Metrics for Wind and Rain Shells

The industry uses a few different metrics for assessing wind and rain shell performance. CFM (cubic feet per minute), which is a measurement of air permeability, or what volume of air can be pushed through a fabric. MVTR (moisture vapor transmission rate), which is a measurement of how much moisture vapor gas can be pushed through a fabric. HH (hydrostatic head), which is a measurement of how tall a cylinder of liquid water a fabric will hold back before leaking. I think the industry has a pretty much right with regards to which of these metrics they use for which garments. Generally, CFM is used for wind shells that are not waterproof. MVTR is generally used for waterproof breathable membrane fabrics because they usually have little to no actual CFM. They can only transmit moisture vapor gas through micro pores in a membrane. HH is generally used for anything meant to be waterproof. I think HH is fairly straightforward and this article will focus on the nuances between CFM and MVTR.

There is some push these days to re-think the models of which metrics we use for what fabrics, but the line of thinking seems to head in different directions from there. Some would argue that we should see all of this data for all different types of fabrics. This I can agree with. It can’t hurt to have more data as long as we are able to use it proerly. The other direction is the idea that CFM isn’t relevant and MVTR is the holy grail. As usual, it just isn’t that simple. For a fabric that has limited CFM, MVTR is really the best option because CFM isn’t high enough to contribute much. Once we get above a certain threshold ( I don’t really know where this point is) CFM really becomes the most important metric. That air permeability we are measuring is also the primary means of transmitting moisture vapor, above a point.

The REALLY big aspect that is overlooked about CFM, as a metric, is what actually happens in real life. Real life is not a static lab test. When we are wearing a high CFM wind shell out on the trail, air is not only moving out of it from our heat and moisture build up, but it is also moving in from the outside. In use, there is very little resistance against air movement through a fabric, into a garment. All the edges of a garment, like the hip, cuff, neck, etc, are not sealed. They are actually very leaky. Therefore the only resistance to air movement into a garment is the CFM of the fabric itself. So if air can move out of the garment at X, it can also move into the garment at X. Since most of the time, either the wearer is moving, or the air is moving, there is constant air transfer going on. This air transfer not only moves moisture, but also moves heat. THIS is key. If we take a waterproof breathable membrane garment with little to no CFM and compare it with a wind shell garment with CFM that happens to have the same MVTR in a lab, the wind shell with CFM will almost always come out the more comfortable and drier garment. CFM regulates temperature. A high MVTR WPB membrane can move a certain amount of moisture, but if it doesn’t move any significant amount of air, then the heat will continue to build up more and more. As heat builds up, the body produces more and more moisture, which inevitably overwhelms the MVTR of the membrane. A high CFM wind shell with the same MVTR in a lab will be more comfortable and more dry because air will move in and push out the hot air and moisture. This cools the body and reduces the amount of moisture produced.

Of course, the reality of a WPB membrane fabric having a similar MVTR to any high CFM wind shell is very unlikely. There is some recent single source lab data suggesting they can match up but this generally contradicts real world performance. We all know very well that bivy sacks built from WPB membrane fabrics commonly have internal condensation issues. It is really just an accepted drawback of this type of shelter. However, the UL backpacking community has really embraced the use of Bivys built from high CFM fabrics like Argon 67 (50cfm) because they can be used in most conditions without the build up of internal condensation. This is because the high CFM moves more air and moisture than the membrane fabrics, even in stagnant air. These results really align with general common sense. Microporous WPB membranes can be transmit vapor while being waterproof simply because the pores are small enough to keep water out but large enough to allow vapor through. If the pores in the Argon 67 fabric are so much larger, as to let 50 CFM of the much bigger air molecules through, then we can easily predict that it will also be letting an increased number of way, way smaller vapor molecules through.

I think it is important to use lab data properly. Real world performance is the the end result that inevitably factors in all the variables. I think the purpose of lab testing is to help explain why we see the real world results that we do. We are getting into trouble when lab results contradict the real world results that we clearly see. One of the benefits of lab testing is that you can control variables, but this is also, in a way, one of its drawbacks. The real world performance of something is a combination of many different variables and conditions. It is very easy to isolate a testing procedure in a lab down to a point where you see results that contradict, or don’t apply at all to the real world use case at all.

It is also very important that any conclusions derived from lab testing comes from multiple sources in order to prove repeatability. Single source lab results that contradict other data should be set aside as an anomaly until there is something to back it up. Single source data that contradicts other data AND real life performance is almost certainly flawed.

9 thoughts on “Metrics for Wind and Rain Shells”

  1. I enjoyed reading your take on this. The variables as you say are many. In the past I have relied on mechanical venting to tune my inner heat/moisture level while climbing and descending if there is sufficient wind. It allows me to dump heat reasonably quickly without repeated gear/clothing changes or removals. Don’t want to give that up. I find the constriction of pack shoulder straps and chest strap to limit the advantage of a full zip when trying to dump heat. I always liked the additional feature of long double zip pit-zips to shed more heat. I wear a Brynje next to skin mesh layer and an Alpha Direct 4008 hoodie. I am looking at your Hyper D wind pullover. When I messaged you a month or so ago you recommended not making this with pit-zips. You mentioned MegaZips in you reply but I am not clear what they are. Nonetheless, I am asking you to make one (Hyper D pullover) for me with both a full zip front and long pit-zips (elbow to halfway between pit and waist, double zips) and drawcords. I recognize you are extremely busy, and would be happy to just get in the queue. Is this possible?

    1. I’ll try to clarify my response regarding a Hyper D wind shell. First, Hyper D is a VERY high CFM wind shell. NOT anything like what most people think of when you say “wind shell”. It is closer to the air permeability of a T-shirt than that of a traditional wind shell. It is a piece that blends between categories. It is meant to provide a cap over Alpha Direct so that it can provide some insulation, but the high CFM causes the system to act more like what we would call a “soft shell” these days. So it provides some insulation, but it has enough CFM to keep you comfortable at high exertion instead of becoming a sweat box like “hard shells” or “wind shells”. The flip side, like soft shells, is that the system will have limited wind resistance, contrary to the category it is in. So, due to all of this, installing mechanical ventilation on a Hyper D might not be necessary. It can be done, of course, and there is nothing wrong with going that route. However, this is to make you aware of what this garment actually is. It is not one of the rain shells that this article is about or a regular wind shell where mechanical ventilation is the only method of regulating temperature. It is a garment that does quite a bit of temperature regulation on its own, without any large mechanical vents.
      The second part of this is that we are not doing short pit zips moving forward. It is either MegaZip or no zips. See the MegaZip products for info on that but they are “pit” zips that go the whole length of the side, from hip to wrist. Two sliders allow adjustment of the opening.
      If someone wanted a MegaZip wind shell they would simply purchase the Silpoly MegaZip and write in the notes box that they wanted Argon or Hyper D, with a color selection. The available colors can be seen on the respective Argon or Hyper D pages.

  2. Thanks for a detailed and very reasonable response. I know you are busy and that is appreciated. I had just finished reading some of Seeger’s opinions re: CFM vs MTVR, specifically his findings on decreased CFM due to the aerodynamics near the body. I think I got carried away. This is not my field. The moisture build-up for me when active always leaves me exasperated, perhaps due to my poor shape or laziness when a change of layers or clothing is called for. I have a somewhat breathable waterproof rain jacket that I can substitute if I find the Hyper D to be too permeable. Again, thank you for the detailed input. I’ll get on the queue for ordering and will be glad to leave honest feedback after hiking in it.

    1. Lab data is good to have, but we need to be cautious about how we interpret it. In real world use, we know for sure, that the 80 – 100 cfm of Hyper D moves a lot of moisture. Far, far more moisture than any membrane tech available today. I can say this without any shadow of a doubt. We also know, without any doubt, that a 20mph wind easily blows right through that cfm from the outside. I can also say, without a doubt, that it moves much more heat through that cfm. These things we know for sure. That said, if you run pretty hot and sweat a lot, some extra forms of mechanical ventilation will likely still be welcome. The only penalty, in this case, would be the weight of the MegaZips, which isn’t really much.

  3. “One of the benefits of lab testing is that you can control variables, but this is also, in a way, one of its drawbacks. The real world performance of something is a combination of many different variables and conditions. It is very easy to isolate a testing procedure in a lab down to a point where you see results that contradict, or don’t apply at all to the real world use case at all.”


    I’m glad to see this discussion finally happen somewhere.

    Having used a Patagonia Airshed pullover (one of the pieces tested by Seeger) for many seasons and now a Timmermade Hyper D pullover (not tested), I can say from personal experience that they both work better hands down for my real world use scenario than the other pieces he tested. The Hyper D pullover in a minimalist design works well over a wide range of conditions. Thanks for making that available!

  4. 1. You write: “Moisture vapor molecules are smaller than the other air molecules.” Air molecules (80% N2 and 20% O2) are not larger than water molecules. The mean bond length of H20 is 96 pm while the mean bond length of O2 is 121 pm and of N2 is 109 pm. But H20 has two bonds at a 104.5º angle while N2 and O2 have only one each.

    2. The volume of the molecules doesn’t actually matter for vapor transmission, regardless. Any pore that passes any gas, including vapor, is, at the very minimum, hundreds of times larger than air or water molecules. Gore, for example, advertises pores as small as 200,000 pm.

    3. Graham’s Law of Effusion describes the relationship between the transmission rates of different molecules through pores. The only important factor is the square root of the ratio of molecular weights of the different molecules and the formula tells us that the difference in transmission rate between water vapor and air is, at maximum, 25% for N2 and 33% for O2. That means even the finest pores passing water vapor will necessarily also be passing at least 79% as much air as they pass water, relative to partial pressure. Since the vapor pressure of water at room temperature is 3 kPa and normal air pressure is 100 kPa, that means you will always be moving at least 26 times more air than water vapor even through a perfect tiny pore membrane. Since water vapor pressure drops very fast at actual cool outdoor temperatures where you might wear a shell, the practical ratio will always be much larger. There is no way in physics to beat this ratio without an outside source of power.

    4. This is not correct at all: “A high MVTR WPB membrane can move a certain amount of moisture, but if it doesn’t move any air, then the heat will continue to build up more and more.” Moving moisture out also carries heat out because the body has to evaporate the moisture before it can move. Furthermore, there is more than one way heat can move. You may block air movement at the shell, but there is still conduction and radiation to carry heat from the body.

    5. There are several different MVTR tests and most of them were originally designed for verifying packaging seals of nearly completely impermeable plastics, not breathability. Manufacturers, of course, pick and choose the tests and invent new ones, always for marketing purposes and not to inform the public. CFM tests can suffer from similar chicanery, but are mostly standardized because they’re measuring a much larger and faster movement of gas across a membrane. Regardless, there should never be a result where MVTR is similar and CFM wildly different, since they’re measuring the same thing—movement of gas under fixed pressure across a membrane that offers resistance. When you see such a result, you can be confident there is a problem with the testing on some level.

    1. Curious data…..but it’s all about relating to a known end result. Generally speaking, membranes do not permit air movement in any significant amount. With most of them you can blow up a balloon out of it and hold air, even under pressure….but at the same time, under ideal circumstances, moisture vapor does move through them. I would entertain the opinion that they do move air in some tiny amount, but it is so small to not really be relevant to this topic.

      Clearly heat moves via conduction and radiation, but the point is that these result in such a tiny amount as compared to the convection provided by air permeable fabrics and mechanical venting. For the purposes of this discussion the sentiment that membrane garments build up more heat than air permeable fabrics, holds true.

      5 is an interesting perspective, which supports the part of my opinion that states that CFM and MVTR are closely related. However, that part of my opinion is directed at air permeable fabrics above a certain threshold of permeability. In regards to membranes that have little to no air permeability, this relationship is typically advertised as much different by manufacturers and I would agree with it. The product exhibits those properties in physical testing and actual use. No perceivable air permeability, with vapor transmission under ideal conditions.

  5. Hey Dan! I’m actually interested in your SilPoly jacket, primarily as a pure rain protection layer, a) because of its “fully impermeable protection,” as you put it, but also b) because of its lack of breathability. I feel as though it would offer me interesting layering combinations together with my other breathable layers, including a merino or alpaca base layer, a fleece layer, and a puffy down layer. There may be a scenario where I purposely want to trap the heat, and/or I most likely wouldn’t be wearing the jacket unless it’s raining. I like the fact that it doesn’t require DWR maintenance either. I a fan of the Montbell Versalite, but it’s almost 2X as heavy as your SilPoly jacket and more than 2X the cost. Does my thinking make sense, or is something like Gore-Tex or Argon 90 more practical? Thanks!

    1. I think it makes sense, but the we aren’t really nailing down a specific use case here. I think the scenarios you mentioned often exist and an impermeable layer is essential in my kit. I use a hyper breathable insulation layer like Alpha Direct or APEX with mesh, then I have a high CFM wind layer to cap that for active use but still allow good breathability, and then I have an impermeable layer with good mechanical ventilation. That way I can use it as a wind blocker if it’s cold / windy and I can also use it for rain protection with vents open to manage moisture inside. A90 layers have mostly been omitted from my kit since they aren’t quite breathable enough for intense activity with those hyper breathable insulation layers. It pretty breathable and a good wind blocker, but personally I would rather have something that is comfy during exertion and if I need better wind protection I can put the impermeable layer over. “Waterproof breathable” membrane stuff like Gore does have a place in my kit, but mostly for experimentation purposes. I have found some to be functional, but I don’t like the drawbacks that come along with them. Due to this, I typically rely on impermeable layers instead.

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