Originally Posted by
Iride01
Bicycle tires will lose pressure. Some quite a bit faster than others. For tubed tires it's more to do with the tube than the tire. Latex typically losing air much faster than butyl. One PSI of air every other day isn't unusual if you keep high pressure in narrow tires. Some say even more.
OP is saying that the tire is getting noticeably softer overnight. That is definitely not normal. 0.5psi a day is not even close to noticeable for a road bike.
I would recommend taking the tube out, inflating it and putting it in a bucket of water. See if there are any air bubbles. But if it's a slow leak, you probably won't be able to see them.
It could be a very small micropuncture in the tube, which can be solved by removing the valve core and putting in some Orange Seal. Run your hand across the inside of the tire to ensure there is nothing there that could repuncture the tube.
It could be that the valve core is faulty or not tight enough in the valve stem.
If it's both of your tires, then something else is up, probably.