What is phenolic foam insulation?
Phenolic foam is a rigid insulation material made from resol resin, a phenol-formaldehyde resin. Foaming produces very fine closed cells, which give the board a low thermal conductivity. It is abbreviated PF, and the European product standard is BS EN 13166. A well-known product range is Kingspan Kooltherm. In Dutch it is called resol hardschuim.
Properties
- Low lambda. For closed-cell phenolic boards, manufacturers typically declare values between about 0.018 and 0.023 W/mK, depending on product and facing. That is usually slightly lower than PIR and clearly lower than EPS and mineral wool.
- Thermoset. It does not melt or drip when exposed to fire.
- Fire classification varies by product. Euroclasses differ widely between products and applications. Some phenolic products achieve B-s1,d0, while others are rated considerably lower. Always check the Declaration of Performance for the exact product, and in the UK check what the Building Regulations allow for the wall type and building height.
- Facings. Glass tissue or foil facings, and for internal lining, laminated boards with vapour control layer and plasterboard.
Application
Phenolic boards are used in cavity walls, roofs, floors and internal wall insulation. The advantage is thickness. Since R = d / λ, a layer with R = 4.5 m²K/W needs about 95 mm at λ = 0.021, against about 158 mm for a material at λ = 0.035. In refurbishment, where cavity width or room depth is limited, that difference can decide the specification.
Practical tips and common mistakes
- Keep boards dry. Manufacturers require dry storage and installation onto a dry substrate, with the work protected from the weather during breaks.
- Use the right lambda. Take the declared value for the product you are buying, not a generic figure.
- Butt joints tightly. A low lambda is wasted if gaps are left between boards or mortar snots bridge the cavity.
- Compare cost per unit of performance. Phenolic is usually dearer per m² than PIR or EPS, so it pays off mainly where thickness is genuinely constrained.
Related terms
- PIR insulation
- EPS
- Mineral wool
- Cavity wall insulation
- Fire classification
- Insulation value (Rd value)
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