Most rain sheets these days say they’re waterproof breathable, but there are no standards to determine what exactly that means. Many are made using a coating applied on the inside of a nylon or polyester outer shell. This coating can have good w/b properties, but it is used mostly in cheaper outdoor clothing for both horses and humans. The coating can peel away from the outer fabric as it gets rubbed against the ground as the horse rolls, and as it gets dirty the pores in the coating that allow water vapor to escape get plugged with dirt particles.
The finest performance waterproof-breathable items are made with a three layer construction- protective nylon outer layer, inner waterproof-breathable layer consisting of a membrane applied to a light nylon fabric and a lining on the inside of the garment to protect the membrane layer. Not only does this keep the membrane away from dirt and abrasion that will tear it, the three layer construction allows for air circulation between the layers, which facilitates the movement of water vapor from a sweaty horse to the outside of the blanket. The outer layer has a durable water repellant coating which helps keep water away from the membrane, although the membrane does have such small holes water can’t soak through. Additionally, seams in the membrane are sealed so that no water can get through the needle holes in the sewn seam.
Our Premium Rain Sheets constructed in this way are not inexpensive, but they offer the best protection for your horse in rainy weather as being wet from sweat on the inside is just the same as being wet from rain soaking through the sheet!