There is one BIG caution when using S-glass. It has a shelf life of only one year. S-glass uses an organic sizing that degrades with time. Once the fibers are sized, it must be woven into cloth, shipped to the suppier, sold and used within a year. We recently tested some 2.5 year old S-glass roving in our lab. Its compression strength and modulus were 30% below that of E-glass. Owens-Corning verified that the sizing was the likely problem.