PURPOSE: To provide the titled emulsion which is useful for rendering a porous substrate and the surface of lumber, water repellent, and sufficiently satisfy the atmospheric quality standard of the environment protection agency, by including a hydrolyzable silane, emulsifying agent or the like a specific compound, and water.

CONSTITUTION: (A) 1-60 wt.%, based on the composition, of a hydrolyzable silane having a determinable pH-stable range (alkylalkoxy silane, etc., having a molecular weight of not more than 600), (B) 0.5-50 wt.%, based on the component A, of an emulsifying agent having an HLB value of 1.5-20 (preferably 4-17) (including alkylphenol.polyethyleneoxy ethanol, or the like), and its mixture, (C) 0.01-5 wt.% based on the composition, of at least one buffering compound to maintain the pH of the composition within the range of 6-8 (preferably 7.5) (e.g.; sodium hydrogenphosphate, etc.), (D) 100 wt.%, based on the composition, of the water, and a biocide are blended, to obtain the desired emulsion containing less than 400 g/l volatile organic compound when cured.