Poured concrete foundations develop cracks. That is just the nature of the beast. When these cracks go all the way through the wall and allow water to penetrate, then you need to act. There are 2 ways to seal foundation cracks.
1) Exterior Multi-Layer Crack Repairs
Whenever possible, it is always best to seal a foundation crack from the OUTSIDE.
By doing this, you are:
a) working with physics by allowing the water pressure to push sealant INTO the crack.
b) providing a multi material covering that has increased tolerance to the acid inherent in soil.
c) building in fault-tolerance by sealing a much larger area than just the crack location, so when the wall shifts and the crack widens, you are still protected.
2) Interior Epoxy/Urethane Injection
Sometimes, you cannot access the foundation crack from the outside. When this is the case, Interior Epoxy or Urethane injection is an option. This allows us to fill the crack using a low pressure injection system that fills the cracks from the inside out. Epoxy injection does not expand after it has cured. Urethane injection can expand, but only up to 20% after curing. That means if the wall moves and the crack widens significantly, the injection repair will have to be done again. This is why we always recommend performing EXTERIOR crack repairs whenever possible.