No nice scientific evidence here I'm afraid, but a few thoughts based on my own experience.
Nights in the piney woods have been in very short supply recently, so I've spent some time recently improving my back yard hanging opportunities and decided to try it out last night. Now we have had some heavy rain here over the past few weeks and the ground is saturated. I set my tarp and hammock and yesterday afternoon, ready for the evening, and a thunderstorm rolled in for the early part of the evening. By the time I crawled into the hammock it had been raining hard for three or four hours, but the inside of my tarp was dry and there were no droplets of rain on my hammock or quilts.
My tarp was pitched quite low with pullouts on both sides. The hammock RL was only inches away from the tarp RL and I have to admit, I was a bit too close to the inside of the tarp, but nothing was touching.
I slept great and was toast warm with temps down to around 4*c and only the slightest breeze, but whilst I was lying there contemplating getting up for a coffee in the morning, it started pouring again. The rain was coming straight down and hard, causing small droplets to fly of the inside of the tarp. It felt like a tingling on my face and was soon visible on top of my TQ. Within ten minutes I was thinking that this could be a show stopper on a multi day trip, because my gear was getting a soaking.
A quick search on here shows a number of similar threads questioning the waterproof ability of silnylon and others claiming it just condensation. My experience showed that there was no misting or wetting of the hammock during the thunderstorm before I climbed under the tarp, and there had been hours of rain, enough to saturate the outside of the tarp. If water was going to pass through the silnylon, then I think it would have happened at that stage. Checking the inside of the tarp after I was up an about showed that the most moisture inside had accumulated in the top corners by the doors and on the doors themselves. There was also more condensation in the depressions at the side pulls where the pitch of the fabric was shallower.
My tarp is a fairly new superfly and hasn't had any abuse, so for test purposes it's as good as new. I would say the water falling from the inside of the tarp had to have been condensation being released by the the percussive action of the rain on the outside of the tarp. Why was it so bad last night? Well I think the saturated ground, low temperatures and lack of any reasonable breeze to promote circulation had caused more condensation than normal. I also think that hanging in the back yard without a canopy of trees above meant the rain was hitting my tarp that bit harder. Nothing suggests that any water passed through the silnylon, but the amount of what appeared to be rain passing through left me wondering.
Some threads have suggested that sil sometimes suffers from misting, and this doesn't happen with PU coated tarps, but surely condensation would form regardless of the tarp fabric. My only thought would be that PU is stiffer and perhaps better at absorbing the percussive affect of heavy rain. Sil is also pitched tighter then PU, giving the fabric a bit more 'bounce' than PU, which might stop condensation bouncing off the inside.
As I said at the start, I have no scientific evidence to back up either theory, but I'm happy to believe that it's just condensation and that the conditions last night made it worse than normal.
If anyone else is worrying about the waterproofness of their silnylon tarp, I hope this helps. For everyone else, sorry for the long winded thread