Paper having a high porosity of the material. In the form of paper fibers, a sizing agent, a filler constituted only a small part, while the rest is occupied by air. Leavening of paper, accounts for about 60-70% of the gap as much as the high density glassine paper (glassine paper), only 15-35% voids. Paper substantive components and void volume ratio affect the paper's hardness, compressibility, elasticity of the absorption liquid, ink, oil and water capacity.

     The spatial paper mainly by: 1 volume of air voids, such gaps may be accessible by the internal surface of the paper sheet; 2 interstices between the fibers; 3 empty, completely through the thickness of the layer of paper space. With high density of voids greaseproof paper and filter paper high compared to most of the uncoated printing and writing paper between the porosity between the above two. In addition, the porosity of coated printing paper and uncoated printing paper with a lot less. The permeability of paper and paper processes. The same process, short-fiber paper out of the paper fibers longer even more closely and low porosity. As a filler in paper occupies pores between the fibers, so the lower permeability. Influence the degree of beating the air permeability of paper hugely increase the degree of beating fiber link between the increased fiber and structure of the paper even more closely, and therefore reduce the permeability of the paper and the paper density. The glue on the surface of the paper, glue on the surface of the pore is blocked, so the porosity also decreased significantly. Wet paper machine press section of the paper will affect the pressure treatment porosity. Paper by calender treatment, since the paper even more closely, hence will reduce the air permeability. Containing mineral pigments applied to the printed paper and will significantly reduce the permeability of paper so the paint layer breathable for printing great influence.