What is PET?
PET stands for polyethylene terephthalate: a thermoplastic from the polyester family. Outside construction everyone knows it as the drinks bottle; inside construction it turns up as fibre insulation, foam core, clear sheet, geotextile and as the carrier fleece in bituminous roofing membranes.
PET is hard, tough, dimensionally stable and absorbs almost no water. It is one of the most readily recycled plastics — recycling code 1 — and that is exactly what makes it interesting on site: many PET building products are made from recycled bottles.
Where PET shows up in building
- PET fibre insulation. Quilts of recycled polyester fibre, used between timber studs, in roof insulation and as acoustic damping. The thermal conductivity is broadly between 0.035 and 0.040 W/m·K — comparable with mineral wool. It does not itch and cuts with a bread knife.
- PET foam. Closed-cell rigid foam used as a core in sandwich panels and composite boards. Lighter than timber, compression-resistant and rot-proof.
- Clear sheet (PET-G). Glycol-modified PET is clear, very impact-resistant and can be cold-bent without heating. It is used for screens, machine guards and rooflight glazing where polycarbonate is too expensive and acrylic too brittle.
- Polyester fleece in roofing felt. Almost every modern bituminous membrane has a PET fleece as reinforcement; it governs the tear and puncture resistance of the sheet.
- Geotextiles. Nonwoven and woven PET fabrics for separation, filtration and reinforcement in groundworks and highways.
Points to watch
- UV. Unprotected PET discolours and embrittles under prolonged sunlight. Geotextiles and external products are UV-stabilised or pigmented; do not leave fabric exposed for weeks before covering it.
- Alkaline environments. Polyester is prone to hydrolysis in a strongly alkaline setting. In direct contact with fresh concrete or lime-rich soil, polypropylene or glass fibre is often the better choice.
- Heat. PET softens at raised temperature. Keep fibre insulation and foam cores away from flues and from downlights without a clearance fitting.
- Fire. As an organic plastic, PET is combustible. Always check the declared reaction-to-fire class of the specific product and of the build-up it was tested in.
Related terms
- Plastic
- Polymer
- Polyester
- Polyethylene
- Polystyrene
- Thermal conductivity
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