CNFs. This result suggests that either the numbers of pores in the paper has reduced, or their sizes have diminished. This hypothesis was confirmed by SEM micrographs.