Polyethylene Sheeting Rolls: Mil Thickness, Widths and Vapour-Barrier Use

Extrusion-grade polyethylene sheeting is manufactured from low-density, linear low-density, or high-density resins with nominal densities from 0.910 g/cm³ to 0.965 g/cm³. The product is supplied in roll form with thickness designated in mils, where 1 mil equals 0.001 inch or 25.4 µm. Construction-grade rolls are commonly slit to widths of 3.05 m, 3.66 m, 4.27 m, and 6.10 m; roll lengths of 30.5 m, 61 m, and 122 m are standard, though converter specifications vary. Polyethylene sheeting intended for vapour-barrier service is evaluated under ASTM D4397, ASTM D2103, and ASTM E96; underslab products are additionally classified under ASTM E1745. The key functional variable is mil thickness, because water-vapour permeance and puncture resistance are both thickness-dependent.

How Does Mil Thickness Shift Permeance and Puncture Behaviour?

Water-vapour transmission through polyethylene occurs by solution-diffusion across the film thickness. Thicker sheet reduces the driving gradient per unit thickness and lowers the measured water-vapour permeance. Under ASTM E96 Procedure A at 23°C and 50% RH, a 6-mil (0.15 mm) LLDPE sheet typically reports a permeance below 0.1 perm; 10-mil (0.25 mm) and 15-mil (0.38 mm) HDPE or coextruded LLDPE/HDPE sheet frequently report values in the 0.02 perm to 0.05 perm range when tested by ASTM F1249 at 37.8°C and 90% RH. The precise value depends on resin crystallinity, thickness uniformity, and any coextruded barrier layer. One US perm is equivalent to 57.2 ng/(Pa·s·m²); therefore a 0.1 perm limit translates to approximately 5.7 ng/(Pa·s·m²).

Puncture resistance follows a steeper response to thickness. ASTM D1709 dart-drop values for 6-mil construction sheet typically range from 200 g to 400 g, while 10-mil and 15-mil sheet can exceed 800 g and 1200 g, respectively, depending on resin blend and orientation. ASTM D882 tensile strength is less directly thickness-dependent because it is normalised to cross-sectional area; however, total load-carrying capacity in a seam increases linearly with thickness. Field substitution of a 6-mil sheet for a specified 10-mil sheet therefore creates a larger failure risk from aggregate puncture than from permeance alone.

The underslab classification system in ASTM E1745 separates vapour retarders into Class A, Class B, and Class C based on physical-property minimums after conditioning; all three classes carry the same water-vapour permeance ceiling. The table below summarises the required values.

Site handling conditions on slab-on-grade projects determine whether Class A, Class B, or Class C sheet is selected. Angular aggregate larger than 19 mm, foot traffic from reinforcing placement, and pump-hose drag produce localised tensile loads that cannot be predicted by water-vapour transmission data alone. ACI 302.1R-18 specifies a minimum 10-mil (0.25 mm) polyolefin vapour retarder complying with ASTM E1745 Class A beneath moisture-sensitive floor finishes; the same document recommends that the retarder be placed directly beneath the concrete slab rather than under a granular blotter layer. In crawl-space practice, the 2021 IRC Section R408.3 permits a minimum 6-mil (0.15 mm) polyethylene ground cover for exposed earth, but many unvented-crawlspace specifications upgrade to 10-mil because the membrane must survive service access and radon-mitigation sealing.

When the 6-mil Ground Cover Is Upgraded to a 10-mil Underslab Retarder

The selection boundary is not solely the numerical thickness. A 6-mil sheet may satisfy the vapour-permeance requirement for a vented crawl space but is not sufficiently robust for direct under-slab installation because of construction-load puncture. ASTM E1643 requires laps not less than 150 mm and sealing with a pressure-sensitive tape that is compatible with polyethylene; the same practice requires that penetrations, plumbing stubs, and footing steps be cut and sealed without bridging. In underslab installations, the membrane is typically placed over a compacted granular base with a surface deviation less than 10 mm over a 3 m straightedge. Puncture holes from reinforcement chairs or worker traffic defeat the vapour-barrier function even when the membrane meets the 0.1 perm limit in an intact laboratory sample.

Width choice interacts with seaming and labour. A 4.27 m roll covers a typical residential footing width with a single sheet and reduces the number of longitudinal seams compared with a 3.05 m roll. Six- and ten-foot-wide rolls are used for hand application; 6.10 m (20 ft) wide rolls are generally specified where mechanical placement is available. Slitting tolerance is commonly held to ±25 mm on width. For vapour-barrier service, folded film is undesirable because fold creases become preferential paths for water-vapour transmission and can reduce tensile strength at the fold line; specifiers therefore require flat-wound rolls for underslab applications.

Published data for extremely thick coextruded underslab membranes under combined soil contact and wet-dry cycling is limited. Specifiers should request ASTM E1745 test reports from the converter rather than relying on nominal thickness alone. Polyethylene sheeting is used as a concrete-curing cover under ASTM C171; in that application, the membrane is placed after final finishing and sealed at edges to prevent moisture loss.

Vapour-Barrier Placement in Walls, Roofs and Controlled-Environment Chambers

Polyethylene sheeting is used as a Class I vapour retarder on the warm side of wall assemblies in cold climates. The 2021 IRC R702.7 classifies vapour retarders by permeance: Class I is 0.1 perm or lower, Class II is greater than 0.1 perm and not more than 1.0 perm, and Class III is greater than 1.0 perm and not more than 10 perm. A 6-mil PE sheet is generally sufficient to meet the Class I limit. However, in mixed or warm humid climates, an interior polyethylene vapour barrier can trap moisture from inward air leakage; hygrothermal analysis under ASHRAE 160 is used before specifying an interior Class I retarder. In roof assemblies, polyethylene is not normally installed as a direct underlayment because of low softening point and slip hazard; it is instead used as a temporary enclosed-space barrier or as a separating layer above insulation in vented attics where local code permits.

Controlled-environment chambers and temporary containment use polyethylene sheeting as an air and moisture barrier for dust control, concrete curing, and temporary isolation of asbestos or mould-remediation zones. In these applications, the sheet is not required to meet ASTM E1745 physical properties, but flame spread and smoke development may be regulated by NFPA 701 or ASTM E84 when used as a temporary enclosure. Fire-testing data for thin PE sheet frequently show high smoke developed indices; therefore, local permits often require flame-retardant grades containing antimony trioxide and halogenated additives.

Adhesive compatibility is limited by the low surface energy of polyethylene, typically 31 mN/m to 36 mN/m. Pressure-sensitive tape adhesion requires an aggressive acrylic or butyl formulation and, where specified, surface priming by corona treatment. Corona treatment on film for vapour barrier is generally avoided because treatment decays within 30 days and can increase roll blocking.

Process Conditions That Govern Roll Flatness and Dimensional Stability

Blown-film extrusion of polyethylene sheeting is performed on lines with annular dies from 100 mm to 600 mm in diameter, blow-up ratios between 2.0:1 and 3.5:1, and melt temperatures from 190°C to 230°C for LLDPE-rich blends. Frost-line height is maintained at 3 to 5 die diameters; internal bubble cooling and automatic air-ring control hold thickness variation to ±5% on production runs. A 3-layer coextruded 10-mil sheet may use an HDPE cap layer for stiffness and an LLDPE core for dart impact, with layer distribution held within ±2% of target by gravimetric feeders. Cast-film lines operating at 240°C to 270°C produce lower machine-direction orientation and higher Elmendorf tear values but require chill-roll temperatures of 15°C to 25°C to prevent blocking.

Slip and antiblock packages are necessary because polyethylene film blocks at roll pressures above 0.7 MPa when surface temperature exceeds 35°C during storage. Erucamide at 500 ppm to 1500 ppm migrates to the surface within 24 h to 72 h and reduces coefficient of friction to 0.2 to 0.4, but the same migratory behaviour can interfere with tape adhesion on seam laps. Therefore, seam adhesion on vapour retarders is tested under ASTM D3330 using the specified tape after 24 h dwell; peel values below 0.5 N/mm are typically rejected for underslab service.

Batch-to-batch variation in LLDPE melt index from 0.5 g/10 min to 2.0 g/10 min alters bubble stability and gauge uniformity. Converters monitor melt-flow rate under ASTM D1238 at 190°C/2.16 kg; a shift below 0.4 g/10 min can produce heavy gels, while values above 2.5 g/10 min lower melt strength and increase sag in wide-web bubble lines.

Unstabilised polyethylene has poor weathering resistance. Continuous outdoor exposure to ultraviolet radiation causes chain scission and a loss of elongation at break within 150 h to 300 h of accelerated exposure under ASTM G154 Cycle 1. Carbon black at 2 wt% to 3 wt% or hindered amine light stabiliser packages are required if the sheet is left exposed for more than 30 days. Polyethylene is compatible with dilute acids and alkalis but softens in aromatic solvents, chlorinated solvents, and some hydrocarbon streams; it is not suitable for direct contact with roofing asphalt or coal-tar pitch because of low heat deformation temperatures. Storage above 40°C or below -20°C can alter roll unwind tension and film flatness. Installation below -5°C is discouraged because pressure-sensitive tape adhesion decreases sharply at low substrate temperatures.