SilPoly Tarp Durability?

I have made tarps out of cuben, silnylon, and the membrane silpoly, The membrane silpoly pitches nice and taunt, It is strong, Now if you do somehow make a tear or puncture in it, it will tear easier than reinforced silnylon, depending on which direction you pull on it. That being said I have a “cigar brown” patrol shaped shelter to sell if you are interested.

Not to hijack the thread, but I’ve had some inquiry’s about my tarp above. It weighs 10.9 ozs. More info in the MYOG thread here: http://backpackinglight.com/forums/topic/107757/

I made a dome tent out of RBTR’s “1.1oz silpoly” (fabric weighs 1.24 osy). The fabric seems quite tough – no concerns with wind or hail or tearing or strength. It doesn’t sag when wet or cold. It was, however, tested to have a lower HH than the Membrane silpoly.

I also made a small kitchen tarp out of Membrane silpoly. No issues so far, but it has admittedly seen much less use than my dome tent. The fabric is strong when undamaged, but weak once torn. I don’t know how it will hold up long-term, and the fabric is so new, I think it could take some time to build up a set of user experiences.

The frequent references to ‘silpoly’ appear to be to silicone coated polyester, not silicone/polyurethane coated fabric. Also, some of the ‘sil’ coatings include a good dose of polyurethane.

According to articles on this site, as well as posts by the forum editor, the conventional silicone coated nylon fabric will be stronger than were it coated with polyurethane. Also, 6,6 Dupont nylon will be stronger than available polyester fabrics of the same weight, given the same coatings. That has been my experience also, although there may be some higher strength polyester formulations out there.

I think your concern about the strength of the polyester fabric vs nylon is a valid one. And agree with John H. that the higher denier polyester, while slightly heavier, may have been a good choice for this reason. Note that Extrem Textil also sells a 1.16 oz/sq/yd incl coating 20D ripstop silnylon that while tested by R. Nisley at around 1400mm HH, tested at only around 770 HH after simulated aging. They may have skimped on the coating to lighten the weight.

To address this concern, I have purchased Terra Nova, GoLite and Sea to Summit mini-ripstop nylon tarps, 15-20 denier, and weighing from 1.05 to 1.16 oz/sq/yd coated, and plan to use them for tents. But it is an expensive way to go, and I will probably later try making a tent with the RBTR membrane to see how it holds up. In Richard Nisley’s tests, it had superior HH, both before and after simulated aging, than the mini-ripstop nylon tarps just mentioned. It also does not sag with cold and humidity, and unlike Cuben, has a good bias stretch that increases strength. But that is little consolation though if it has significantly less strength than nylon, in terms of tearing, puncturing, abrasion and wear.

The sil coatings on most of these newly available 15-20 denier nylon and polyester fabrics, with the exception of the above mentioned Terra Nova nylon, all adhere to polyurethane sealers such as Seam Grip, so appear to contain polyurethane. Seam Grip will just peel off of conventional silnylon when dry. No idea whether these silicone/polyurethane coatings will retard strength, as previously suggested for conventional polyurethane coatings.

Sorry if all this is confusing. It confuses me also. Agree with John H. that it could take some time to build up experiences with the lighter 15-20D fabrics becoming available for MYOG. Cuben has been debated here for years, and still there is no clear consensus.

Jimmyjam, your patrol style tarp is interesting, and pretty light at 10.9oz – but I saw in the original thread that there was a second tarp made with the 1.1oz SilyPoly that weighed 12.3oz. Have you decided to keep the tarp made from the SilPoly ripstop? Does that mean that for you, the less than 2oz difference in weight is not worth the decreased tear resistance?

John, Sam – I’m mostly interested in the PUx000 coated SilPoly fabrics from RBTR, it looks like the Silicone coated 1.1 is stronger, but less waterproof than the PU4000 coated 1.1:

I wonder if it is possible to get a polyester fabric where the ripstop grid is made of nylon, or more interestingly, spectra/dyneema? Then you’d have the sag/stretch benefits of polyester with the stronger ripstop grid for tear resistance.

In any case, it looks like maybe I should go with the 1.1 SilPoly over membrane – the difference in weight for an 11×8 tarp would only be about 4 oz, and the membrane’s polyester fabric and lack of a ripstop grid leaves it very vulnerable to tearing.

I would worry that the overall durability/usability of SilPoly is less than SilNylon.

1. Impact resistance
2. Overall strength of the fabric
3. Tear resistance

These things all conspire to produce a low durability in use with PU4000. Typically, I will pull my SilNylon tarp fairly tightly (given I use Hair Ties on all corners and the ridge lines) and ignore it. I will be limited on how far I can stretch my guy lines before bulling the tent too tight. There is a narrow range, compared with SilNylon. Generally, SilPoly will be harder to set-up.

Wind Hammer or flapping becomes concerning due to these features of SilPoly. In heavy winds. I can envision wind hammer loosening stakes if it is over-tensioned and/or splitting fabric around any flaws. Similar to opening a plastic snack bag, a nick or tear will likely continue until the pressure is relieved. Whereas SilNylon benefits from elastic self-tensioners to prevent over sagging in wet weather, the self-tensioners become almost mandatory to prevent actual damage to a SilPoly tarp.

The PU coating on a tarp is NOT repairable. It requires special adhesives and not a .1 or .2oz/yd re-coat with mineral spirits and caulk as with SilNylon. Yes, such a coating adds 2-3oz to the overall weight, but it extends the life to 10 years or more. Duct tape can be used to patch up holes and some tears in SilPoly, but I am afraid that a slight slit or hole will ruin a tent before it can be repaired. (Such as sitting out a rain storm and a small branch punctures the tarp. Well, you might loose a good portion of your weather protection…) Also, the PU/Silicone mix on the PU4000 really provides no other benefit except water resistance. It looks real good in this category, but there is no long term data. After 5 years of use, after 10 years of use. Even cuben starts failing at the end of 5 years of use.  SilNylon is simply re-coated and patches can be stuck on with the coating. The ripstop grid will prevent catastrophic failures. Yes, I do worry about tear strength also. I agree.

So, like cuben, you need deep pockets to accept this as your main line shelter. It will need to be replaced every 3-5 years. At the cost of a couple tenths of an oz to recoat SilNylon, it will be durable for ten years or more.

UV damage may be a concern. Generally, SilNylon will eventually die due to this. SilPoly is much less susceptible to UV degradation. It can last far longer in a sunny environment than SilNylon.

I believe you will find the PU coating to be very susceptible to abrasion, though. And the base fabric has about 1/4-1/3 less abrasion resistance than SilNylon. I think this is an error in the chart. PU coatings are subject to cracking when tightly folded and packing & unpacking will need to be treated fairly gently, compared with SilNylon…the basic materials are just not flexible enough.

Overall, I don’t think I would choose SilPoly for my tarps. But, I would be tempted with a ripstop and full siliconized fabric version.

I believe you will find the PU coating to be very susceptible to abrasion, though. And the base fabric has about 1/4-1/3 less abrasion resistance than SilNylon. I think this is an error in the chart. PU coatings are subject to cracking when tightly folded and packing & unpacking will need to be treated fairly gently, compared with SilNylon…the basic materials are just not flexible enough.

Well, according to the aged HH tests done by Richard Nisley, the PU-coating used by RBTR is actually more durable than most silcone treatments. The aged HH values remain very high for RBTR’s PU-coated fabrics. See the thread “The Optimal non-Cuben Tent Fly material” thread for more data:

http://backpackinglight.com/forums/topic/105800/

Wind Hammer or flapping becomes concerning due to these features of SilPoly. In heavy winds. I can envision wind hammer loosening stakes if it is over-tensioned and/or splitting fabric around any flaws. Similar to opening a plastic snack bag, a nick or tear will likely continue until the pressure is relieved. Whereas SilNylon benefits from elastic self-tensioners to prevent over sagging in wet weather, the self-tensioners become almost mandatory to prevent actual damage to a SilPoly tarp.

SilPoly *does* stretch, particularly along the bias. It stretches almost like silnylon; however, it does not sag when wet or cold or otherwise exposed to moisture. Once you setup a Silpoly tarp nice and taut, it will not need to be re-tensioned.

The water resistance of RBTR’s wares varies a lot, as shown by Richard Nisley’s tests posted on BPL, so it is impossible to generalize. The more resistant fabrics appear to be those with the “PU2000′ or ‘PU4000’ coatings.

As forum moderator Roger Caffin has often pointed out, adding ripstop threads to a fabric, will usually reduce the HH. Roger has sent me some Suter testing photos that illustrate this well. It would be great if BPL would commission him to do an article on water resistant coatings on various fabrics. As he has posted, he has a career background in fabrics. We would benefit from a more educated discourse in threads about these topics.

If Roger is correct, a ripstop weave should require a heavier PU coating to achieve an equal HH, and the ripstop weave will also add weight to the fabric. Thus the Membrane can be lighter for the same HH. Which raises the question of how much does a ripstop weave actually increase the strength of the fabric. Roger has expressed some doubt about this. In cutting light ripstop fabrics for many MYOG projects, I’ve never noticed any greater resistance to cutting from the ripstop grids, so am inclined to think Roger is correct. I use sharpened shears that cut cleanly through the material without the need for any actual scissoring.

That’s much of why I posted above that we need to give the Membrane fabrics a try and see how they perform. The weight, at well under 1.1 oz/sq/yd with coating will take 3-4 oz off a spacious solo tent compared with one made of 30 D silnylon weighing around 1.4 oz/sq/yd. Please note that the Sea to Summit nylon tarp material I mentioned above is about the same weight as the Membrane, and was tested by Richard at over 3500mm initial HH, and has a mini-ripstop grid. I tend to doubt how much that adds to important indicia of strength, like puncture resistance, and being nylon, it will sag in the cold and humidity.

Agree with John H. regarding the distinction between sag and stretch, and regarding the different characteristics of the newer sil/PU coatings compared with conventional PU coatings. I may use polyester Membrane for the front and rear end covers, or vestibules, of a side-entry tent with a main canopy of the Sea to Summit 1.05 oz sil/PU nylon fabric, and make a completely separate inner tent of netting or fabric 0.8 oz or under, much in the way that is popular with Cuben tarp tents. Then if the Membrane fails, it should be a fairly simple matter to remove and replace the vestibules with a stronger material. If it doesn’t fail, I’ll have a nice set of no-sag vestibules.

John,  PU coatings are susceptible to moisture problems. They can get damaged just by packing them up wet. PU picks up water and can actually break down in the presence of water. When wet, they are far more susceptible to abrasion. Especially with cheap fabrics, some has been noted to be damaged before a single season is done (Outdoor Gear Lab.) http://www.outdoorgearlab.com/Backpacking-Tent-Reviews/Buying-Advice “<span class=”reviewer”>How to Choose the Best Tent for Backpacking,” Jessica Haist</span> and Max Neale,  September 22, 2015

Nylon is considerably stronger than Poly fabrics, and has a much higher abrasion resistance. A damaged silicone coating is simply re-coated to restore it, though I agree silicone coatings are weaker than poly coatings. In thin areas a slit will act to score it and cut it. But, silicone also has a “self healing” property that PU lacks. It has an affinity to itself, even totally dry. We had a discussion about that a couple years ago and talc was recommended to prevent this self bonding or self healing.

“SilPoly *does* stretch, particularly along the bias. It stretches almost like silnylon; however, it does not sag when wet or cold or otherwise exposed to moisture. Once you setup a Silpoly tarp nice and taut, it will not need to be re-tensioned.”

Yes,I agree with that. But, like any membrane material, even ripstop nylon, a slit, puncture or stratch becomes the weak point. Example: In construction, most materials, even steel, that is abraded or “scored” is broken at the score line. Poly is much worse at this than Nylon. I cannot envision going camping and NOT getting a few scratches on my tarp.

The stretch of Poly is is less. If you pull it taught, an “impact” will be a problem. A dropped stick will likely penetrate a poly tarp set up tightly as you suggest. Or, tripping on a guy line at night. Or…well lots of things can go wrong here. Nylon is more elastic in all dimensions, not simply the bias. Did you ever see that program on shooting arrows through silk? Loose, it prevents the arrow from going through, it simply gives in in all directions. Anchor it taught and the arrow hardly slows down as it goes through. Stretch in all directions is an important consideration to impact resistance on a tarp. If stretch is controlled, nylon is a better choice than poly. Nylon simply gives, poly is too tight. Wind Hammer is another item related to this. Nylon stretches preventing high impacts from wind hammer on the stakes. Poly doesn’t have the range of stretch, if taught, then it is close to it’s limit. It becomes a problem in rain storms (usually 40mph gusts, too.) Rain loosened soil, staking pressure reduced, then add some wind hammer from a gust with little stretch to cushion it…not a good scenario.

SilNylon is not the best for wet weather sagging. PU/poly cloth is much better. To me, this is a small price to pay when using SilNylon. I will gladly pay .5oz per yard and wet weather stretch, over, weaker, stiffer less resilient materials.

Durability means maintenance to me. If my car needs work, I pay for it to put it in like new condition (cars last 150-200 thousand miles.) If my house needs work, I pay for a new roof, or new house paint (houses last for a couple hundred years.) If my tarp coating goes bad, I expect to be able to re-coat it. This is no worse than painting my house.  Like any durable item, I expect a certain amount of maintenance. PU coatings do not give me that option, they are simply disposable. While silicone impregnation of the PU is “supposed” to keep the PU waterproof, it really does not work that well, mostly because of other issues.

And, the clincher IS the stretch. Stretch can damage PU/Sil coatings. If the poly stretched as much as nylon, it could easily damage the coating. If it doesn’t stretch, then we are back to other problems. SilNylon can be repaired easily. What about PU coatings? And if they resist stretch, they are no better than cuben. What the hell? There is little market for that stuff outside of some experimental stuff. Lab reports do NOT equal usability.

If it were half the cost of SilNylon, I *might* do more with it simply because it is lighter. I can get SilNylon for $4.61/yd.  PU4000 would be $7.50 at Ripstop By the Roll.  And, I think that Silnylon is better for the job I want it to do. Even economics are against it.

James, I see a substantial lack of data, either lab-measured or user experience, about how your claims are relevant to newer PU-coated fabrics like SilPoly from RBTR.

The stretch of Poly is is less

Have you used SilPoly? Have you measured its mechanical properties?

Did you ever see that program on shooting arrows through silk? Loose, it prevents the arrow from going through, it simply gives in in all directions. Anchor it taught and the arrow hardly slows down as it goes through

Thank you for the safety tip. I will pitch my silk tarp loose during bow-hunting season now

If stretch is controlled, nylon is a better choice than poly. Nylon simply gives, poly is too tight. Wind Hammer is another item related to this. Nylon stretches preventing high impacts from wind hammer on the stakes

I would be interested in documentation showing poly exerts higher force on stakes. My guess is at high rates of strain, these coated fabrics (already pre-stressed from being pitched) act in a viscoelastic manner and don’t even have time to elongate, and that any difference in stretch for “wind hammer” is negligible.

And, the clincher IS the stretch. Stretch can damage PU/Sil coatings. If the poly stretched as much as nylon, it could easily damage the coating. If it doesn’t stretch, then we are back to other problems

I am also interested in this. Is there evidence that a few percent stretch can damage fabric coatings?

I’m not saying SilPoly is definitively awesome and better than Silnylon. But it could be. And it could work just fine as a shelter material, all fears aside. It’s a lightweight, highly waterproof fabric that is pretty affordable. I agree with Sam F:

That’s much of why I posted above that we need to give the Membrane fabrics a try and see how they perform

His plan to use Membrane SilPoly for the vestibules seems sound. For myself, I may have to purposely cut a small slit into my Membrane SilPoly tarp and pitch it in the backyard the next time a windy storm blows through. This would help convince myself that SilPoly is a suitable shelter fabric. If I’m lucky, maybe even a small branch will fall on it.

Edit: RBTR now has 0.93 osy fabric (called Membrane SilPoly). The “old” Membrane SilPoly that I have is now called “Membrane SilPoly PU4000”

Very interesting discussion, if rather confusing!

I thought I’d give a heads up to the views of Mike Cecot-Scherer. He’s a freelance designer of backpacking tents, with over 250 designs to his name for many of the major brands. He has access to their labs and repair departments, so is in an usually strong position to cast light on this issue.

He is a strong proponent of 20d poly as a fly fabric, particularly with a sil/PU coating.

Some take-home points from his site:

• Sag when wet: silnylon can stretch as much as 4″ across the longest dimension of a small tent, leading to serious flapping and increased wind load. Silpoly retains its pitch in the wet. He sees this as a big deal.
• Tear strength: while nylon is a few percent stronger in theory, in practice strength will vary a lot between manufacturers and batches. After a few days of UV exposure, the silpoly may well overtake the nylon because of its much superior UV resistance. And in any case, repair departments report that fabric tears aren’t a significant cause of failure.
• Puncture & abrasion resistance: he argues that these are terrible on all lightweight tent fabrics, so there isn’t much to choose between them. Provided you reinforce the stress points, he feels that all you can do is carry a good repair tape in case of issues.
• Waterproofing: he feels that a 1500-2000 coating is fine for both flies and floors. If you want a bit more robustness, you could take the floor up to 3000. Anything more is overkill. In his experience, users often mistake condensation or vapour transmission (lightweight fabrics breathe) for leakage. He prefers a sil/PU coating as you can seal the seams reliably. He doesn’t discuss the longevity of the PU coating, so I’ve dropped him a line and will post if he replies.

He’s recently developed his own comfort-oriented range of freestanding backpacking tents with some bleeding-edge features. He subjected them to multi-directional wind tests – possible the most comprehensive yet conducted. They were stable and quiet up to Force 8. When tested to failure, it was the DAC poles that went, not the fabric. There are videos of the tests on his site, which is well worth exploring:

http://thetentlab.com/MoonLightTents/MoonLightIntro.html

I find his arguments quite convincing, but would be very interested in any rebuttals based on practical experience…

“would be very interested in any rebuttals based on practical experience”

Yes, confusing because these are new materials that have not been tested in the field, or in tests of all desired capabilities. Look for reports on the WP and durability of the lightest new Nemo tents, though.

Was put off because at this stage of tent design, a tent should not require the conventional inner that goes up with the poles, over which the fly must be placed.
That is just a disaster in driving rain.

He is right about the no sag, as shower tests on You-tube have shown that if the more horizontal surfaces of the fly sag and form pools, they become much more susceptible to water penetration in a driving rain. There is also a much drier exit and entry with a sagless canopy.

The equivalency between polyester and good nylon like Dupont nylon 6,6 is BS IMO. Snow Peak has mfg’d a light denier polyester in its own factories in Japan that is a little stronger; but generally, my experience is that polyester of the same denier will be significantly weaker every time.

A big issue with waterproofing is whether there is a ripstop weave. Roger Caffin, whose career has been in fabrics, has posted almost endlessly here about substantial drops in WP caused by ripstop. Alternatively, if the ripstop is omitted, as on the ‘membrane,’ the waterproofness is much better, especially after aging; but as jimmyjam, showing evidence of his experience, posted above, it “will tear easier than reinforced silnylon.”

Submit that the answer to this confusion is to find the material with the lowest denier that has satisfactory (to you) puncture and tear resistance and send it to Richard per his protocol for WP testing, both before and after aging. Please check his extensive posts here first, though, as he may have tested it already. Richard has tested a number of the RBTR fabrics and posted VERY differing results on this site.

A simple answer would be nice, but with what is known presently, there is a lot of guesswork that must be done in selecting fabrics.

my aliexpress 3F 100$ USD mid tent (inner and outer) is made out of 15-20D (sources differ) sil poly … they claim its silnylon but its seam taped on the inside and the outside feels like sil while the inside feels like poly

http://www.aliexpress.com/store/product/Free-shipping-3Ful-Gear-Pedestrian-3-season-2-layer-1-resident-professional-silicon-PU-coating-outdoor/1302938_2006039576.html

i cant tell you about high winds … but for rain the thing is VERY waterproof … ive tested it on PNW continuous 30mm+ rain days over multiple days, and sprayed it down with a garden hose … ive even setup the inner which has the same material in the groundsheet in a small stream

however the silpoly it uses doesnt compress as much as silnylon so it takes up more pack space … and there is still some sag when it gets wet in prolongued rain

as a side note for shorter folks or kids its probably the best deal ever for a double walled tent … a few mods and its quite functional

;)

I have used my DIY rsbtr silpoly tarp for around 400 nights now with no issues at all compared to silnylon. The only difference is that it sags less when wet, which is exactly why I made it.

Andre – can you expand on the weight and coating of your RSBTR poly? Was it sil/sil or sil/PU?

Sam

When you say that in your experience poly is significantly weaker, do you mean that you’ve experienced failure in actual use? I’m building a tarp-tent for use in exposed places, so this is an issue for me.

As I understand it, Mike’s main point is that both fabrics are plenty strong enough to withstand storm forces, so any differences in lab-style tests are academic. As I said, in his wind tests nothing ripped, even above Force 8, and it was the poles that failed.

I’ll quote him at greater length, and would value your views about his argument:

… But here’s the thing: no one actually bases their tear strength requirements or even their desires on what’s NEEDED because no one knows what that too-low number is. So we’ve gotten into a sort of tear strength arms race even though it’s obvious that it’s the tensile strength that a tent primarily needs (and all fabrics available are WAY overkill).

So I’ve taken it upon myself to ask repair departments of my clients (and Kelty, of course, when I was there) what, if any repairs they saw were caused by the tear strength being too low. Tents age, UV damages fabric, surely there should be some indication of a lower limit to tear strength in use. But the answer was always no. No repairs ever because tear strength was too low. Actually is was quite striking. There were even tents sent in for repair that you could literally push your finger through which weren’t being sent back to be fixed because of that; the customer hadn’t damaged it and it was sent back for something else entirely!

As to abrasion resistance or puncture resistance, ALL the lightweight fabrics we use are just terrible – it takes the barest swoosh against a sharp rock or a gnarled tree to put a bunch of holes or a tear into a rainfly. About the only thing one can say about higher tear strength is that there’s the theoretical possibility (and hope) that a tear won’t propagate as much in a higher tear strength fabric. But prevent a tear? No way. This is when having some of the truly excellent repair tapes available can really save the day. I love modern repair tapes.

For me, this is reinforced by the performance of the Stephenson WarmLites. They only use 20d nylon, but have proved themselves at some scary wind-speeds. There are quite a few accounts on the web of very old WarmLites, which must have experienced UV damage, still performing well. So my feeling is that the 20d poly will be strong enough, but I don’t want to find out that I’m wrong when the tent disintegrates around me!

Hi Geoff,   I used the 1.1 gen 2 silpoly as I understand it is Sil/pu coated on both sides.  I sealed the ridge line with atsko silicone spray as I do with every tarp I make and have no issues.

Good point Geoff…

I have a 1979 or 1980 Stephensons Warmlite 2R that is still going strong, and is bombproof above treeline.

Have wondered what a Warmlite tent might look like/perform like with Cuben…or SilPoly, for that matter.

Cheers

Well, Mike has responded with great generosity.

I asked him about the longevity of PU coatings on lightweight fabrics, and also to look over this thread and give his responses. As you read this, it’s worth bearing in mind that he’s one of the most experienced people in the industry.

The durability of PU coatings can be all over the map and they can’t be distinguished between without some very fancy equipment. The most common PU coatings are called polyester-based polyurethanes. These will degrade with water (!) over time through a process called hydrolysis which basically means that water causes the polymer chains to break apart. It’s what’s responsible for sticky and stinky coatings and short tent life on all kinds of fronts.

A far better type of PU is poly-ether based PU (sometimes called PEU) which I’ve found to be very durable and very well behaved in general: soft, not sticky or stinky, reliably adheres to fabric, folds well, and so on. Mountain Hardwear once made a big deal about this kind of coating but it’s been a while since I’ve seen it on their website (though I could just be missing it). I want to say that all good brands use it but it’s so easy for them to forget to spec it in and with staff turnover – who knows. Also, Asian fabric finishers worry more about UV and yellowing resistance than water stability. They just don’t ‘get’ our concerns; tent makers are oddballs in the market. So they default to polyester-based PUs.

As to basic coating quality, like questions of tear strength, it seems mostly to be a matter of process control – expertise to make the coating without pinholes or too thick or not to wreck the tear strength of a fabric. It’s not an exaggeration to say that the tear strength of a particular fabric depends more on the finisher than on the plastic the fabric is made from.
Too much coating is a HUGE problem for makers concerned with weight. So as with tear strength, an established source with a good track record is important. But, unless someone goes out of their way to tell you who they use for fabrics, it all seems like a big black box at this end (leading to all kinds of false generalities).

I took a quick look at the thread you mention. Nice to see people care. Some notes:
• Ripstops can easily be found with very flat weaves. So coating and abrasion on those materials are just like non-ripstops. There’s no bumps to make wear points or have thin spots in the coating. Feel a fabric; if it feels ribbed, then it fits the old stereotypes. If it feels essentially flat, it’s good to go. I would say that a flat ripstop weave is one of the selection criteria for tent fabrics that experienced tent designers use when evaluating fabrics.

• Plenty of overlap between inner tent and rainfly has proven itself to handle very harsh rain conditions. What they offer is the ability to take the fly off when it’s hot or to sky watch or even just to dry the tent faster (where polyester IS faster than nylon). Also less condensation when those conditions occur.

• 20D polyester can easily have enough tear strength. Again the finisher can’t be throwing it away. Obviously it’s more prone to little holes and tears from unknown causes than heavier materials – “wear and tear.” Some repair tape is a great idea to prevent little things from getting bigger.

• The impact resistance argument is complete nonsense. No winds can hit with the speed and force that is required to get into the realm of “impact” in the engineering, materials science sense. Also note that the actual impact strength of an oriented plastic fiber is very good. Finally, note that the concept of impact is often conflated with notch sensitivity and here again, woven fabrics have no notches in the engineering sense.

• Related to the notion of impact is the idea that a stretchy guyline is helpful. Again this is simply not true. A stretching line makes a scooping fabric shape which leads to heavier loads on the tent (and fiercer flapping). I highly recommend completely non-stretch dyneema lines or near-completely non-stretch polyester lines. Neither of these lines will loosen when wet either.

• I can’t track down a reference but think “wind hammer” is an idea from sailing that crucially depends on a boat’s rocking motion to compound the “impact”. In other words it occurs when a mast and sail swings against the wind. If so, please note that the whole idea here depends on a rigid connection of the boat to the mast to make “wind hammer” a destructive thing. Tent poles (the mast of a tent) flex relative to the ground (the boat) so the concept really doesn’t apply.

• I know of no difference between re-coating nylon versus re-coating polyester fabrics. Both plastics have plenty of binding sites for adhesives and presumedly you’re bonding on the PU anyway. But trying anything but a silicone on the siliconed side will fail.

• All conventional woven fabrics have mechanical stretch in the bias direction. A “triaxial” fabric is available in the composite’s industry but it is not available for anything we would call lightweight or affordable.

In the light of Mike’s advice above, I contacted RSBTR and they have confirmed that their PU coating is polyether/PEU.

So their Poly/PU fabrics are beginning to look like an attractive option: no stretch in the wet, lower bias stretch, good UV resistance, and very high HH. Plus they have it in MARPAT camo, which is a plus for stealth camping.

Got to be worth a try…

“Sam – When you say that in your experience poly is significantly weaker, do you mean that you’ve experienced failure in actual use?”

Certainly. Have been backpacking many decades. Plenty of failures in actual use.

But I am not an expert, Roger is. And he has covered all these issues if one cares to mine BPL. It’s all there.

There are great clips of Warmlite and other tents on You-Tube showing them being ripped apart. Again, it is just a question of how much high altitude camping you want to do. For CDT and similar trekking, without having to retreat much below timber line, a more robust fabric (min 15 D – some thread reinforcement – 2K-3K HH AFTER aging) is key. Much of what your expert says is OK, but I have been reading Roger for years here. He’s the man, period, IMO, if you want to stay dry and safe. His article here, ‘When Things Go Wrong,” is a classic.

Am now leaving for the CDT and vicinity in CO for a month. Best time now with the bugs on the wane. Hope to see you in the Fall, with an OP about a new tent, and detailed reasons for the choice of fabric. Will print a full confession if I get all wet. See you then.

I would submit that with careful detailing the silpoly is plenty strong enough. Fabric failure in tents or tarps is pretty rare. – almost always it is the poles that fail if anything does. But careful sewing and detailing of the reinforcements and guyline attachment points is critical regardless of the fabric, as that is the weak point as far as the fabric goes. Nylon is stronger for a given weight and denier, but in reality what that gives you is more of an extra margin for extreme cases than a needed level of strength. Stretch can be an issue in maintaining a taut pitch; but it can also be the patternmaker’s friend. A lower stretch fabric is less forgiving of errors in patterning, cutting, and sewing. If I were making a tarp or tent fly, I think I’d go for the membrane silpoly – for me, the superior coating and lighter weight are more important than the slightly greater strength. Also in my mind is the general sense that the quality of the coating (or more properly, impregnation) on the widely available silnylons is not what one would hope it was.

Paul

I suspect you’re right that design and reinforcement are more important than outright fabric strength. This is what Mike was saying on the basis of his extensive industry experience. And we shouldn’t lose sight of his point that the strength advantage of nylon in the lab won’t last long in the field because of its inferior UV resistance.

So I think it’s reasonable to assume that for all practical purposes silpoly will equal silnylon in the field in terms of fabric failure – especially as the pitch will be better in the wet so the shelter can work as intended. If we make this assumption, there’s convincing evidence that a well-designed 20d silpoly shelter should be plenty strong enough for above-the-treeline use.

I quoted the case of the Stephenson WarmLite routinely surviving major storms with a 20d silnylon fly. Sam claims he’s seen videos of WarmLites shredding but I think he’s mistaken – I’ve only seen praise for its wind performance despite its large unsupported panels. Mike’s freestanding 20d sil/PU poly designs did very well in multi-directional tests, even though they weren’t guyed. And Nordisk test their 20d sil/PU nylon designs to 90 mph (Force 12), which is something to behold: http://www.nordisk.eu/media-center/media-center-detail/media-category/wind-test/tent/svalbard-1.html

Once you drop below 20d the evidence is less comforting. Mike advises 20d as the current limit for silpoly. I know they have to cover themselves, but RSBTR go out of their way to emphasise that the 15d Membrane is pushing the boundary strength-wise for a shelter. And Nordisk only test their 10d silnylon ultralites to 56 mph, so we can assume that something failed beyond that.

I think that for my own alpine shelter project I’ll go for the 20d sil/PU so I can sleep more easily at night!