so before I rely on it.... stood in home shower with a Rab Quantum Windtop over a dark cotton t-shirt (to show where water is getting in) and ran the shower head at about the pressure you'd get from a typical English downpour (not quite bouncing, but one of those 4 hour fronts), trying to avoid the collar (intention is have a head solution e.g. hat).
I rotated body and the width of the water jet was about half my chest width so I stood rotating for 20 minutes to synthesise a 5 minute rain shower with rain coming from all angles (e.g. blustery, walking).
Conclusion - it's the zip!
The t-shirt was bone-dry apart from a little where "rain" got under the collar behind me, but a big wet patch beginning at where the smock's zip finished. On looking at the Windtop, the zip is not waterproof, it has a flap behind (so inside) which simply terminates, so that flap is acting as a channel, the "rain" enters at any point on the zip and runs down the channel and pours into your abdomen.
So how to improve.... work on the zip (how?), or lean into the wind with a cap/hat which drips water to below where the zip stops, to reduce what water reaches the zip. Smocks have less of this weakness (zips), but arguably if the zip ran to the bottom to make a channel which ran to the bottom, then the rain would pour out of the fabric, and hit whatever is at the bottom there (shorts). Or.... stitch the zip's flap to the outer fabric and have a "hole" there to empty the channel into the outside? I think in the real-world the zip-flap will not be simply uniform structure so it will drain in all directions over its length so act to make the enter chest+abdomen damp (progressively worse lower down).
The fabric itself (Pertex Quantum) seems waterproof enough in this experiment, the rain did not have sustained pressure to get into the fabric and the seams were low enough for the water to be already rolling down over the seams.
Next.....
I stood in a dry place (i.e. not in the shower) and let the wet fabric dry naturally, jumping a little to synthesise the act of walking to encourage the beads to roll off. The fabric itself dried very fast, in about 5 minutes, but the last parts to dry were the "Pertex" label which was actually wicking water into the seam making the inside of the fabric damp at the seam, but that label is fairly low down and would eventually if in sustained rain drip down the inside of the seam about 6cm. More worrying is the "Rab" label on the chest, it absorbed a lot of water and this is high-up and could wick to the inside high-up, but as it's fairly close to the zip-flap, it's adding water to that front-central area.
How to improve - well cut off the "Pertex" label, and probably paint-on something to the "Rab" label to make it shed (dab a little Nikwax TXDirect?).
I'd conclude the best Pertex garments are label-less and with minimal zips, or better zips (like Rab put on their waterproofs), but it is a £27.50 (from Joe Brown's) garment not intended to be waterproof.
Thanks for the link, still in the market for something thin for my legs.
FYI - the cotton T-shirt (used to find Pertex weakspots) still wet long long after the Pertex windtop finished drying. We all know cotton is bad idea for baselayers, but just a reminder they hold a lot of water which takes time+energy to evaporate.
Not cutting off labels just yet..... may be a warranty return but after some months will prune carefully....
Loony? I didn't document how long it took for Paramo Velez worn over a soaked cotton T to determine its pumping transfer rate. (I just know the T was dry in an hour). That experiment was to know if I could use Velez as a belay jacket if I tried to rough it through a shower when cycling over a steep pass (e.g. Windermere to Patterdale).
No, if I return it for some manufacturing wear. I had a TrekMates equivalent, it simply decomposed in just a few wears. This Rab windproof isn't intended to be used waterproof, just wanted to know its tolerance.
has the seam between the collar and the armpit, i.e. a high seam. I rejected such as seams are a weakspot. Seams need to be lower down on the flat so water is well flowing at that point in its journey. It could be an optical illusion due to how its draped, but in which case I don't see a seam on the chest.
The Rab has the seams at the chest, you can just make it out here at top of the sturnum just above the "Rab" label.
I have similar experiences of the windproofs that I have used in heavy rain. I recently went for a longish run in heavy rain wearing a Montane Featherlite Smock over a polypropylene t-shirt. It coped very well with the rain except around the zip. I wasn't wearing anything to keep the water out of my collar, but there wasn't much getting in down the back so I don't think this was the culprit.
I also use an excellent Helly Hansen Motive jacket and this seems to have less of a problem with the zips, probably due to the storm-flap running the full length on the inside. Water getting in most likely runs down the channel behind the zip. It's a much better fit for me too with a shorter, narrower collar which probably helps. This, and the HH Motive Pants, are my windproofs of choice for winter running as they use a mixture of ripstop and stretch panels that make for an excellent fit. Unfortunately they have been replaced by the HH Mars. It's amazingly light (beats everything else I have seen) and water resistant thanks to a PU coating but less robust and significantly less breathable.
I like the Pertex fabric of the Montane Featherlite, but the cut is too baggy for me unless using it under a full weight fleece (the purpose for which I bought it!). And the zip-pull flaps into my face when I run... so it will eventually get acquainted with my scissors...
It is strange that the few grammes of a storm-flap are missed when they apparently make such a useful contribution to water-resistance.
Can you make a storm-flap to put over the zip? Or, is there anything you can squeeze into the zip to make it more water-resistant?
I have a P***** Fuera Smock, it also has the inevitable zip weakness.
Montane fits, agreed they seem most strange, it's as if their tall models are fatter and their shorter models have no muscles, which is annoying as they have good zips.
Hoods, if not excellent they just act to help water get inside the collar (wicking from the hair, or ingress around the face), so need to be good or don't bother.
Now, you see, you all laughed at my resurrected 25yr old Peter Storm Cagoule, but it's not so stupid... The front zip has a triangular valance behind it, and the bottom of the zip isn't sewn in, so water can drain.
Experiment repeated, this time with an OutdoorDesigns Topptur Lite peaked waterproof cap.
Well the zip problem is reduced, if I aimed for the peak to drip around the zip to produce some protective cover, then no actual ingress via the zip, I could see water coming from the shoulders running down towards the zip but seems it was not as keen to get into the zip, so the zip vulnerability seems more due to direct rain rather than running water past the zip.
However, in this experiment, more water came in via the collar, it could have been the variability of moving head around and a gap between cap and collar, but effectivelty the water that didn't enter zia smock zip just entered the collar instead.
Conclusion? probably needed a pertex head under the waterproof cap to recieve the cap's run-off and keep it going down outside.
Pertex itself seems quite waterproof, but rain just finds the vulnerabilities in the jacket's design.
I wouldn't be too confident about the water repellant properties of the Pertex. I find that shells like this keep the water out pretty well if you don't move around much. Once you start walking, however, the movement (and some rubbing of straps/sleeves etc.) seems to help the water get through the jacket more readily. In windy conditions the water also gets through more easily.
ok thanks for the warning. I got a Pertex outer Montane Prism, seems to be able to cope with downpours, walking in it last night in wet+windy, but that is brand-new, I suspect will lose water-resistance with age and I've not worn it with backpack just yet.
However, I'd rather wear-down cheaper garments, seeking to minimise wearing of waterproofs til required. I now have a Rab Drillium to pack, you're talking twice the cost of a Prism and over 3 times the cost of a Fuera and slightly more expensive than Paramo Velez.
The Rab Quantum is really meant for warm dry days when a cool wind or evening means a t-shirt isn't enough, useful to know it's capabilities against rain, if that occured. I now know it is not be trusted for when chance of showers and where I may be away from cover for some time. It is only 70g I have a Paramo Fuera more for deliberate windproof protection, I know that's not waterproof, it seems less waterproof than Pertex, packs down to slightly smaller than my Rab Drillium. I suspect the lining has a lot to do with water-resistance, something not so wishing to suck water in. I find human hair seems to wish to suck in water, or is the oils from the skin fighting the beading?
I have a Paramo Fuera more for deliberate windproof protection, I know that's not waterproof, it seems less waterproof than Pertex, packs down to slightly smaller than my Rab Drillium. I suspect the lining has a lot to do with water-resistance, something not so wishing to suck water in. I find human hair seems to wish to suck in water, or is the oils from the skin fighting the beading? 
