A mortgagor in possession of the mortgaged property is not liable to the mortgagee for allowing the property to deteriorate; but he must not commit any act which is destructive or permanently injurious thereto, if the security is insufficient or will be rendered insufficient by such act.

Explanation.—A security is insufficient within the meaning of this section unless the value of the mortgaged property exceeds by one-third, or, if consisting of buildings, exceeds by one-half, the amount for the time being due on the mortgage.