i'll start with hydrostatic head. higher is supposed to be better for waterproofs and many choose these on the basis but seem to be quite happy to spend the night in a tent with a comparatively very low hh.

so, does it really matter to have such high hh for clothing?

Only for areas of clothing that may be subjected to pressure; shoulder areas, seat, knees, etc.

A tent flysheet isn't subjected to such weight-induced pressure.  The groundsheet might be, but hopefully, the ground under the groundsheet should be fairly dry...

HH values for fabrics used in clothing are probably far in excess of what they need to be.  I can't remember what the BS* figure for 'waterproof clothing' is, but it's something like 1500mm or 3000mm.  Not 20m.

* British Standard, that is, not b*llsh*t...

The pressure that's going to force water through the fabric isn't caused by weight, but by slowing your raindrop down from 30mph to zero in not very far.

Working out what the pressures are, bearing in mind the deformation of the raindrop and the movement of the fabric under the impact is a non-trivial task that I'll leave up to you. 

> The pressure that's going to force water through the fabric isn't caused by weight

It is when you're sitting, kneeling or wearing a heavy rucksack.

I'd venture that in all these cases, the pressure is higher than that caused by raindrop impacts.  Tents get raindrop impacts, too...

the sciecne bit of rucksack mesh backs - do they really make a difference for a "cooler" back or is it mostly hype.

does a mesh back with a t shirt merely let you decide between a very wet back and an exceedingly wet back.

once you wear it over a shell, is there any real difference?

There are a whole range of waterproof fabric tests; many of them with European standards associated . I used to have a very good pdf explaining them and their relative merits but I can't find it anymore.

This link is almost as good though:

http://www.bicycleapparel.com/RainTechArt2.html

read this one too for more industry info from Patagonia via the very-informative Mr Verber:

http://www.verber.com/mark/outdoors/stash/patagonia-testing.html

Anyway, the list includes the well known Hydrostatic Head test,  the Rain Tower test, Bundesmann test and the particularly revealing centrifuge test in falling rain to analyse how a fabric reacts to high-speed droplets.

Tests are conducted on standardised samples of the fabric (HH, Bundesman & its pin-prick alternatives etc) or on garments in a Rain tower.

What my long-lost pdf said was that some fabrics scored highly on the HH test but were poor contenders in the centrifuge. Likewise, the pin-prick recovery of certain fabrics (presumably P***** style stuff) was far better than of others meaning that they might not be as waterproof new, but abused for a while and they'll still be doing very well.

Unfortunately, the market as a whole isn't using standard tests except for HH. Not even for breathability and some of the MVT (moisture vapour transfer) rates claimed on fabrics (in ml per metre-square per 24hrs) are massively exagerated. 

The rain tower or 'cloud burst' tests are especially revealing of poor design and construction and I think that this would be more useful to the average punter than the HH info.

Here's a possibly related question:

You're walking hard in a windshirt, say an uncoated pertex one,  and a layer or two underneath. It starts to rain. Should you take your windshirt off before you put your waterproof on, or leave it on?

I usually find it  more comfortable to take it off, but why should that be?

ALS,

under a waterproof, a wind shell is only adding to the resistance to vapour leaving your body. It doesn't have much intrinsic insulation beyond it's ability to stop the wind from removing all the warm air from your insulation layers. Once the waterproof is over the top, this benefit is lost. In fact, the wind shell could also restrict the movement of air inside the jacket which would reduce it's ability to vent also.

Chris,

indeed MVT isn't very helpful but I suspect the units have been chosen by the textile industry to give 'BIG NUMBERS' to stick on the labels. If you took a typical claimed MVT of, say, 10 000 ml per m2.day and converted it into something more honest, like ml per hour over a typical torso it would be stated as nearer 400 ml/hr per jacket. A person puffing up a hill in mild, damp weather could easily produce double that in sweat so it's obvious why the jackets can't cope and will eventually saturate. Moreover, their MVT is measured under some very unrealistic lab conditions and represents a best case value IMO.  It's also why I still look for jackets with good venting capability even if it does mean that they might let in a few drips and splashes. At least vents can be closed!

John, I often wear a windshirt under a waterproof and find that it provides just the right amount of insulation. It doesn't seem to increase the amount of condensation. Condensation that does form tends to stay between the windproof and the waterproof, keeping inner layers dry.

I agree on MVT - the figures are designed to look impressive but are not actually very informative as to real world performance. 

Chris,

I sometimes wear a windshirt under my waterproofs (over a thin baselayer) because I also run hot. But I think ALS was implying that wearing it over other insulation layers wasn't comfortable in this context. I can understand that as fleeces etc need to allow air to escape in order to pass vapour. How much difference it will make is probably totally personal anyway, and I suspect that some waterproof breathable fabrics (Gore Tex Paclite ?) might even benefit from the buffer that pertex would provide.

John: I usually find it noticeably clammier if I keep the windshirt on - like you suggest, it seems to inhibit the ability of the system to vent/breathe -  whatever you want to call it. This may be exacerbated by the fact that, since it has been rained on, it will be wet to some extent, and that dampness will be trapped inside too. Usually I will just have a base layer underneath. Trouble is, if I do take my windshirt off, I find I am much less likely to brave the elements if I later on need to add another layer.

Most people don't realize that Pertex's DWR is similar to EPIC's but applied in a different process.  Both terribly inhibit the breathability of the fabric, despite 99% of people exclaiming how breathable Pertex is.  In reality, it's simply more breathable than other forms of similar DWR-coatings that impregnate a polyamide in the fabric, but still not-very-breathable on my scale.

Pete2s, which type of Pertex are you talking about? Pertex is a family of fabrics not just one fabric. Pertex Microlight is very breathable, Pertex Quantum is very breathable, Pertex Endurance is very breathable.

The DWR on Pertex is nothing like the process of encapsulation with EPIC treated fabric. EPIC is not a DWR but the coating of each fibre with silicone. I've found EPIC treated fabrics less breathable than untreated Pertex but far more water resistant.

I find my Montane Lightspeed helpful under my Paclite - condensation inevitably forms, but does so between the two. Not so good under a Paramo though - it maybe hinders the wicking of the Paramo liner - tends to leave me sweatier than the Paramo would do.

What I'd like to know is, given the usual foam padding in footwear, and the ubiquitous Gore-tex that always starts leaking after a while (I'm inclined to ask, why bother?), why doesn't anyone use the Paramo analogy fabric? 

Bill, I've found the same. The Litespeed is my favourite windshirt and I've found it comfortable under Paclite, XCR, eVENT and other fabrics but not Paramo. Usually I just wear a base layer under a Paramo jacket unless it's windy and below freezing when I'll wear either a mid or heavyweight base layer or a light fleece.

I don't know if Nikwax Analogy would work in footwear. You couldn't just sandwich it between two other layers as  you can with a membrane but if it was exposed it would wear very quickly.

CB,

 IIRC someone from Paramo said a while ago they were looking to use the materials in footwear (and gloves) but there were problems.

Prehaps if Skippy or Nick sees this they could upate us - (IIRC of course).