Feldon, unfortunately (and I have researched this) porch screening doesn't block thrips as well as products designed for thrip exclusion. The weave is too large.
Thrips aren't even blocked from many commercial insect screening --I have linked a study that shows alot of screening sold is no better than the type of screening you mentioned.
Only two types No Thrips and Bugbed 123 rate high for blocking thrips.
Screening for thrips has such a tight weave greenhouses can run into air circulation problems. Bugbed 123 is a little better in this regard than NO Thrips screening.
http://www.ces.ncsu.edu/depts/hort/f...97/Apr1997.pdf
http://www.green-tek.com/PDFs/Insect.PDF
With regular fiberglass window screening you would get around 50% exclusion as compared to 90% with No Thrips and Bugbed 123. Maybe a little is better than none but I did not want you to think that porch screening would solve a thrip problem completely.