A Guide to Ultralight Films: Polycro, Polycryo, Window Insulation Films, and More
Updated: Oct 2
Glossary of Ultralight Films: Technical Properties, Differences, and Applications
In ultralight hiking, terms such as Polycro and Polycryo are often used as though they describe one defined material. They do not. Window-insulation film, packaging film and a specified outdoor film can look similar, yet differ in thickness, layer construction, mechanical properties and intended use.
This guide explains the common film categories and, where a technical data sheet is available, the figures that make a comparison meaningful.
Why use film at all?
In the ultralight and long-distance hiking scene there is a constant search for lighter, more efficient materials, and for some years film has been part of that search. Film is light, waterproof and packs small. In its advanced form – cross-linked polyolefin film – it is considerably lighter than traditional outdoor fabrics.
It is not better in every respect, though. At 18 gsm, XFOIL reaches a tensile strength in the range of an equally light 7D ripstop nylon (170 / 110 N per 5 cm under ISO 13934-1; XFOIL converted: about 128 / 119 N), and unlike that fabric it is waterproof. Woven fabrics have the advantage in tear resistance, and film’s puncture resistance is generally lower as well – which affects how much repair work you should expect on a trip.
Why a material name is not enough
A product called Polycro may be a household window-insulation film, a cut-down packaging film or a cross-linked multilayer polyolefin film. The name alone does not tell you its polymer blend, thickness, tensile strength, elongation, puncture resistance or whether it is supplied as one layer or a folded double layer.
For an ultralight groundsheet, those details matter more than the label. A useful question is not “Is this Polycro?” but “What is this film specified to do?”
Where the names Polycro and Polycryo come from
Both names most likely come from the ultralight hiking community, where they describe a thin plastic film used as a groundsheet. “Poly” probably refers to polyolefin, the polymer family the material belongs to. “Cryo” – cold – probably points to its origins in window insulation film, and the “cro” in Polycro may come from “cross-linked”. The exact origin isn’t certain, but the names have stuck.
XFOIL technical data
XFOIL is a cross-linked, multilayer polyolefin film. Its technical data sheet documents the following values for the material:
Thickness: 19 μm.
Tensile strength: 135 N/mm² in machine direction (MD) and 125 N/mm² in transverse direction (TD).
Elongation at break: 130% in MD and 120% in TD.
Puncture resistance: 22 N.
Film-to-film coefficient of friction (COF): 0.20.
Material classification: LDPE 04; PVC-free.
These values describe XFOIL’s film. They should not be assumed for every product sold as Polycro, Polycryo or window film.
The 160 × 240 cm sheet weighs 67 g / 2.4 oz and covers most one- and two-person tents: XFOIL Ultralight Groundsheet, 160 × 240 cm
How to read the figures
MD means machine direction: the direction in which the film was manufactured. TD means transverse direction: the direction across the film. Tensile strength describes resistance to a controlled pulling load; elongation describes how far a sample stretches before it fails. Puncture resistance describes performance in a defined puncture test.
None of these individual figures makes a 19 μm film puncture-proof or abrasion-proof in every campsite. They also do not support direct claims against Tyvek, Silnylon, DCF or a particular window film unless the competing materials were tested to the same standard, with the same sample geometry and conditions.
LDPE 04 is a material classification, not a guarantee that every local household recycling system accepts the film. Check local collection rules before disposal.
Why we sell a plastic product at all, and what that means at the end of its life, is something we discuss openly in Let’s start with an uncomfortable truth: we sell plastic.
What tensile strength, tear strength and puncture resistance actually measure – and which test standards sit behind these figures – is explained in How strong is Polycro really? If you want to check thickness and weight figures yourself, see Microns, mil, gauge and grams per square meter.
Cross-linked polyolefin films
Cross-linking connects polymer chains within the material. In a thin outdoor film, that construction is part of a design aimed at stable mechanical behaviour at very low weight. XFOIL’s film is 19 μm thick, so it remains a specialist ultralight material that benefits from careful campsite selection and early repair of any damage.
Material: high-grade cross-linked polyolefin, usually multilayer
Typical use: demanding ultralight use – groundsheets, tarps and other protective layers where the weight-to-strength ratio matters
Strengths: very low weight with a tensile strength on a par with equally light woven fabrics; cross-linking adds robustness under mechanical load
Weaknesses: lower tear and puncture resistance than woven fabrics
Repairability: easy to moderate – tape is often enough for a lasting repair, depending on the film’s thickness and structure
Window-insulation films
Window-insulation films are a popular low-cost source of very light groundsheet material. Their exact construction varies by product. Some are supplied as folded or double material; separating layers can change their practical behaviour. If no data sheet is available, compare thickness, construction and intended use carefully rather than assuming all window films are equivalent.
Some specialist insulation films carry a metallised layer, such as aluminium, to reflect radiant heat; others insulate by trapping a layer of air. Which of these you are actually buying is often unclear, and whether the film is cross-linked is almost never stated. Because window films are meant as temporary insulation, they are usually not cross-linked: cross-linking makes a film tougher and more wear-resistant, but also stiffer.
Material: polyethylene, occasionally polypropylene
Typical use: a popular low-cost groundsheet because it is light and waterproof
Strengths: very light, waterproof, easy to buy
Weaknesses: usually not cross-linked, which limits robustness; designed for the low loads of a window, not for rough outdoor conditions
Repairability: easy – patches well with tape
Packaging and stretch films
Mattress packaging, furniture packaging and pallet wrap are often repurposed by hikers. They can be useful for experiments or temporary protection, but their thickness, resin blend and intended use vary widely. Weight alone does not predict performance on the ground.
Mattress packaging from DIY stores
Material: LDPE (low-density polyethylene)
Typical use: a cheap alternative; robust and waterproof, but heavier and thicker than specialist outdoor films
Strengths: very stable and hard-wearing
Weaknesses: too heavy and thick for ultralight use, especially compared with cross-linked films
Repairability: moderate – strong tape works, but the film’s thickness can get in the way
Furniture packaging film (e.g. IKEA)
Material: LDPE or HDPE
Typical use: packaging, for example from mattresses, that some ultralight hikers reuse as a groundsheet
Strengths: very robust, tear-resistant and water-repellent
Weaknesses: too thick and heavy for ultralight use – it is made to protect goods where weight doesn’t matter
Repairability: moderate – strong tape works, though the thickness makes handling harder
Stretch film (pallet wrap, more common in the US)
Material: LLDPE (linear low-density polyethylene)
Typical use: wrapping pallets; some hikers use it for light protective covers
Strengths: very stretchy, light and cheap
Weaknesses: low tear and puncture resistance; not suitable for rough outdoor use
Repairability: difficult – it is so thin and stretchy that repairs don’t hold well
When is a film groundsheet the right choice?
Film is a good groundsheet option when minimum weight and pack size matter, and when the user is prepared to inspect the campsite. Remove sharp thorns and sticks where possible, cut the sheet slightly smaller than the tent floor, and avoid letting it extend beyond the shelter where it could collect rainwater.
On abrasive or thorny ground, a heavier footprint or another material may be the more appropriate choice. There is no universal winner: terrain, tent-floor construction, expected weather and risk tolerance determine the right trade-off.
In practice, a single sharp stone is rarely enough to damage a cross-linked film, and plenty of thru-hikers have walked long trails such as the PCT on a film groundsheet. The deciding factor is checking the campsite for thorns first. Which sheet size suits which tent is covered in our groundsheet sizing guide, and a side-by-side look at film, Tyvek, DCF and other materials is in our groundsheet materials comparison.
Groundsheets and tarps are different applications
A groundsheet lies flat and carries little tension. A tarp transfers wind loads into its tie-out points, where forces concentrate. A film tarp therefore needs deliberately designed reinforcement, sensible line angles and careful pitching. The film body is only one part of the system.
Repairability on the trail
Small holes and tears in a film can often be patched with suitable tape. Clean and dry the area first, round the tape corners to reduce lifting, and repair damage before it grows. Repair success depends on the tape, surface condition, temperature and the load at that point.
Cross-linked films generally respond well to tape repairs, which makes them practical for longer trips. Films that are not cross-linked tend to be less robust and need repairs more often. Unlike woven fabric, a torn film can often be patched quickly and permanently, as long as the damage isn’t too extensive. Which tapes work on film is covered in our adhesive tape guide.
Conclusion
Polycro is a useful community term, not a technical standard. For a meaningful comparison, ask for a data sheet with thickness, tensile strength, elongation, puncture resistance and material classification. The XFOIL specification provides those figures; other films should be judged by the same questions, not by name alone.
Every film has its place. If your current groundsheet does its job, it makes sense to keep using it for as long as possible rather than switching to a new material straight away. This guide isn’t meant to talk any material down – it’s meant to bring some clarity to the terminology.
A final note: the material details given here for other films – for example, that mattress packaging is made of LDPE – reflect our current knowledge and may not apply to every product on the market. Manufacturers often don’t state the exact material, which makes films hard to compare. If you have your own experience with any of these films, share it in the comments below.
XFOIL ships worldwide for a flat €2. See the 2.4 oz groundsheet.
Comments