The layering of the running jacket depends on what you need. They come in 2-layer, 2.5-layer, and 3-layer options.
3-Layer Materials
The 3-layer rain jackets are the most durable and the most waterproof. They consist of an outer layer of material, a middle waterproof membrane, and a soft inner layer.
The outer layer stops the waterproof membrane from getting damaged due to abrasion, while the inner layer prevents sweat and body oils from reducing its effectiveness.
2.5-Layer Materials
2.5-layer jackets are similar to 3-layer jackets, with an outer layer to protect the waterproof membrane from damage.
However, there is only a light coating on the inside of the waterproof membrane, which still protects it from body oils and perspiration but is less robust and slightly more lightweight.
2-Layer Material
Although they still last, the 2-layer running rain jackets are the least durable. They have an inner layer and a waterproof membrane only.