Silnylon vs silpoly: which is better for a tent or tarp?

Silpoly is the better fabric for a tent or tarp that has to stay taut through a wet night, because polyester takes up roughly a tenth of the water nylon does and barely changes length. Silnylon is the better fabric where tear strength per gram matters, such as a fly that takes high wind loads or the lightest possible shelter. Polyester's other selling point, better resistance to sunlight, is not supported by the only test made on lightweight tent fabrics.

Silnylon or silpoly: which to choose#

Each fabric wins one property clearly, and the others are close or unproven, so choose by the job.

Why does silnylon sag in the rain and silpoly not?#

Nylon fibre absorbs water and polyester barely does. Moisture regain, the water a dried fibre takes back up from the air as a percentage of its dry weight, is 0.4 to 0.8 percent for polyester, 2.8 to 5.0 for nylon 6 and 4.0 to 4.5 for nylon 6,6. The same 4.5 and 0.4 figures sit behind the re-tensioning habit in our pitching guide.

Water swells a nylon fibre and lets it relax under load, so a nylon panel lengthens as the air gets wetter. Both suppliers whose sheets are used here say so. Extremtextil's silnylon pages say "This fabric will stretch when wet", and its silpoly page describes a fabric that "does not stretch or stretches very little in wet conditions". Ripstop by the Roll sells its silpoly on "lower water absorption" to "minimize sag/stretch when wet".

How much a wet nylon fly lengthens is the number nobody publishes with a method. Durston Gear's X-Mid page says wet nylon "expands (or 'sags') about 4%" and "weakens by about 10% when wet". Those are a tent maker's figures, given without a test, and we use 4 percent below only as an upper case.

The coating matters as well. SlingFin's coatings article says silicone keeps nylon from soaking up water, so "fabric stretch due to moisture is greatly reduced", while polyurethane is hydrophilic. A sil/sil nylon sags less than a sil/PU nylon. It still sags more than silpoly, because the water goes into the fibre itself.

How much sag does a few percent of stretch make?#

More than intuition suggests. When an edge between two fixed points gets slightly longer, the extra length has nowhere to go but down, and for a shallow curve the drop grows with the square root of the strain.

Because it is a square root, the first fraction of a percent does most of the damage: a quarter of a percent, 6 mm on a 2.5 m edge, already allows nearly 8 cm of sag. Wherever the gap between fly and inner is smaller than that, the fly touches the inner, and contact is where water comes through. The fix is equally small. Taking 2.5 cm out of a guy line restores the pitch at 1 percent, which is why people who re-tension after an hour of rain are content with silnylon.

Silpoly's advantage is that the night-time trip outside mostly goes away. On a tarp, a mid or a trekking pole shelter, where nothing elastic takes up the slack, that is a real gain.

Is silpoly weaker than silnylon?#

Weaker to tear for its weight, on the one set of like-for-like data sheets we found. Tear strength decides whether a puncture or a stressed guy point becomes a split.

The silpoly is 25 percent heavier than the 20 denier silnylon and still tears at less in its weaker direction, 4 kg against 4.5, so per gram it keeps about 71 percent of the silnylon's tear strength. In a straight pull it is different: per gram in the weak direction the silpoly (35.5 kg over 45 g/m2, about 0.79) slightly beats the silnylon (27.5 over 36, about 0.76). Polyester loses once a tear has started.

The mechanism is stretch. SlingFin's fabric article explains that nylon deforms around a guy point and shares the load, while in a fabric that does not stretch "all the force is concentrated at the reinforcement". Extremtextil says silnylon's slight elasticity "prevents peak loads". The stretch that makes silnylon sag is the stretch that makes it hard to tear, and no choice of fibre gets one without the other.

These are three fabrics from one mill's range. A heavier silpoly can out-tear a lighter silnylon outright, and SlingFin notes that designers often use heavier, thicker polyester to match nylon's strength. The table shows the direction of the difference, not a fixed ratio.

Is silpoly really more UV resistant?#

It is widely claimed and has not been shown at tent weights. Ripstop by the Roll lists "increased UV resistance compared to Silnylon", and Extremtextil writes "Polyester is more UV-resistant than nylon".

In 2022 SlingFin sewed swatches of tent fabric into panels on its studio roof and pulled strips apart every few weeks. The company calls the method "dubiously scientific" and "not to any standard, ASTM or otherwise", but it is the only published comparison on lightweight tent fabrics we found.

SlingFin concluded that "polyester is not necessarily more UV-resistant than comparable weight nylon, at least at the weights and with the coatings typically found in tents". Its three worst performers were all sil/PE, and sil/sil beat sil/PE on the same 20D nylon. The limit: it had no sil/sil polyester to test. So the evidence does not say silpoly is worse in sun. It says the usual reason for choosing it has not been demonstrated, and that the coating matters at least as much as the fibre.

For scale, SlingFin estimates a five month thru-hike, with the tent up for just under three daylight hours a day, at about one month of its rooftop exposure. That weakens a lightweight fly without finishing it. A base camp tent left up for weeks is a different case, whatever its fibre.

Sil/sil, sil/PU, sil/PE: what the label does not say#

The fabric name gives the fibre but not always the coating. SlingFin warns that phrases like "silicone-treated ripstop nylon" can hide sil/PU or sil/PE fabric, and that you sometimes have to ask the maker.

This is where most of the confusion comes from. A sil/PE polyester fly and a sil/sil nylon fly differ in two ways at once. The X-Mid, whose maker credits it with the industry's shift to polyester, specifies "15D Sil/PE Polyester 3500mm HH". SlingFin's worst polyester in sun was sil/PE too.

• Sticky old tents are a PU problem. A sil/sil fly of either fibre will not go tacky as described in why is my tent sticky, and that article's recoating method does not work on silicone.
• Factory taped seams mean a non-silicone face. Tape does not hold on silicone.
• A sil/sil fly carries no flame retardant, since the chemicals go into PU or PE. Why tents were treated at all is on the CPAI-84 page.

Weight and waterproofing, side by side#

At the same nominal weight, silnylon and silpoly finish within about 2 g/m2 of each other, so grams are not the deciding factor. The membrane silpoly's head falls to about 200 mm after 10 home washes under AATCC 135, a strong reason never to machine wash a sil/sil fly. SlingFin says it is hard to get water columns "much higher than 1500mm" with silicone on lightweight fabric, and that this has not caused it problems. What those millimetres mean in rain is in waterproof ratings explained.

Does a wet nylon fly really carry a pound of water?#

Not inside the fibre. Durston says nylon leaves a tent "heavy with up to a pound of water in the fabric". A 5 square metre fly in 37.3 g/m2 base cloth holds about 187 g of nylon, and at a regain of 4.0 to 4.5 percent that fibre holds 7 to 8 g of water from the air. Polyester at 0.4 to 0.8 percent holds 1 to 1.5 g. A fibre in rain takes up more than in air, but 450 g would need the nylon to absorb more than twice its own weight.

A heavy wet fly is carrying water on its surface, in the weave and in any polyurethane coating. SlingFin says a sil/sil fly can be "almost totally dried out just by giving it a good shake". The fibre difference in water weight is real, and measured in grams.

How to check a fabric before you buy, and test one you own#

Before buying, ask for the fibre and denier, the coating on each face, and the tear strength with its test method. ISO 13937-2 and ASTM D1424 are different tear tests, so compare figures only within one method. Then, on a shelter you own:

1. Coating, by feel. Both faces slippery points to sil/sil. One face smoother or slightly rubbery points to sil/PU or sil/PE.
2. Wet sag, by measurement. Pitch dry on a still evening and tension fully. Measure the height of the middle of the longest unsupported edge above the ground. Wet the whole fly with a hose or sprayer, wait 30 to 60 minutes, and measure again without touching the guy lines. Read the drop against the sag table: about 8 cm on a 2.5 m edge is a quarter of a percent, about 15 cm is 1 percent.
3. Clearance. Measure the dry gap between fly and inner at the same point. If the wet sag is bigger than the gap, the fly will touch the inner in rain unless you re-tension.

Common mistakes#

Buying silpoly to stop condensation. The fabric does not change the water two sleepers breathe out. See tent condensation.

Treating "silpoly" as one fabric. A 15D sil/PE polyester and a 30D sil/sil polyester share a name and little else.

Assuming polyester means sun-proof. A lightweight fly of either fibre loses much of its strength in a few months of continuous sun. Store tents in the dark, as gear storage explains.

Over-tensioning silpoly to stop flapping. It does not stretch to spread the load, so a hard pull goes straight into the guy point. Use wide, reinforced tie-outs, as tents in wind describes.

Patching sil/sil with adhesive tape. It lets go. Use a silicone-based sealant or patch.

Frequently asked questions#

Does silpoly sag when wet?#

Very little. Polyester's moisture regain is 0.4 to 0.8 percent, against 2.8 to 5.0 for nylon 6 and 4.0 to 4.5 for nylon 6,6, so the fibre barely changes length when wet. A silpoly pitch can still loosen as guy lines stretch or stakes shift in soft ground, so a check before sleeping remains worthwhile.

Is silnylon stronger than silpoly?#

In tear strength per gram, yes. On one supplier's sheets using the same ISO 13937-2 method, a 36 g/m2 silnylon tore at 4.5 kg and a 45 g/m2 silpoly at 4 kg in its weaker direction, about 30 percent less per gram for the polyester. In a straight tensile pull the two were close, with the silpoly slightly ahead per gram.

Is silpoly more UV resistant than silnylon?#

Not demonstrably at tent weights. Suppliers say so, but SlingFin's 2022 rooftop test found a 20D sil/PE polyester lost strength slightly faster than a comparable 20D sil/PE nylon, keeping 14 percent after 104 days. Coating and weight mattered more than fibre. No sil/sil polyester was tested, so the question stays open for that fabric.

Which is better for a tarp, silnylon or silpoly?#

Silpoly, for most people. A tarp has no inner tent and nothing elastic to absorb slack, so wet stretch turns straight into sag and flapping, and silpoly holds its pitch overnight. Choose silnylon if minimum weight matters most, or for a tarp pitched taut in strong wind, where its tear strength protects the tie-outs.

How do I seam seal silnylon or silpoly?#

With a liquid silicone-based seam sealer on a clean, dry, pitched fly. Seam tape and urethane sealers do not bond to silicone and peel off. If the fabric is sil/PU or sil/PE, the PU or PE face can take tape or a urethane sealer, which is why factory taped seams indicate a non-silicone face.

Can you machine wash a silnylon or silpoly tent?#

Avoid it. One supplier's data sheet shows its membrane silpoly falling from about 1,500 mm hydrostatic head new to about 200 mm after 10 home washes under AATCC 135. Clean the fly by hand instead, and dry it fully before it goes into storage.