Anyone using these new fiberglass windows with sips? I heard they are a heavier window than vinyl but seem to have better specs. Any input would be helpful.  Thanks

I am building with 4 1/2" urethane wall SIPs. I will be installing Marvin's Integrity Fiberglass windows. There should be absolutely no concern with installing a fiberglass window in a SIP home. Vinyl and aluminum also work fine. Be sure you have 2x bucking installed in all window and door opening. And finally, be sure you review your window Rough Opening dimensions with your manufacturer (assuming you're getting pre-cut panels). Triple check each dimension because if you're off, field modification will be time consuming.

I used fiberglass windows in my new sip house and found them to be great. they are very square and are stronger than wood and vinyl. From what I understand many window makers like Pella are working on a line of fiberglass as well, I also used the Marvin Integrity double hung windows and that would be my first choice if I built again. Vinyl is a thermoforming plastic so if it gets hot it moves and bends where as fiberglass is a thermosetting plastic that cures( with a hardener) and don't change until it burns. I never saw Chevrolet make a Corvette out of vinyl.

A couple of other things. If, by chance, the rough openings are too small, it is better to find this out BEFORE putting the 2x bucking in. It is much easier to cut the panel without the bucking (take it from a guy who didn't on a couple of windows). Also, if you get jamb extensions already on the window, make sure they are the correct width. I have 6.5 " walls but told the window distributor it was 6" walls. I had to make half inch jamb extensions on all my windows. Pretty stupid on my part.