Waterproof clothing for use in active sports has a hard job. It must stop you getting wet from the outside but allow the moisture generated by your body to escape. All breathable waterproof clothing will stop you getting wet from the rain but top quality fabrics breathe much better than budget items. Vents help by letting the wind blow moisture away but they're not a subsitute for better fabric. Breathable waterproof clothing will only work if the moisture given off by your body is still vapour as it passes through the membrane. To allow this, you need to wear the right clothing underneath and keep insulation to a minimum to reduce perspiration. Technical base layers and fleece fabrics will transmit moisture away from your skin and let it escape through the outer layer. Absorbent fabrics like cotton will stop this process as will wearing the waterproof right next to your skin. As the moisture vapour passes out through the outer layer, it needs to pass into air - not water. That's why the outer fabric is treated with a water repellant finish, so it stays dry. If this finish gets worn off, you need to replace it.