Polystyrene Mouldings: Are They Any Good?
The short answer
It depends who made them and how.
Cheap polystyrene mouldings from a DIY shop are light foam with a thin skin of paint. Knock them and they dent. Leave them out for a few winters and the coating cracks.
A properly made exterior moulding starts with the same base material, but that's where the likeness ends. Ours use a high-density EPS core (29 to 31 kg/m³), cut on a CNC hot-wire machine to the drawing, then coated in the factory with a polymer-modified, fibre-reinforced basecoat.
What EPS actually is
EPS stands for expanded polystyrene. It's the same family of material used as insulation in walls and floors across the UK. In building terms it's light, stable, doesn't rot and doesn't take up water like timber or some stone.
On its own it isn't tough enough for a facade. That's why the coating matters more than anything else.
What makes the difference
The core. We use high-density EPS, not the lighter grades used for packaging. Higher density means a firmer, sharper profile and better arrises.
The cut. Every profile is cut on our CNC hot-wire machine from the approved drawing, so the section is accurate along the whole length and every piece matches.
The coating. Every profile gets the same polymer-modified, fibre-reinforced basecoat, applied in our factory under controlled conditions. The basecoat is covered by UKTA-0836-22/6381 (BBA) for VPI RHÉATHERM 600, and we hold written authorisation from VPI to use it.
The finish. Profiles can be supplied basecoated for the render contractor to finish, or fully finished in a stone-effect texture or a smooth painted finish with the look of Portland stone.
The fixing. Profiles are bedded on a 3-strip adhesive bed with Insta-Stik secondary fixing. Where a profile is large enough to need it, mechanical fixing is reviewed at design stage.
Why lightweight
Weight is the big advantage. A long cornice or a run of coping stones can be carried and fitted by hand, with no extra loading on the wall. Profiles are cut from your drawing, so a one-off section doesn't need a mould making first.
Where they're used
Cornices, parapet copings and cappings, garden wall copings, window cills, window heads, keystones, quoins, string bands, pediments and window surrounds. On new builds they're fixed to the blockwork with the render brought up to them. On existing houses they can be fixed to sound brickwork, stonework or render.
Buying for a wall top or a corner? Read our quick guides to coping stones and quoin stones.
Common questions
Will polystyrene mouldings last outside?
A coated, high-density moulding that's fixed and sealed properly is built for exterior use. The coating and the detailing are what protect it, so joints, top surfaces and the junctions with the wall need to be sealed and kept maintained like any other facade element.
Can you tell it's polystyrene once it's up?
Not from the street. Once finished and fixed, it reads as a stone or rendered moulding.
Talk to us
Send us your drawings or a photo of the house and we'll tell you what we'd make and how it would be fixed. Free finish samples are sent within 5 working days.