A Guide to Ultralight Films: Polycro, Polycryo, Window Insulation Films, and More

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 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?”

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: 132 N/mm² in machine direction (MD) and 141 N/mm² in transverse direction (TD).

Elongation at break: 125% in MD and 115% 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.

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.

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.

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.

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.

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.

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.

XFOIL ships worldwide for a flat €2. See the 2.4 oz groundsheet.