Hydrostatic Head Explained - Hydrostatic Head (HH) is a way of measuring how waterproof a fabric is. The test is completed using a long tube that will hold water - the piece of fabric under test is fastened across the open end at the bottom. The tube is then filled with water until water can be seen to penetrate the fabric - at which point the length of water in the tube is read off - for the best fabrics it can be as high as 20,000mm.
Applying this to Garments - In the UK manufacturers are allowed to claim a fabric is waterproof if the HH is 1500mm but most jackets exceed this comfortably and figures of 10,000 upto 30,000mm are not uncommon.
Fabrics for use in garments require higher levels of waterproofing because garments are subjected not only to driving rain but also to pressure applied by straps and hipbelts of rucksacks. A HH figure of 10,000 will deal with most conditions, 20,000 is excellent, 30,000 is only required for the most demanding conditions. A higher HH does not guarantee a better garment since breathability is usually a compromise when water proof performance is maximised.
MVTR Explained - MVTR stands for moisture vapor transmission rate. It measures the rate at which moisture permeates through a fabric measured in grams/meter /day. As it turns out, this is a very hard measurement to make and therefore figures should be used as a rough guide only. The difficulty of making the measurement is why some manufacturers won't quote figures, a comparison between an MVTR of 10,000 and 12,000 is probably meaningless, though a jacket with MVTR of 20,000 will be more breathable than one with 10,000 MVTR. Some fabrics are rated 30,000 which is very high indeed.
Applying this to Garments - Using MVTR as a starting point garment design will also have an impact on overall breathability. The main culprit is pockets, extra layers of fabric compromise breathability heavily, though mesh lined ones less so than pockets made from the same fabric as the jacket outer. Seams also play an important part, seams represent a concentration of layers of fabric which will not be breathable like a single layer of fabric, and the seams are are then 'taped' which reduces the breathability further. Breathability is enhanced when venting features are present such as Pit zips.
Compare the overall weight - Waterproof & Breathable fabrics are usually made in layers, the outer layer is made from a woven nylon based fabric to which is 'bonded' a waterproof/breathable membrane layer. A third layer is added to the inside of the jacket to protect the membrane from dirt, sweat and damage. A good guide to durability is the weight of the jacket itself - since durability is derived from the outer nylon layer a heavier outer fabric will result in a more durable jacket - though remember when comparing two jackets; features like hoods and pockets add weight so you can only make this comparison test for jackets with similar features and in the same size.
2.5 layer fabrics versus 3 layer - In a drive to make ever lighter waterproofs fabric manufacturers developed 2.5 layer fabrics - with these fabrics the innermost layer (that protects the membrane) is not a 'fabric' layer, its referred to as a 'print', in this instance the depositing of a print provides the protection for the membrane. 2.5 layer garments are not as durable as 3 layer due to this method of construction, however they have a proven performance record and are very common in lightweight garments.