Both latex and Mylar (foil) balloons will deflate over time if not chemically treated on the outside to prevent the internal gas from escaping.
Both untreated latex and foil balloons are not airtight. They both have tiny microscopic holes within the material itself. If the gas molecules inside the balloon are smaller than the holes in the material, the gas will effuse and leave the balloon slowly.
Effusion is the process of molecules going through holes without colliding with other molecules.
The gas molecules inside the balloons move randomly around until they come into contact with something, either another molecule, or in this case, the wall of the balloon. Because the balloon has tiny holes in it, if the molecules are smaller than the holes, instead of hitting the side of the balloon, the molecule will go through the opening.
Speed of Deflation
The rate at which balloons deflate depends on the balloon material itself as well as which gas is used to fill the balloon.
According to Balloons Are Taking Off, a balloon supply company in Scotland, foil balloons inflated with helium will float for weeks but will begin to deflate after about 3 to 5 days.
A latex balloon, also filled with helium, floats for about 17 hours and, if treated, can float for weeks.
Both foil and latex balloons filled with regular air will not float, but they will stay inflated longer than balloons filled with helium. This is because the molecular structure of air is much bigger than the holes inside the balloon material. Instead of passing straight through them, the molecules will bounce off the smaller openings.