The craftsman blog
I have searched for the best and most cost effective solution for sash cord for over 20 years. I've tested various brands and strengths over the years before settling on two favorites.
Sash rope only lasts so long. Quality sash rope can last a long time (in most cases 50-70 years!) so you’ll likely only re-rope a window once in your lifetime. The cheap rope will likely give out in less than a decade.
Get my free downloadable ebook for a simple primer on the tips I have found most useful from decades of this kind of work.