What is polyethylene?
Polyethylene (PE) is a thermoplastic built from long chains of the monomer ethylene. It is tough, chemically very resistant, waterproof and cheap — which makes it the most widely used plastic in construction.
The main types
- LDPE / LLDPE (low density). Soft and stretchy. This is what building film, protective sheeting and vapour control membranes are made from.
- HDPE (high density). Stiffer and stronger, with a density around 0.95 g/cm³. Used for pipes, chambers, dimpled membranes and sheet material.
- PE-X (cross-linked). The chains are chemically tied to one another, so the pipe keeps its shape at higher temperature and pressure — the standard for underfloor heating and hot and cold water pipework.
PE floats: its density is below 1 g/cm³. That is a simple way to identify it.
Applications in construction
- Building film and vapour control layers. PE film, typically 0.2 mm, beneath ground-bearing slabs and on the warm side of insulation. Note: it is a vapour control layer, not insulation, and the laps must be overlapped and taped.
- Pipework. PE pressure pipe for water (blue in the UK for potable mains, or black with blue stripes) and gas (yellow). The designation PE100 describes the long-term strength of the material; the SDR value the ratio of diameter to wall thickness, and therefore the permitted pressure.
- PE-X underfloor heating, with an oxygen diffusion barrier to prevent corrosion in the boiler and manifold.
- Geomembranes. Thick HDPE sheets as a waterproof barrier under ponds, tanks and landfill.
- Dimpled membranes and drainage sheets against basement walls.
- PE foam. Perimeter strips at screed edges, spacers and backer rod in sealant joints.
Points to watch
- UV. Unprotected PE embrittles in sunlight. External products are pigmented black with carbon black, which makes them UV-resistant. Do not leave clear film out in the sun for months.
- You cannot glue it. PE has a low surface energy; adhesives and sealants barely bond to it. Joints are made by electrofusion, butt fusion welding or mechanical couplings.
- Thermal movement. PE expands substantially with temperature change. On long runs allow for expansion loops or unclipped lengths.
- Temperature. Ordinary PE softens well below 100 °C. For hot water you need PE-X or a different material.
- Permeation. PE is watertight, but some substances — solvents and mineral oil — can diffuse through the wall. In contaminated ground a PE water main without a barrier sleeve may not be acceptable.
Related terms
- Plastic
- Polymer
- Polypropylene
- Vapour control layer
- PVC
- Dimpled membrane
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