My only thoughts on open cell vs. closed cell backer rod is that I hoped I used the right stuff.
I asked the counter guy at the place I bought it and he wasn't sure - no one had ever asked before.
This was a major contractor's supply house - Cook Iron Store - http://www.heavydutystore.com/About_Us_sp_3.html
which has been around since 1907, so I just trusted them that it would work.
I assumed that if they carried it, then no one was complaining about it, so it must be OK. Could be a bad assumption, but at 5-6 cents a foot in bulk instead of the packaged prices I'd seen elsewhere, I put my trust in them and bought it.
I don't know how long it would take to outgas, but it's been 5 months and I haven't noticed any bubbles, although I'll admit I haven't inspected every caulk line on every window. Now you've got me thinking I should, although I don't know what I would do if I found bubbles. It's not like I'm gonna pull all the caulk and re-do it.