The only blade that will hold up to cutting fiberglass is Carbide tipped. I use a Lennox Tri-Master, 1/2", 3 TPI. I have cut out at least 100 bows with one blade. Do not buy the 3/8" blade as the blade thickness is too great for your 14" saw and you will experience frequent blade breakage. The blades are pricy, around $150, but well worth it.