What is polyurethane?

Polyurethane (PUR, also PU) is formed by the reaction between a polyol and an isocyanate. By varying the formulation an extremely wide range of products results: from rock-hard insulation foam to an elastic joint sealant.

That is why the abbreviation turns up in very different places on site, while the properties differ completely from product to product.

Forms used in construction

1. PUR rigid foam insulation. Closed-cell foam with one of the most favourable thermal conductivities among common insulants: broadly 0.022 to 0.028 W/m·K. That gives a high insulation value in little thickness. Used as board in roofs, walls and floors, and as the core of sandwich panels.
2. PIR. A variant with modified chemistry (polyisocyanurate), with comparable thermal performance but better behaviour in fire. In the board trade, PUR has largely been replaced by PIR.
3. Spray foam. Foam sprayed in situ, in open-cell or closed-cell grades. Widely used for insulating floors from below and roof slopes from the inside. Note that spray foam applied to a roof structure has attracted scrutiny over inspection access and lender acceptance — check the position before specifying it in an existing roof.
4. Gun foam / expanding foam. The one-component canister foam that cures on atmospheric moisture. Intended for filling joints — not as insulation and not as an air seal.
5. PU adhesive. Moisture-curing construction adhesive for timber, including the water-resistant D4 class. It expands while curing; clamping is essential.
6. PU sealant. Elastic, paintable joint sealant with good adhesion, widely used in movement joints.
7. PU coating and varnish. Hard-wearing floor coatings and finishes for timber and metal.

Points to watch

• Isocyanates. This is the key point. Isocyanates can cause respiratory problems and sensitisation. In the EU, training has been mandatory since August 2023 for professional and industrial use of products containing more than 0.1 per cent diisocyanates; check the equivalent requirement in your own jurisdiction. Always work with respiratory protection, gloves and good ventilation, and keep others out of the area during application and curing.
• Fire performance. PUR foam is combustible and must be covered in almost every application. Assess the reaction-to-fire performance for the complete construction, not for the board alone.
• UV and heat. Unprotected foam discolours and chalks in the sun. Gun foam left uncovered loses its properties.
• Gun foam is not a seal. It is neither airtight nor vapour-tight. Always combine it with the appropriate membrane or sealant line.
• Recycling. Cured PUR is hard to separate and recycle. Weigh that up when choosing between rigid foam and a mineral or bio-based insulant.

Related terms

• PIR
• Polyurea
• Plastic
• Thermal conductivity
• Cavity wall insulation
• Expanding foam

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.